Tectonic plates and the interior of the Earth Plate Tectonics

Theory A historical perspective Why is the Earth so restless? What causes the ground to shake violently, volcanoes to erupt with explosive force, and great mountain ranges to rise to incredible heights? Scientists, philosophers, and theologians have wrestled with questions such as these for centuries. Until the 1700s, most Europeans thought that a Biblical Flood played a major role in shaping the Earth's surface. This way of thinking was known as "catastrophism," and geology (the study of the Earth) was based on the belief that all earthly changes were sudden and caused by a series of catastrophes. However, by the mid-19th century, catastrophism gave way to "uniformitarianism," a new way of thinking centered around the "Uniformitarian Principle" proposed in 1785 by James Hutton, a Scottish geologist. This principle is commonly stated as follows: The present is the key to the past. Those holding this viewpoint assume that the geologic forces and processes -- gradual as well as catastrophic -- acting on the Earth today are the same as those that have acted in the geologic past. The belief that continents have not always been fixed in their present positions was suspected long before the 20th century 1 . However, it was not until 1912 that the idea of moving continents was seriously considered as a full-blown scientific theory -- called Continental Drift -- introduced in two articles published by a 32-year-old German meteorologist named Alfred Lothar Wegener. He contended that, around 200 million years ago, the supercontinent Pangaea began to split apart. Alexander Du Toit, Professor of Geology at Witwatersrand University and one of Wegener's staunchest supporters, proposed that Pangaea first broke into two large continental landmasses, Laurasia in the northern hemisphere and Gondwanaland in the southern hemisphere. Laurasia and Gondwanaland then continued to break apart into the various smaller continents that exist today. Wegener's theory was based in part on what appeared to him to be the remarkable fit of the South American and African continents, first noted by Abraham Ortelius three centuries earlier. Wegener was also intrigued by the occurrences of unusual geologic structures and of plant and animal fossils found on the matching coastlines of South America and Africa, which are now widely separated by the Atlantic Ocean. He reasoned that it was physically impossible for most of these organisms to have swum or have been transported across the vast oceans. To him, the presence of identical fossil species along the coastal parts of Africa and South America was the most compelling evidence that the two continents were once joined. 1 This notion was first suggested as early as 1596 by the Dutch map maker Abraham Ortelius in his work Thesaurus Geographicus. Ortelius suggested that the Americas were "torn away from Europe and Africa . . . by earthquakes and floods" and went on to say: "The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents]." Ortelius' idea surfaced again in the 19th century. 1 In Wegener's mind, the drifting of continents after the break-up of Pangaea explained not only the matching fossil occurrences but also the evidence of dramatic climate changes on some continents. For example, the discovery of fossils of tropical plants (in the form of coal deposits) in Antarctica led to the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow. Other mismatches of geology and climate included distinctive fossil ferns (Glossopteris) discovered in now-polar regions, and the occurrence of glacial deposits in present-day arid Africa, such as the Vaal River valley of South Africa. The theory of continental drift would become the spark that ignited a new way of viewing the Earth. But at the time Wegener introduced his theory, the scientific community firmly believed the continents and oceans to be permanent features on the Earth's surface. Not surprisingly, his proposal was not well received, even though it seemed to agree with the scientific information available at the time. A fatal weakness in Wegener's theory was that it could not satisfactorily answer the most fundamental question raised by his critics: What kind of forces could be strong enough to move such large masses of solid rock over such great distances? Wegener suggested that the continents simply plowed through the ocean floor, but Harold Jeffreys, a noted English geophysicist, argued correctly that it was physically impossible for a large mass of solid rock to plow through the ocean floor without breaking up. Undaunted by rejection, Wegener devoted the rest of his life to doggedly pursuing additional evidence to defend his theory. He froze to death in 1930 during an expedition crossing the Greenland ice cap, but the controversy he spawned raged on. However, after his death, new evidence from ocean floor exploration and other studies rekindled interest in Wegener's theory, ultimately leading to the development of the theory of plate tectonics. Plate tectonics has proven to be as important to the earth sciences as the discovery of the structure of the atom was to physics and chemistry and the theory of evolution was to the life sciences. Even though the theory of plate tectonics is now widely accepted by the scientific community, aspects of the theory are still being debated today. Ironically, one of the chief outstanding questions is the one Wegener failed to resolve: What is the nature of the forces propelling the plates? Scientists also debate how plate tectonics may have operated (if at all) earlier in the Earth's history and whether similar processes operate, or have ever operated, on other planets in our solar system. Developing the theory Continental drift was hotly debated on and off for decades following Wegener's death before it was largely dismissed as being eccentric, preposterous, and improbable. However, beginning in the 1950s, a wealth of new evidence emerged to revive the debate about Wegener's provocative ideas and their implications. In particular, four major scientific developments spurred the formulation of the plate-tectonics theory: (1) demonstration of the ruggedness and youth of the ocean floor; (2) confirmation of repeated reversals of the Earth magnetic field in the geologic past; (3) emergence of the seafloor-spreading hypothesis and associated recycling of oceanic crust; and 2 (4) precise documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges. Ocean floor mapping About two thirds of the Earth's surface lies beneath the oceans. Before the 19th century, the depths of the open ocean were largely a matter of speculation, and most people thought that the ocean floor was relatively flat and featureless. However, as early as the 16th century, a few intrepid navigators, by taking soundings with hand lines, found that the open ocean can differ considerably in depth, showing that the ocean floor was not as flat as generally believed. Oceanic exploration during the next centuries dramatically improved our knowledge of the ocean floor. We now know that most of the geologic processes occurring on land are linked, directly or indirectly, to the dynamics of the ocean floor. Measurements of ocean depths greatly increased in the 19th century, when deep-sea line soundings (bathymetric surveys) were routinely made in the Atlantic and Caribbean. In 1855, a bathymetric chart published by U.S. Navy Lieutenant Matthew Maury revealed the first evidence of underwater mountains in the central Atlantic (which he called "Middle Ground"). This was later confirmed by survey ships laying the trans-Atlantic telegraph cable. Our picture of the ocean floor greatly sharpened after World War I (1914-18), when echo-sounding devices -- primitive sonar systems -- began to measure ocean depth by recording the time it took for a sound signal (commonly an