geol unit1 pt3
Compositional Layers
Earth resembles hard-boiled egg, with three principal layers:
1. A not-so-dense crust (the eggshell) primarily (95%) composed of igneous and metamorphic rocks; remaining 5% is sedimentary rock
2. A denser solid mantle in the middle (the “white”)
3. A very dense core (the “yolk”) composed of metal
Silicates contain silicon (Si) and oxygen (O)
Compositional Layers: Crust
When you stand on the surface of Earth, you are standing on top of its outermost layer, the crust
The crust is our home and the source of all our resources
The crust consists of a variety of rocks that differ in composition (chemical makeup) from underlying mantle rock
Base of crust (or crust-mantle boundary) = Moho (Mohorovicic seismic discontinuity)
Geologists distinguish between two fundamentally different types of crust:
1. oceanic crust forming the floor of the deep oceans
2. continental crust underlying virtually all the land surface and the shallow seas
Crust: Continental Crust
Thick: 30 - 70 km thick
Old (up to 4,400,000,000 years)
Heterogeneous composition
– Felsic, intermediate, and mafic igneous rocks
– Sedimentary rocks
– Metamorphic rocks
Light (~2.7 g/cm3 )
Highly deformed by folding
Crust: Oceanic Crust
Thin: about 7-10 km thick
Young (<200,000,000 years)
Homogenous composition
-Mafic igneous rock (basalt and gabbro) with a thin layer of sediments/sedimentary rock on top
Dense (~ 3.0 g/cm3 )
Relatively undeformed
–Why?
Compositional Layers: Mantle
~2900 km thick
Constitutes the great bulk of Earth
– 82% of its volume
– 68% of its mass
More dense than crust
– Why?
Mantle contains more iron, magnesium, and calcium than the crust
– Ultramafic silicates
Compositional Layers: Mantle
The upper mantle is typically composed of peridotite, a rock dominated by the minerals olivine and pyroxene
– This means that peridotite, though rare at Earth’s surface, is the most abundant rock in our planet!
Compositional Layers: Core
16% of Earth’s volume, but 32% of its mass
We cannot see or measure Earth's core directly
– Why not? Core lies unreachably far below Earth’s crust and mantle
Geoscientists infer the presence of a metallic core
– Iron alloy (iron mixed with tiny amounts of other elements, particularly nickel)
Layering by Differing Physical Properties Mechanical (or physical) properties of a material tell us:
–How it responds to force
–How weak or strong it is
–Whether it is a solid or a liquid
Lithosphere
crust and the uppermost part of the mantle constitute a single rigid unit known as the lithosphere (from lithos, the Greek word for ‘rock’)
varies greatly in thickness – why??? Thin below ocean, thick below continental
Asthenosphere
Below the lithosphere, the mantle is relatively weak, although its chemical composition is the same
– This weak zone has been named the asthenosphere (using the Greek word for weak)
Solid, but “plastic” region of the upper mantle
– Asthenosphere is entirely in the upper mantle
Highly viscous; mechanically weak
Outer Core (Liquid)
Circulation of liquid iron alloy in the outer core generates Earth’s magnetic field
Inner Core (Solid)
– Pressure from the rest of the planet is so great that the iron cannot melt
Extremely high pressure Extremely hot
– 5,000-7,000°C (9,000- 13,000°F)
Has the properties we would expect of solid iron mixed with a small percentage of nickel
From Continental Drift to Plate Tectonics
In the early 1900’s, most geologists believed that major features of Earth’s surface were fixed and permanent, having been formed during the formation of the planet
First detailed theory of Continental Drift, a hypothesis that challenged this belief, put forth by Alfred Wegener in 1912
• In the late 1960’s, technological advancements led to the development of the Theory of Plate Tectonics • The positions of the continents and ocean basins are not fixed
Took ~50 years for the idea of plate tectonics to be accepted…
WHY DID IT TAKE SO LONG?
True revolution in thinking about Earth, and that was difficult for many established geologists to accept
Political gulf between main proponent of Continental Drift (German Alfred Wegener) and the geological establishment of the day, which was mostly centered in UK and US
Key evidence and understanding of Earth that would have supported plate tectonic theory simply didn’t exist until the middle of the 20th century (we had to wait for the technology to catch up)
“All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident.”
-Arthur Schopenhaur
Wegener’s Continental Drift Hypothesis (REJECTED)
Alfred Wegener
1880 – 1930
Ph.D., Astronomy, University of Berlin, 1904
Meteorologist by profession
First presented his theory of “continental displacement” at a meeting of the Geological Association in Frankfurt in 1912
Published The Origin of Continents and Oceans in 1915
What was Wegener’s idea?