electrically generated "ping") from the ship to bounce off the ocean floor and return. Time graphs of the returned signals revealed that the ocean floor was much more rugged than previously thought. Such echo-sounding measurements clearly demonstrated the continuity and roughness of the submarine mountain chain in the central Atlantic (later called the Mid-Atlantic Ridge) suggested by the earlier bathymetric measurements. In 1947, seismologists on the U.S. research ship Atlantis found that the sediment layer on the floor of the Atlantic was much thinner than originally thought. Scientists had previously believed that the oceans have existed for at least 4 billion years, so therefore the sediment layer should have been very thick. Why then was there so little accumulation of sedimentary rock and debris on the ocean floor? The answer to this question, which came after further exploration, would prove to be vital to advancing the concept of plate tectonics. In the 1950s, oceanic exploration greatly expanded. Data gathered by oceanographic surveys conducted by many nations led to the discovery that a great mountain range on the ocean floor virtually encircled the Earth. Called the global mid-ocean ridge, this immense submarine mountain chain -- more than 50,000 kilometers (km) long and, in places, more than 800 km across -- zig-zags between the continents, winding its way around the globe like the seam on a baseball. Though hidden beneath the ocean surface, the global mid-ocean ridge system is the most prominent topographic feature on the surface of our planet. Magnetic striping and polar reversals Beginning in the 1950s, scientists, using magnetic instruments (magnetometers) adapted from airborne devices developed during World War II to detect submarines, began recognizing odd magnetic variations across the ocean floor. This finding, though unexpected, was not entirely surprising because it was known that basalt -- the iron-rich, volcanic rock making up the ocean floor-- contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. This distortion was recognized by Icelandic mariners as early as the late 18th century. More 3 important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor. Early in the 20th century, paleomagnetists (those who study the Earth's ancient magnetic field) recognized that rocks generally belong to two groups according to their magnetic properties. One group has so-called normal polarity, characterized by the magnetic minerals in the rock having the same polarity as that of the Earth's present magnetic field. This would result in the north end of the rock's "compass needle" pointing toward magnetic north. The other group, however, has reversed polarity, indicated by a polarity alignment opposite to that of the Earth's present magnetic field. In this case, the north end of the rock's compass needle would point south. How could this be? This answer lies in the magnetite in volcanic rock. Grains of magnetite -- behaving like little magnets -- can align themselves with the orientation of the Earth's magnetic field. When magma (molten rock containing minerals and gases) cools to form solid volcanic rock, the alignment of the magnetite grains is "locked in," recording the Earth's magnetic orientation or polarity (normal or reversed) at the time of cooling. As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with reversed polarity. The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping. Seafloor spreading and recycling of oceanic crust The discovery of magnetic striping naturally prompted more questions: How does the magnetic striping pattern form? And why are the stripes symmetrical around the crests of the mid-ocean ridges? These questions could not be answered without also knowing the significance of these ridges. In 1961, scientists began to theorize that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. This process, later called seafloor spreading, operating over many millions of years has built the 50,000 km-long system of mid-ocean ridges. This hypothesis was supported by several lines of evidence: (1) at or near the crest of the ridge, the rocks are very young, and they become progressively older away from the ridge crest; (2) the youngest rocks at the ridge crest always have present-day (normal) polarity; and (3) stripes of rock parallel to the ridge crest alternated in magnetic polarity (normal-reversed-normal, etc.), suggesting that the Earth's magnetic field has flip-flopped many times. By explaining both the zebralike magnetic striping and the construction of the mid-ocean ridge system, the seafloor spreading hypothesis quickly gained converts and represented another major advance in the development of the plate-tectonics theory. Furthermore, the oceanic crust now came to be appreciated as a natural "tape recording" of the history of the reversals in the Earth's magnetic field 2 . 2 Additional evidence of seafloor spreading came from an unexpected source: petroleum exploration. In the years following World War II, continental oil reserves were being depleted rapidly and the search for offshore oil was on. To conduct offshore exploration, oil companies built ships equipped with a special drilling rig and the capacity to carry many kilometers of drill pipe. This basic idea later was adapted in constructing a research vessel, named the Glomar Challenger, designed specifically for marine geology studies, including the collection of drill-core samples from the deep ocean floor. In 1968, the vessel embarked on a year-long scientific expedition, criss-crossing the Mid-Atlantic Ridge between South America and Africa and drilling core 4 A profound consequence of seafloor spreading is that new crust was, and is now, being continually created along the oceanic ridges. This idea found great favor with some scientists who claimed that the shifting of the continents can be simply explained by a large increase in size of the Earth since its formation. However, this so-called "expanding Earth" hypothesis was unsatisfactory because its supporters could offer no convincing geologic mechanism to produce such a huge, sudden expansion. Most geologists believe that the Earth has changed little, if at all, in size since its formation 4.6 billion years ago, raising a key question: how can new crust be continuously added along the oceanic ridges without increasing the size of the Earth? This question particularly intrigued Harry H. Hess, a Princeton University geologist and a Naval Reserve Rear Admiral, and Robert S. Dietz, a scientist with the U.S. Coast and Geodetic Survey who first coined the term seafloor spreading. Dietz and Hess were among the small handful who really understood the broad implications of sea floor spreading. If the Earth's crust was expanding along the oceanic ridges, Hess reasoned, it must be shrinking elsewhere. He suggested that new oceanic crust continuously spread away from the ridges in a conveyor belt-like motion. Many millions of years later, the oceanic crust eventually descends into the oceanic trenches -- very deep, narrow canyons along the rim of the Pacific Ocean basin. According to Hess, the Atlantic Ocean was expanding while the Pacific Ocean was shrinking. As old oceanic crust was consumed in the trenches, new magma rose and erupted along the spreading ridges to form new crust. In effect, the ocean basins were perpetually being "recycled," with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously. Thus, Hess' ideas neatly explained why the Earth does not get bigger with sea floor spreading, why there is so little sediment accumulation on the ocean floor, and why oceanic rocks are much younger than continental rocks. Concentration of earthquakes During the 20th century, improvements in seismic instrumentation and greater use of earthquake-recording instruments (seismographs) worldwide enabled scientists to learn that earthquakes tend to be concentrated in certain areas, most notably along the oceanic trenches and spreading ridges. By the late 1920s, seismologists were beginning to identify several prominent earthquake zones parallel to the trenches that typically were inclined 40-60° from the horizontal and extended several hundred kilometers into the Earth 3 . The study of global seismicity greatly advanced in the 1960s with the establishment of the Worldwide Standardized Seismograph Network (WWSSN) to monitor the compliance of the 1963 treaty banning above-ground testing of nuclear weapons. The much-improved data from the WWSSN instruments allowed seismologists to map precisely the zones of earthquake concentration worldwide. But what was the significance of the connection between earthquakes and oceanic trenches and ridges? The recognition of such a connection helped confirm the seafloor-spreading hypothesis by pin-pointing the zones where Hess had predicted oceanic crust is being generated (along the ridges) and the zones where oceanic lithosphere sinks back into the mantle (beneath the trenches). 3 These zones later became known as Wadati-Benioff zones, or simply Benioff zones, in honor of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States. samples at specific locations. When the ages of the samples were determined by paleontologic and isotopic dating studies, they provided the clinching evidence that proved the seafloor spreading hypothesis. 5 Plate tectonics: the cornerstone theory in Geology In geologic terms, a plate is a large, rigid slab of solid rock. The word tectonics comes from the Greek root "to build." Putting these two words together, we get the term plate tectonics, which refers to how the Earth's surface is built of plates. The theory of plate tectonics states that the Earth's outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material. Plate tectonics is a relatively new scientific concept, validated in the late 50s, early 60s, but it has revolutionized our understanding of the dynamic planet upon which we live. The theory has unified the study of the Earth by drawing together many branches of the earth sciences, from paleontology (the study of fossils) to seismology(the study of earthquakes). It has provided explanations to questions that scientists had speculated upon for centuries -- such as why earthquakes and volcanic eruptions occur in very specific areas around the world, and how and why great mountain ranges like the Alps and Himalayas formed. Understanding plate motion Scientists now have a fairly good understanding of how the plates move and how such movements relate to earthquake activity. Most movement occurs along narrow zones between plates where the results of plate-tectonic forces are most evident. There are four types of plate boundaries: o Divergent boundaries -- where new crust is generated as the plates pull away from each other. o Convergent boundaries -- where crust is destroyed as one plate dives under another. o Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other. o Plate boundary zones -- broad belts in which boundaries are not well defined and the effects of plate interaction are unclear. Divergent boundaries Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle. Picture two giant conveyor belts, facing each other but slowly moving in opposite directions as they transport newly formed oceanic crust away from the ridge crest. Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip of Africa, is but one segment of the global mid-ocean ridge system that encircles the Earth. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years. This rate may seem slow by human standards, but because this process has been going on for millions of years, it has resulted in plate movement of thousands of kilometers. Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today. 6 Convergent boundaries The size of the Earth has not changed significantly during the past 600 million years, and very likely not since shortly after its formation 4.6 billion years ago. The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created, as Harry Hess surmised. Such destruction (recycling) of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks (is subducted) under another. The location where sinking of a plate occurs is called a subduction zone. The type of convergence -- called by some a very slow "collision" -- that takes place between plates depends on the kind of lithosphere involved. Convergence can occur between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates. o Oceanic-continental convergence: if by magic we could pull a plug and drain the Pacific Ocean, we would see a most amazing sight -- a number of long narrow, curving trenches thousands of kilometers long and 8 to 10 km deep cutting into the ocean floor. Trenches are the deepest parts of the ocean floor and are created by subduction. Even though the oceanic plates are sinking smoothly and continuously into the trenches, the deepest part of the subducting plate breaks into smaller pieces that become locked in place for long periods of time before suddenly moving to generate large earthquakes. Such earthquakes are often accompanied by uplift of the land by as much as a few meters. Oceanic-continental convergence also sustains many of the Earth's active volcanoes. The eruptive activity is clearly associated with subduction, but scientists vigorously debate the possible sources of magma: Is magma generated by the partial melting of the subducted oceanic slab, or the overlying continental lithosphere, or both? o Oceanic-oceanic convergence: as with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed. The Marianas Trench (paralleling the Mariana Islands), for example, marks where the fast-moving Pacific Plate converges against the slower moving Philippine Plate. Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The trenches are the key to understanding how island arcs such as the Marianas and the Aleutian Islands have formed and why they experience numerous strong earthquakes. o Continental-continental convergence: the Himalayan mountain range dramatically demonstrates one of the most visible and spectacular consequences of plate tectonics 4 . When two continents meet head-on, neither 4 The collision of India into Asia 50 million years ago caused the Indian and Eurasian Plates to crumple up along the collision zone. After the collision, the slow continuous convergence of these two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights. Most of this growth occurred during the past 10 million years. The Himalayas, towering as high as 8,854 m above sea level, form the highest continental mountains in the world. Moreover, the neighboring Tibetan Plateau, at an average elevation of about 4,600 m, is higher than all the peaks in the Alps except for Mont Blanc and Monte Rosa, and is well above the summits of most mountains in the United States. 7 is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. Transform boundaries The zone between two plates sliding horizontally past one another is called a transform-fault boundary, or simply a transform boundary. Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes. However, a few occur on land, for example the San Andreas fault zone in California. Plate-boundary zones Not all plate boundaries are as simple as the main types discussed above. In some regions, the boundaries are not well defined because the plate-movement deformation occurring there extends over a broad belt (called a plate-boundary zone) 5 . Because plate-boundary zones involve at least two large plates and one or more microplates caught up between them, they tend to have complicated geological structures and earthquake patterns. Hotspots: mantle thermal plumes The vast majority of earthquakes and volcanic eruptions occur near plate boundaries, but there are some exceptions. For example, the Hawaiian Islands, which are entirely of volcanic origin, have formed in the middle of the Pacific Ocean more than 3,200 km from the nearest plate boundary. How do the Hawaiian Islands and other volcanoes that form in the interior of plates fit into the plate-tectonics picture? In 1963, J. Tuzo Wilson, the Canadian geophysicist who discovered transform faults, came up with an ingenious idea that became known as the "hotspot" theory. Wilson noted that in certain locations around the world, such as Hawaii, volcanism has been active for very long periods of time. This could only happen, he reasoned, if relatively small, long-lasting, and exceptionally hot regions -- called hotspots -- existed below the plates that would provide localized sources of high heat energy (thermal plumes) to sustain volcanism. Specifically, Wilson hypothesized that the distinctive linear shape of the Hawaiian Island-Emperor Seamounts chain resulted from the Pacific Plate moving over a deep, stationary hotspot in the mantle, located beneath the present-day position of the Island of Hawaii 6 . Heat from this hotspot produced a persistent source of magma by partly melting the overriding Pacific Plate. The magma, which is lighter than the surrounding solid rock, then rises through the mantle and crust to erupt onto the seafloor, forming an active seamount. Over time, countless eruptions cause the seamount to grow until it finally emerges above sea level to form an island volcano. Wilson suggested that continuing plate movement eventually carries the island beyond the hotspot, cutting it off from the magma source, and volcanism ceases. As one island volcano becomes extinct, another develops over the hotspot, and the cycle is repeated. This process of volcano 6 Although Hawaii is perhaps the best known hotspot, others are thought to exist beneath the oceans and continents. More than a hundred hotspots beneath the Earth's crust have been active during the past 10 million years. Most of these are located under plate interiors (for example, the African Plate), but some occur near diverging plate boundaries. Some are concentrated near the mid-oceanic ridge system, such as beneath Iceland, the Azores, and the Galapagos Islands. A few hotspots are thought to exist below the North American Plate. Perhaps the best known is the hotspot presumed to exist under the continental crust in the region of Yellowstone National Park in northwestern Wyoming. Here are several calderas (large craters formed by the ground collapse accompanying explosive volcanism) that were produced by three gigantic eruptions during the past two million years, the most recent of which occurred about 600,000 years ago. 5 One of these zones marks the Mediterranean-Alpine region between the Eurasian and African Plates, within which several smaller fragments of plates (microplates) have been recognized. 8 growth and death, over many millions of years, has left a long trail of volcanic islands and seamounts across the Pacific Ocean floor. What drives the plates? The tectonic plates do not randomly drift or wander about the Earth's surface; they are driven by definite yet unseen forces. Although scientists can neither precisely describe nor fully understand the forces, most believe that the relatively shallow forces driving the lithospheric plates are coupled with forces originating much deeper in the Earth. From seismic and other geophysical evidence and laboratory experiments, scientists generally agree with Harry Hess' theory that the plate-driving force is the slow movement of hot, softened mantle that lies below the rigid plates. This idea was first considered in the 1930s by Arthur Holmes, the English geologist who later influenced Harry Hess' thinking about seafloor spreading. Holmes speculated that the circular motion of the mantle carried the continents along in much the same way as a conveyor belt. However, at the time that Wegener proposed his theory of continental drift, most scientists still believed the Earth was a solid, motionless body. We now know better. Both the Earth's surface and its interior are in motion. Below the lithospheric plates, at some depth the mantle can flow, albeit slowly, in response to steady forces applied for long periods of time. Just as a solid metal like steel, when exposed to heat and pressure, can be softened and take different shapes, so too can solid rock in the mantle when subjected to heat and pressure in the Earth's interior over millions of years. The mobile rock beneath the rigid plates is believed to be moving in a circular manner somewhat like a pot of thick soup when heated to boiling. The heated soup rises to the surface, spreads and begins to cool, and then sinks back to the bottom of the pot where it is reheated and rises again. This cycle is repeated over and over to generate what scientists call a convection cell or convective flow. While convective flow can be observed easily in a pot of boiling soup, the idea of such a process stirring up the Earth's interior is much more difficult to grasp. While we know that convective motion in the Earth is much, much slower than that of boiling soup, many unanswered questions remain: How many convection cells exist? Where and how do they originate? What is their structure? Convection cannot take place without a source of heat. Heat within the Earth comes from two main sources: radioactive decay and residual heat. Radioactive decay, a spontaneous process that is the basis of "isotopic clocks" used to date rocks, involves the loss of particles from the nucleus of an isotope (the parent) to form an isotope of a new element (the daughter). The radioactive decay of naturally occurring chemical elements -- most notably uranium, thorium, and potassium -- releases energy in the form of heat, which slowly migrates toward the Earth's surface. Residual heat is gravitational energy left over from the formation of the Earth -- 4.6 billion years ago -- by the "falling together" and compression of cosmic debris. How and why the escape of interior heat becomes concentrated in certain regions to form convection cells remains a mystery. Until the 1990s, prevailing explanations about what drives plate tectonics have emphasized mantle convection, and most earth scientists believed that seafloor spreading was the primary mechanism. Cold, denser material convects downward and hotter, lighter material rises; this movement of material is an essential part of convection. Thus, subduction processes are considered to be secondary, a logical but largely passive consequence of seafloor spreading. In recent years however, the tide has turned. Most scientists now favor the notion that forces associated with subduction are more important than seafloor spreading. The gravity-controlled sinking of a cold, denser 9 oceanic slab into the subduction zone (called "slab pull") -- dragging the rest of the plate along with it -- is now considered to be the driving force of plate tectonics. We know that forces at work deep within the Earth's interior drive plate motion, but we may never fully understand the details. At present, none of the proposed mechanisms can explain all the facets of plate movement; because these forces are buried so deeply, no mechanism can be tested directly and proven beyond reasonable doubt. The fact that the tectonic plates have moved in the past and are still moving today is beyond dispute, but the details of why and how they move will continue to challenge scientists far into the future. 