Continents, Wegener said, wander about
They bump into each other
Collisions cause mountains
Continents were once joined together; have subsequently broken up and moved enormous distances apart
Continental Drift: An Idea Before Its Time
Continental Drift hypothesis
supercontinent, consisting of all of Earth’s landmasses, once existed
Wegener named his supercontinent Pangaea, meaning “all lands”
During the Mesozoic, ~200 million years ago, Pangaea began splitting (drifting) apart
Continental Drift: An Idea Before Its Time
Continental Drift hypothesis
supercontinent, consisting of all of Earth’s landmasses, once existed
Wegener named his supercontinent Pangaea, meaning “all lands”
During the Mesozoic, ~200 million years ago, Pangaea began splitting (drifting) apart
Continental Drift: Supporting Evidence
Fit of the Continents
fit of the continents is very good, especially of the coastlines of Africa and South America
Wegener concluded that the fit of coastlines was too good to be coincidence
His conclusion = the continents once did fit together!
Wegener published papers and books explaining his ideas of continental drift until shortly before his death in 1930. In this series of maps published in 1929, he illustrated the gradual breakup of Pangea (labels are the German names for portions of the geologic time scale
“Bullard's fit” of the continents, 1965
Wegener’s fit of opposing coastlines
1. good, but not great
2. not terribly meaningful
Fit better/more meaningful if you use the continental slope instead
real 'edge of the continent' is the continental slope
although Wegener had realized this geological reality, he did not pursue this vital line of inquiry
“Bullard's fit” of the continents, 1965
In 1965 paper, Bullard et al. pointed out that, in geological terms, the real edge of the continent is the continental slope
Used early computers to fit continents together using continental slopes (not coastlines)
A most excellent fit if you use the continental slope, not the coastline! Gap Overlap Sir Edward Bullard’s computer-aided solution to the Atlantic jigsaw Note- Bullard’s fit came after Continental Drift had been rejected!
Continental Drift: Supporting Evidence
Matching Rock Units
Continuity of Ancient Mountain Belts
Appalachian Mountains same as mountains in Greenland, Scandinavia, UK, and NW Africa
2 billion year old rock region in western Sahara (Africa) continues into the São Luis region of northeastern Brazil
Continental Drift: Supporting Evidence
Matching Rock Units
Continuity of Major Faults
Wegener’s reassembly of the continents also brought together major crustal fractures or faults
Great Glen Fault of Scotland continuation of the Cabot Fault of Newfoundland
major crustal fractures that reach the coast in the bight of Africa line up with similar fracture zones in eastern Brazil
Continental Drift: Supporting Evidence
Matching Rock Units
Not only do the outlines of the torn pieces fit together, but the printing on them (analogous to the ages and structural features of the continents) also matches across their edges
Continental Drift: Supporting Evidence Distribution of Climate Belts
Wegener’s father-in-law was Wladimir Peter Köppen
developed Köppen climate classification system
with some modifications, still commonly used!
Continental Drift: Supporting Evidence
Distribution of Climate Belts
Observed deposits of coal, desert sandstone, rock salt, wind-blown sand, gypsum, and glacial deposits laid down ~300 million years ago near the end of the Paleozoic Era
Each deposit indicates a specific climatic condition at the time of its formation
Continental Drift: Supporting Evidence
Past Glaciations
As a glacier flows, it carries sediment of all sizes
Grains protruding from the base of the moving ice carve scratches, called striations, into the substrate
When the ice melts, it leaves behind sediment in deposits called till
Continental Drift: Supporting Evidence
Late Paleozoic glacial deposits found only in Southern Hemisphere and India
areas now close to the tropics
present-day cold latitudes in Northern Hemisphere show no evidence of glaciation at this time
Continental Drift: Supporting Evidence
Arrows show direction of ice movement was from the sea toward the land-- this flow direction is impossible!!