10 The interior of the Earth The structure of Earth's deep interior cannot be studied directly. But geologists use seismic (earthquake) waves to determine the interior of the Earth. Different types of seismic waves behave differently The two principal types of seismic waves are P-waves (pressure; goes through liquid and solid) and S-waves (shear or secondary; goes only through solid - not through liquid). As we know from physics, all waves change direction when they pass through layers of different density (refraction). That is makes seismic waves travel in curved paths through the Earth (because of the increasing pressure, materials are more dense towards the core, travel velocity of seismic waves increases). The study of the behaviour of the P and S waves allows us to calculate the position of major boundaries in the Earth's interior, as well as giving us information about the solid vs liquid character of the various layers, and even about some of their physical properties. The precise speed that a seismic wave travels depends on several factors, most important is the composition of the rock (also on the mineral phase and packing structure, temperature and pressure). We are fortunate that the speed depends on the rock type because it allows us to use observations recorded on seismograms to infer the composition or range of compositions of the planet But the process isn't always simple, because sometimes different rock types have the same seismic-wave velocity, and other factors also affect the speed, particularly temperature and pressure. Temperature tends to lower the speed of seismic waves and pressure tends to increase the speed. Pressure increases with depth in Earth because the weight of the rocks above gets larger with increasing depth. Usually, the effect of pressure is the larger and in regions of uniform composition, the velocity generally increases with depth, despite the fact that the increase of temperature with depth works to lower the wave velocity. Seismic waves move more slowly through a liquid than a solid. Molten areas within the Earth slow down P waves and stop S waves because their shearing motion cannot be transmitted through a liquid. Partially molten areas may slow down the P waves and attenuate or weaken S waves. When seismic waves pass between geologic layers with contrasting seismic velocities (when any wave passes through media with distinctly differing velocities) reflections, refraction (bending), and the production of new wave phases (e.g., an S wave produced from a P wave) often result. Sudden jumps in seismic velocities across a boundary are known as seismic discontinuities. The first discontinuity is known as the Moho. It is the boundary between the mantle and the crust Properties of the Crust Continental Crust: Depth to Moho: 20 to 70 km, average 30 to 40 km; Composition: felsic, intermediate, and mafic igneous, sedimentary, and metamorphic rocks; Age: 0 to 4 b.y.; Summary: thicker, less dense, heterogeneous, old 11 Oceanic Crust: Depth to Moho: ~7 km; Composition: mafic igneous rock (basalt & gabbro) with thin layer of sediments on top; Age: 0 to 200 m.y.; Summary: thin, more dense, homogeneous, young Seismic velocities tend to gradually increase with depth in the mantle due to the increasing pressure with depth. Theoretical analyses and laboratory experiments show that at these depths (pressures) ultramafic silicates will change phase (atomic packing structure or crystalline structure) from the crystalline structure of olivine to tighter packing structures 7 . Seismic velocities gradually increase with depth in the mantle. However, at arc distances of between about 103° and 143° no P waves are recorded. Furthermore, no S waves are record beyond about 103°. Gutenberg (1914) explained this as the result of a molten core beginning at a depth of around 2900 km. Shear waves could not penetrate this molten layer and P waves would be severely slowed and refracted (bent). Between 143° and 180° from an earthquake another refraction is recognized (Lehman, 1936) resulting from a sudden increase in P wave velocities at a depth of 5150 km. This velocity increase is consistent with a change from a molten outer core to a solid inner core. The material the core is made of must be must be denser than the mantle, and it must be dense enough to account for the rest of the mass of the Earth. Since the core makes up about one-third of the Earth's mass it must be a material that is common in the solar system. It must account for the observed seismic velocities. It should also be a material with magnetic properties to account for the Earth's magnetic field. Iron is the obvious candidate 8 . Iron, whether liquid or solid, is a conductor of electricity. Electric currents would therefore flow in molten iron. Moving a flowing electric current generates a magnetic field at a right angle to the electric current direction (basic physics of electromagnetism). The magnetic field is oriented around the axis of rotation of the Earth because the effects of the Earth's rotation on the moving fluid (coriolis force). 8 There are several kinds of meteorites that are found on Earth. One class are called differentiated meteorites. They are thought to represent a planetesimal(s) that was forming with Earth and the other planets. The planetesimal attained a large enough size to become partly/largely molten and segregate into silicate mantle and metallic core which then slowly cooled and crystallized. But the growing planet broke up because of the conflicting gravitational tugs of the Sun and Jupiter. The remains lie in orbit between Mars and Jupiter. Some of the pieces that fall to Earth are stony (mafic and ultramafic silicates) and some are iron. Iron meteorites are presumably the remains of the planetesimal's core. 7 A discontinuity at around 670 km depth is particularly distinct. The 670 km discontinuity results from the change of spinel structure to the perovskite crystalline structure which remains stable to the base of the mantle. Perovskite (same chemical formula as olivine) is then the most abundant silicate mineral in the Earth. The 670 km discontinuity is thought to represents a major boundary separating a less dense upper mantle from a more dense lower mantle 12 Evolution in the light of the Earth’s geological history The traditional Judeo-Christian version of creationism in the 19th century maintained that the earth and life on it are only about 6000 years old. It held that God created an infinite and continuous series of life forms, each one grading into the next, from simplest to most complex, and that all organisms, including humans, were created in their present form relatively recently and that they have remained unchanged since then. Given these strongly held beliefs, it is not surprising that European biology until then consisted mainly of the description of plants and animals as they are with virtually no attempt to explain how they got to be that way. Thanks to the pioneering work of geologists in the early 1800s it was possible to organise rock formations into a single colossal record of Earth’s history. Many geologists saw in this record a stormy epic, one in which our planet had been convulsed repeatedly by abrupt changes. Mountains were built in catastrophic instants, and in the process whole groups of animals became extinct and were replaced by new species. Earth’s history might not fit a strict Biblical narrative any longer. The vision of a very important geologist called Charles Lyell (1797-1875) had an equally profound effect on our understanding of life’s history. His book was an attack on the common belief among geologists and other Christians that unique catastrophes or supernatural events -- such as Noah's flood -- shaped Earth's surface. According to this view, a once-tumultuous period of change had slowed to today's calmer, more leisurely pace. Lyell argued that the formation of Earth's crust took place through countless small changes occurring over vast periods of time, all according to known natural laws. His "uniformitarian" proposal was that the forces moulding the planet today have operated continuously throughout its history. He influenced Darwin so deeply that Darwin envisioned evolution as a sort of biological uniformitarianism. Evolution took place from one generation to the next before our very eyes, he argued, but it worked too slowly for us to perceive. Geology contributed to Darwin's theory of evolution by providing evidence for an Earth that was much older than previously thought. Fossil evidence provides a record of how creatures evolved and how this process can be represented by a 'tree of life', showing that all species are related to each other. Fossils, rocks and time We study our Earth for many reasons: to find water to drink or oil to run our cars or coal to heat our homes, to know where to expect earthquakes or landslides or floods, and to try to understand our natural surroundings. Earth is constantly changing nothing on its surface is truly permanent. Rocks that are now on top of a mountain may once have been at the bottom of the sea. Thus, to understand the world we live on, we must add the dimension of time. We must study Earth's history. 13 When we talk about recorded history, time is measured in years, centuries, and tens of centuries. When we talk about Earth history, time is measured in millions and billions of years. We keep track of time with the calendar, which is based on the movements of Earth in space. People who study Earth's history also use a type of calendar, called the geologic time scale. It looks very different from the familiar calendar. In some ways, it is more like a book, and the rocks are its pages. Some of the pages are torn or missing, and the pages are not numbered, but geology gives us the tools to help us read this book. The geologic time scale Fossils are the recognizable remains of past life on Earth. Long before geologists had the means to recognize and express time in numbers of years before the present, they developed the geologic time scale. This time scale was developed gradually, mostly in Europe, over the eighteenth and nineteenth centuries. Earth's history is subdivided into eons, which are subdivided into eras, which are subdivided into periods, which are subdivided into epochs. The names of these subdivisions, like Paleozoic or Cenozoic, may look daunting, but to the geologist there are clues in some of the words. For example, zoic refers to animal life, and pa/eo means ancient, meso means middle, and ceno means recent. So the relative order of the three youngest eras, first Paleozoic, then Mesozoic, then Cenozoic, is straightforward. Fossils are the recognizable remains, such as bones, shells, or leaves, or other evidence, such as tracks, burrows, or impressions, of past life on Earth. Fossils are fundamental to the geologic time scale. The names of most of the eons and eras end in zoic, because these time intervals are often recognized on the basis of animal life 9 . Relative dating We study Earth's history by studying the record of past events that is preserved in the rocks. The layers of the rocks are the pages in our history book. Most of the rocks exposed at the surface of Earth are sedimentary formed from particles of older rocks that have been broken apart by water or wind. The gravel, sand, and mud settle to the bottom in rivers, lakes, and oceans. These sedimentary particles may bury living and dead animals and plants on the lake or sea bottom. With the passage of time and the accumulation of more particles, and often with chemical changes, the sediments at the bottom of the pile become rock and the animal skeletons and plant pieces can become fossils. To tell the age of most layered rocks, scientists study the fossils these rocks contain. Fossils provide important evidence to help determine what happened in Earth history and when it happened. Relative Dating of rocks is determining the age of materials by putting them in a sequence in order of which event took place from most recent to oldest (comparative not exact). 9 Rocks formed during the Proterozoic Eon may have fossils of relative simple organisms, such as bacteria, algae, and wormlike animals. Rocks formed during the Phanerozoic Eon may have fossils of complex animals and plants such as dinosaurs, mammals, and trees. 14 ● Law of Superposition: the lower layers in any particular cross section of rock are older than the upper layers in that cross section. ● Principle of original horizontality: sedimentary layers are horizontal, or nearly so, when originally deposited. Cross sections that are not horizontal have been deformed by movements of the Earth’s crust (folds, faults, erosion). ● Principle of crosscutting relations: geologic features, such as faults, and igneous intrusions are younger than the rocks they cut. ● Principle of faunal succession: groups of fossil plants and animals occur in the geologic record in a definite and determinable order. If there are fossils embedded in the rock, these fossils (inclusions) are younger than the rock they are embedded in but older than the rock above them. Detailed studies of many rocks from many places reveal that some fossils have a short, well-known time of existence. These useful fossils are called index fossils. ● Principle of inclusion: a rock body that contains inclusions of preexisting rocks is younger that the rocks from which the inclusions came from. Absolute dating Thus far we have been discussing the relative time scale. How can we add numbers to our time scale? Nineteenth-century geologists and paleontologists believed that Earth was quite old, but they had only crude ways of estimating just how old. The assignment of ages of rocks in thousands, millions, and billions of years was made possible by the discovery of radioactivity. Now we can use minerals that contain naturally occurring radioactive elements to calculate the numeric age of a rock in years. The basic unit of each chemical element is the atom. An atom consists of a central nucleus, which contains protons and neutrons, surrounded by a cloud of electrons. Isotopes of an element are atoms that differ from one another only in the number of neutrons in the nucleus. For example, radioactive atoms of the element potassium have 19 protons and 21 neutrons in the nucleus (potassium 40); other atoms of potassium have 19 protons and 20 or 22 neutrons (potassium 39 and potassium 41). A radioactive isotope (the parent) of one chemical element naturally converts to a stable isotope (the daughter) of another chemical element by undergoing changes in the nucleus. 15 The change from parent to daughter happens at a constant rate. The half-life of a radioactive isotope is the length of time required for exactly one-half of the parent atoms to decay to daughter atoms. Each radioactive isotope has its own unique half-life. Through radiometric dating, the age of a sample can be determined from: ● The half-life (t1/2) of a given radioactive isotope contained in a sample. ● The proportion of parent isotopes which have not transformed into daughter products yet. Precise laboratory measurements of the number of remaining atoms of the parent and the number of atoms of the new daughter produced are used to compute the age of the rock. For dating geologic materials, four parent/daughter decay series are especially useful: carbon to nitrogen, potassium to argon, rubidium to strontium, and uranium to lead. Radioactive dating works best with igneous rocks. Sedimentary rocks are formed from material that came from other rocks. For this reason, any measurements would show when the original rocks were formed, not when the sedimentary rock itself formed. Just as uranium 235 can be used to date igneous rocks, carbon 14 can be used to find the ages of the remains of some things that were once alive. Index Fossils Fossils contained within sedimentary rock can offer clues about the age of the rock. An organism that was fossilized in rock must have lived during the same time span in which the rock formed. Using information from rocks and other natural evidence, scientists have determined when specific fossilized organisms existed. If people know how long ago a fossilized organism lived, then they can figure out the age of the rock in which the fossil was found. Fossils of organisms that were common, that lived in many areas, and that existed only during specific spans of time are called index fossils. These characteristics of index fossils make them especially useful for figuring out when rock layers formed. Using Absolute and Relative Age Scientists must piece together information from all methods of determining age to figure out the story of Earth’s past. ● Radioactive dating of igneous rocks reveals their absolute age. ● Interpreting layers of sedimentary rock shows the relative order of events. ● Fossils help to sort out the sedimentary record. It is not possible to date sedimentary rocks with radioactivity directly. Geologists, however, can date any igneous rock that might have cut through or formed a layer between sedimentary layers. Then, using the absolute age of the igneous rock, geologists can estimate the ages of nearby sedimentary layers. 