glaciers flow from centers of accumulation on the continents outward toward the sea
Continental Drift: Supporting Evidence
both location of late Paleozoic glacial deposits and ice flow directions easily explained if:
- continents restored to their former positions according to Wegener’s theory of continental drift
- On Pangaea, areas with glacial deposits fit together with a southern polar cap
Continental Drift: Supporting Evidence
Fossils Matching Across the Seas
By 1900, most geologists and biologists accepted Charles Darwin’s theory of species evolution
Darwin noted that the offspring of various creatures, isolated from each other and exposed to different environments, evolve into quite different beings with the passage of time
Continental Drift: Supporting Evidence
Bison arose on the North American plains
Wildebeest fills a similar ecological niche in Africa
Both form huge herds, mostly survive by grazing (eating grass and seeds), but also by a little browsing (munching on the odd shrub)
Both animals have manes, wild beards, and both look like trouble
Continental Drift: Supporting Evidence
Fossils Matching Across the Seas
you would instantly distinguish a bison from a wildebeest
significant changes have taken place in the millions of years since the two animals shared a common ancestor
with very little practice, you would quickly discern the fossilized bones of each
no African creatures evolved to exactly resemble the American bison and no wildebeest herd ever roamed the Kansas grasslands
similar, but distinct – though they have common ancestors
Continental Drift: Supporting Evidence
Not “similar, but distinct” – fossils on continents separated by oceans were identical
Mesosaurus
Cynognathus
Lystrosaurus
Glossopteris
Continental Drift: Supporting Evidence
Identical fossil organisms found on continents now separated by vast oceans
If you accept Darwin’s Theory of Evolution, how can you possibly explain this?
Problem!
Geologists pointed out the most serious flaw in Wegener’s grand idea – continents couldn’t be moved by any force weaker than God
HOW did the continents wander about?
Rejection of Continental Drift Hypothesis
Objections to the continental drift hypothesis:
Wegener’s inability to identify a credible mechanism for continental drift
Incorrectly proposed the gravitational forces of the Moon and Sun were capable of moving the continents
Incorrectly suggested that continents broke through the ocean crust like icebreakers
There was strong opposition to this hypothesis from all areas of the scientific community, and it was rejected!
By analogy with a court of law, Wegener’s evidence was viewed by the jury as circumstantial and insufficient to secure a conviction
The death of Alfred Wegener (age 50)
On November 2, 1930 Wegener and Rasmus Villumsen set off to deliver supplies to a small outlying camp which had been cut off by bad weather
overtaken by a blizzard – never heard from again
Wegener’s body not found until the following spring, on May 12, 1931
lying upon a reindeer hide, placed there by Villumsen, who was never found
Theory of Plate Tectonics (our current understanding of how Earth works)
Plate Tectonics
During/following World War II, oceanographers with new equipment explored the seafloor (new technology!)
Findings:
1. Oceanic ridge system winds through all of the major oceans
2. No oceanic crust older than ~180 million years old
3. Thin sediment accumulation in the deep oceans
These developments and others led to the Theory of Plate Tectonics
Plate Tectonics
In 1965, JT Wilson published “A New Class of Faults and their Bearing on Continental Drift”
introduced plates for the first time
“mobile belts, which may take the form of mountains, mid ocean ridges and ‐ major faults with large horizontal movements…are connected into a continuous network of mobile belts about the Earth which divide the surface into several large rigid plates.”
Plate Tectonics
Rigid Lithosphere Overlies Weak Asthenosphere
Lithosphere
Earth’s strong, rigid outer layer
Asthenosphere
hotter, weaker region of the upper mantle beneath lithosphere
Because of differences in physical properties, lithosphere is effectively detached from asthenosphere, allowing layers to move separately
What Drives Plate Motions? Convection
Geologists agree that mantle convection is the ultimate driver of plate tectonics
Convection is a type of heat transfer that involves the actual movement of a substance
Plate Tectonics: Plate Movement
Plates move as somewhat rigid units relative to each other
Most interactions and deformations occur along plate boundaries
Types of plate boundaries:
Divergent plate boundaries (constructive margins)
plates move apart
Convergent plate boundaries (destructive margins)
plates move together
Transform plate boundaries (conservative margins)
plates grind laterally past each other without the production or destruction of lithosphere
Changing Plate Boundaries
Although Earth’s total surface area does not change, the size and shape of individual plates are constantly changing
Plate boundaries migrate
Plate boundaries are created and destroyed
Testing the Plate Tectonics Model
Evidence from the Ocean Floor
bathymetric (water depth) measurements
1. initially, soundings with hand lines
2. Sonar (sound navigation and ranging) revolutionizes study of ocean floor
Key finding = ocean floor much more rugged than previously thought!
Mid-ocean ridge systems located in every major ocean basin
Sonar allows us to “see” underwater
Key events in development/advancement of Sonar:
1. Loss of HMS Titanic on April 15, 1912
2. Allied shipping losses to U-boat attacks during WWI
Echo sounders calculate water depth by measuring the time it takes for the acoustic signal to reach the bottom and the echo to return to the ship
Discovery of Mid Ocean Ridges
1925-1927
German scientific ship Meteor equipped with early Sonar produced first detailed survey of southern Atlantic Ocean floor
found that a continuous mountainlike ridge runs through the Atlantic
unfortunately it was not realized at the time that this finding supported Alfred Wegener's theory of continental drift
Marie Tharp and Bruce Heezen (1953)
Tharp and Heezen created map of the ocean floor
Discovered huge (rift) valley in the center of (previously discovered) mid ocean ridge
recognized (rift) valley as spreading center
however both Tharp and Heezen tended to consider this a result of an expanding globe (they got the explanation wrong)
Marie Tharp (actually did the work) and Bruce Heezen (published/took credit for Tharp’s work)
We finally figure out the “HOW?”