16 A short history of the Earth in geologic time divisions Precambrian Time: The Dawn of Life Geologic history begins with Earth’s formation 4.6 billion years ago, which is the start of Precambrian time. Earth’s oldest rocks are just under 4 billion years old. However, some mineral inclusions have been dated at 4.4 billion years. Scientific evidence indicates that Earth and the solar system formed at the same time. However, our planet is too active to preserve its oldest surface materials. The processes of plate tectonics along with metamorphism, weathering, and erosion have destroyed Earth’s original crust. Organisms alive during the Precambrian time did not have hard parts, like shells and skeletons. Therefore, they did not form easily identifiable fossils. Precambrian time makes up about 88 percent of Earth’s history. Living things evolved in the oceans. The ocean water provided them with food and protected them from harmful solar radiation. Earth’s early atmosphere probably was mostly carbon dioxide and nitrogen. It was similar in composition to the present atmosphere of Venus and Mars. Single-celled Precambrian organisms developed photosynthesis. They used carbon dioxide to store energy, releasing oxygen as a waste product. The addition of oxygen to the atmosphere had two very important results. The ozone layer formed. Oxygen in the form of ozone absorbs harmful radiation, such as ultraviolet rays, from outside Earth. Oxygen also is necessary for air-breathing animals. These developments, roughly 2 billion years ago, allowed living things to move out of the oceans. 4.6b years ago EARTH IS BORN Earth grew from a cloud of dust and rocks surrounding the young Sun. Earth formed when some of these rocks collided. Eventually they were massive enough to attract other rocks with the force of gravity, and vacuumed up all the nearby junk, becoming the Earth. The Moon probably formed soon after, when a planet-sized chunk of rock smashed into the Earth and threw up a huge cloud of debris. This condensed into the Moon. 4-3.5b years ago THE ORIGIN OF LIFE Nobody knows exactly when life began. The oldest confirmed fossils, of single-celled microorganisms, are 3.5 billion years old. Life may have begun a bit earlier than that, but probably not while huge rocks were still raining down on Earth. 3.4b years ago LIFE HARNESSES THE POWER OF SUNLIGHT (Photosynthesis) All life needs energy to survive, and the biggest source of energy for life on Earth is the Sun. Some of the early microorganisms evolved a way to use the energy from sunlight to make sugars out of simpler molecules. This process is called photosynthesis. The first photosynthetic bacteria absorbed near-infrared rather than visible light and produced sulfur or sulfate compounds rather than oxygen. Their pigments were predecessors to chlorophyll. 3b years ago? THE BEGINNING OF PLATE TECTONICS Today, Earth's surface is divided into a few dozen plates of rock, one of which sometimes ploughs under another to be destroyed in the planet's molten heart. This process, called plate tectonics, is thought to have begun around 3 billion years ago. 2.4b years ago THE GREAT OXIDATION EVENT (Breathable air) For the first half of Earth's history, there was hardly any oxygen in the air. But then some bacteria began harnessing sunlight to make sugar from carbon dioxide and water, just like green plants today. These microbes pumped out oxygen as a waste product, creating the 17 oxygen-rich atmosphere we have today. Free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have wiped out most of the Earth's anaerobic inhabitants at the time. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history. Additionally, the free oxygen reacted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the longest snowball Earth episode in the Earth's history. 2-1b years ago ENDOSYMBIOSIS (Complex cells) The first organisms were simple cells like modern bacteria, but some of them became much more internally complex. These 'eukaryotes' developed lots of specialised equipment within their cells. They also had a new source of energy: sausage-shaped objects called mitochondria that were once free-living bacteria, but which were absorbed in a process called endosymbiosis. Every animal and plant you've ever seen is a eukaryote. 1.2b years ago? THE FIRST SEX (Origin of mating) Between 1.8 billion and 800 million years ago, the fossil record looks fairly dull – so much so that the period is called the 'Boring Billion'. But behind the scenes plenty was happening. For one thing sex may have evolved for the first time. It's not clear why, or when, some organisms stopped simply dividing in two and started sex. But it was definitely going on 1.2 billion years ago: there are fossils of red algae from that time that were clearly forming specialised sex cells such as spores. 1b years ago? MULTICELLULAR LIFE (Big organisms) For the first time, life was not just made up of single cells. Now cells were teaming up to form larger organisms with things like mouths, limbs and sense organs. It's hard to say when this happened: there are fossils of large organisms dating back 2.1 billion years, but these may simply have been colonies of bacteria. Different groups of organisms probably evolved multicellularity independently. 850-635m years ago SNOWBALL EARTH (A frozen world) Earth froze over again, twice, in the space of 200 million years. The ice may well have stretched all the way from the poles to the equator. This second Snowball period may have triggered the evolution of the first complex animals. The first complex organisms, weird tube- and frond-shaped things called the Ediacarans, appeared soon after. Paleozoic Era: The Origin of Complex Life-Forms Paleozoic means “the time of early life.” This era began a little more than half a billion years ago. The presence of many fossils marks the beginning of the Paleozoic Era. This abundance of life was a result of rapid evolution. Skeletons and shells allowed some organisms to move rapidly in search of food. These parts allowed other organisms to protect themselves from becoming food. In the oceans, trilobites and the first fish appeared early in the Paleozoic Era. Trilobites evolved many variations in shape before they became extinct at the end of the Paleozoic Era. Plants and amphibians that inhabited the land also appeared in the Paleozoic Era. Amphibians lay their eggs in water; but as adults, many move onto the land. Reptiles, which can lay eggs on land, followed the amphibians. At the end of the Paleozoic Era, 95 percent of living species went extinct. Scientists do not know the cause of this extinction. They suspect some dramatic change in the world’s climates. The change might have been related to the formation of the supercontinent Pangaea. It may have been the impact of a large meteorite. There may have been great volcanic eruptions. Whatever the cause, this catastrophic event led to the appearance of new lifeforms in the next era. 535m years ago THE CAMBRIAN EXPLOSION 18 Soon after animals evolved, evolution went through a major growth spurt. In the Cambrian Explosion, it seems almost every group of modern animals appeared within tens of millions of years. This apparent 'explosion' may be partly down to better fossilisation, as many animals now had hard shells. 465m years ago PLANTS COLONISE THE LAND Some animals ventured onto land as far back as 500 million years ago, but they only visited briefly – perhaps to lay eggs in a place without predators. Plants were the first to take up permanent residence on land. The first land plants were relatives of green algae, but they rapidly diversified. 460-430m years ago THE FIRST MASS EXTINCTION The Ordovician period was a time when life flourished. But towards its end, the world cooled dramatically and ice sheets spread from the poles. The deep freeze led to the first-worst mass extinction on record, the Ordovician-Silurian. Most life was still confined to the sea, and 85% of marine species were wiped out. In the aftermath, fish became much more common. 375m years ago FISH THAT WALK ON LAND With plants well-established on land, the next step was for animals to move out of the water. Insects were among the first, around 400 million years ago. But they were followed soon after by big, backboned animals such fish that looked a bit like a salamander. Fish like that would eventually evolve four limbs, and give rise to amphibians, reptiles and mammals. Soon afterwards the Late Devonian Extinction wiped out many marine animals, including some terrifying-looking armoured fish. 320m years ago DAWN OF THE REPTILES When the first reptiles appeared, Earth was in the middle of a long cold snap called the Late Paleozoic Ice Age. Reptiles evolved from newt-like amphibians. Unlike their ancestors they had tough, scaly skin and laid eggs with hard shells that did not have to be left in water. Thanks to these advantages, they quickly became the dominant land animals. 300m years ago PANGAEA For the last time, all Earth's continents came together to form one giant supercontinent. Known as Pangaea, it was surrounded by a world-spanning ocean called Panthalassa. It lasted until 175 million years ago, when it began to tear itself apart over tens of millions of years. Its shattered remnants became the familiar modern continents. 252m years ago THE GREAT DYING (Permian extinction) Just as the reptiles were flourishing, life on Earth faced perhaps its greatest challenge. The Permian extinction was the worst mass extinction in the planet's history, obliterating up to 96% of marine species and similar numbers of land animals. We don't know for sure what caused it. In the aftermath, the first dinosaurs evolved. Mesozoic Era: The Age of Dinosaurs Mesozoic means “middle life.” This era began about 251 million years ago, following the end of the Paleozoic Era. Some forms of fish, insects, and reptiles had survived the Paleozoic Era extinction. Mammals appeared in the Mesozoic Era. However, they remained small creatures. The first birds may have evolved from flying dinosaurs, such as Archaeopteryx, in the Mesozoic Era. Dinosaur evolution produced a wide variety of animals that ruled the land during the Mesozoic Era. These animals inhabited nearly every terrestrial environment. Dinosaurs were very successful; they existed for more than 150 million years. Some may have been remarkably intelligent. Dinosaurs became extinct at the end of the Mesozoic Era. Recent evidence links that extinction to the impact of an asteroid and climatic change. 19 220m years ago THE FIRST MAMMALS At the same time that the dinosaurs were spreading and diversifying, the first mammals evolved. Early mammals were small and probably only active at night. This may have spurred them to evolve warm-bloodedness: the ability to keep their body temperature constant. 201m years ago THE TRIASSIC EXTINCTION The dinosaurs were flourishing on land, and in the sea giant reptiles called ichthyosaurs had become the top predators. Then another disaster struck. We’re not sure what caused the Triassic extinction, but it killed off around 80% of species. In the aftermath, the dinosaurs became the dominant land animals and eventually reached titanic sizes. 160m years ago THE FIRST BIRDS Birds evolved from feathered dinosaurs – modern birds are essentially Velociraptors with beaks instead of snouts and wings instead of arms. The most famous early bird, Archaeopteryx, lived 150 million years ago. 130m years ago FLOWER POWER This may sound strange, but flowers are a recent invention. There have been land plants for 465 million years, yet there were no flowers for over two-thirds of that time. Flowering plants only appeared in the middle of the dinosaur era. The equally-familiar grasses appeared even more recently. The oldest fossil grasses are just 70 million years old, although grass may have evolved a bit earlier than that. 65m years ago DEATH OF THE DINOSAURS 65 million years ago, a huge chunk of rock from outer space smashed into what is now Mexico. The explosion was devastating, but the longer-term effects were worse. Dust was thrown into the upper atmosphere and blocked out sunlight, and in the ensuing cold and darkness Earth suffered its fifth and last mass extinction. The dinosaurs were the most famous casualties. Cenozoic Era: The Age of Mammals Cenozoic means “recent life.” The Cenozoic Era began 65.5 million years ago and continues to the present. With the extinction of dinosaurs, mammals evolved as the most successful group of vertebrate animals. Mammals inhabit nearly every terrestrial environment. Whales, dolphins, and seals live in the seas, and bats fly through the air. The first humans evolved in the late Cenozoic Era. The oldest human fossils were found in Africa and are about 2 to 4 million years old. A few geologists have proposed that a new era be added. The Anthropocene Era: The Age of Humans would begin with the twentieth century, during which human technology caused dramatic changes worldwide. Extinctions of species have been fast and numerous. New life-forms have been created through genetic engineering. Climate changes, caused mostly by the use of fossil fuels, may lead to rapid changes in Earth’s climates. 60-55m years ago THE FIRST PRIMATES EVOLVE Almost immediately after the dinosaurs were wiped out, mammals evolved the ability to nourish their young inside their wombs using a placenta, just like modern humans. Soon, some of these early placental mammals evolved into the first primates. They would ultimately give rise to monkeys, apes and humans. But the first ones were small creatures. They lived in the hot and humid rainforests of Asia. 32-25m years ago SUPERCHARGED PLANTS (C4 photosynthesis) Plants have been busily harnessing sunlight to make sugar for hundreds of millions of years – a process called photosynthesis. But fairly recently, some plants have found a 20 better way to do it. C4 photosynthesis is far more efficient than normal photosynthesis, allowing C4 plants to cope with harsh conditions. 13-7m years ago THE FIRST HOMININS The first apes appeared in Africa around 25 million years ago. Then at some point, the group split into the ancestors of modern humans and the ancestors of modern apes. It's hard to say exactly when, but thanks to modern genetics and a host of fossil discoveries, we have a rough idea. 200,000 years ago THE HUMAN RACE Our species, Homo sapiens, is ridiculously young. We have only existed for a fifth of a million years. In that time we have expanded from our African birthplace to reach every continent, and even outer space. Our activities have precipitated the sixth mass extinction and unleashed the fastest episode of climate change in Earth's history.