Harry Hess proposes sea-floor spreading
Harry Hess and Seafloor Spreading (aka the “How?”)
1962 paper titled "History of Ocean Basins“
one of the most important contributions in the development of plate tectonics
introduced concept of seafloor spreading
outlined the basics of how seafloor spreading works
molten rock (magma) oozes up from the Earth's interior along the mid-oceanic ridges, creating new seafloor that spreads away from the active ridge crest and, eventually, sinks into the deep oceanic trenches
Seafloor Spreading: The Key to Plate Tectonics
continuous spreading produces fractures in the rift valley, into which magma from the mantle is injected to become new oceanic crust
convection currents in the mantle carry the continents away from the oceanic ridge and toward deep-sea trenches
there, the oceanic crust descends into the mantle, with the descending convection current, and is reabsorbed
in this way, the entire ocean floor is completely regenerated in ~200 million years
Testing the Plate Tectonics Model
Evidence from Paleomagnetism
Probably the most persuasive evidence
Paleomagnetism = ancient magnetism preserved in rocks
Paleomagnetic records show:
1. Apparent polar wandering (evidence that continents have moved)
2. Paleomagnetic Reversals
Recorded in rocks as they form at ocean ridges
Testing the Plate Tectonics Model
Evidence from Paleomagnetism
Basaltic rocks contain magnetite (Fe3O4 ), an iron-rich mineral influenced by Earth’s magnetic field
When the basalt cools below Curie point, iron-rich minerals become magnetized and align with existing magnetic field
magnetite is then “frozen” in position and, like a compass needle indicates the position of the magnetic north pole and magnetic inclination angle at the time the lava solidified
Testing the Plate Tectonics Model
Evidence from Paleomagnetism
When the history of Earth’s magnetic field was first examined the results were startling
Rather than pointing to the present-day magnetic poles as expected, the tiny magnetite compass needles in ancient basalt lavas were found to point in many different directions
These data indicate that either the poles or the continents (or both) moved through geologic time Spoiler alert- it was the continents that were moving!
Testing the Plate Tectonics Model
Evidence from Paleomagnetism
Paleomagnetic Reversals
Reversals are not instantaneous
they happen over a period of hundreds to thousands of years, though recent research indicates that at least one reversal could have taken place over a period of one year!
Rocks having same magnetism as present magnetic field exhibit normal polarity
Rocks having opposite magnetism exhibit reverse polarity
Once this concept was confirmed, geologists established a timescale for these occurrences --magnetic time scale
noticed symmetrical pattern of magnetic stripes on either side of the mid ocean ridges
Testing the Plate Tectonics Model
Evidence from Paleomagnetism
Basaltic rocks contain magnetite (Fe3O4 ), an iron-rich mineral influenced by Earth’s magnetic field
When the basalt cools below Curie point, iron-rich minerals become magnetized and align with existing magnetic field
magnetite is then “frozen” in position and, like a compass needle indicates the position of the magnetic north pole and magnetic inclination angle at the time the lava solidified
Testing the Plate Tectonics Model
Evidence from Paleomagnetism
When the history of Earth’s magnetic field was first examined the results were startling
Rather than pointing to the present-day magnetic poles as expected, the tiny magnetite compass needles in ancient basalt lavas were found to point in many different directions
These data indicate that either the poles or the continents (or both) moved through geologic time
Spoiler alert- it was the continents that were moving!
Testing the Plate Tectonics Model
Evidence from Ocean Drilling
1000’s of wells drilled through layers of sediment that blanket the ocean floor and the basaltic ocean crust
Findings:
1. sediments increase in age with increasing distance from the ridge crest
2. sediments almost absent on ridge crest; sediments thickest furthest from the spreading center
Pattern of distribution is expected with Hess’s seafloor spreading hypothesis being correct
Testing the Plate Tectonics Model
Evidence from Seismic Surveys
Seismic surveys use acoustic waves to create images of the subsurface
oil and gas industry routinely uses 2D and 3D seismic surveys to locate crude oil and natural gas in the subsurface
acquisition of seismic survey using ship, air guns and hydrophones
Testing the Plate Tectonics Model
Evidence from Seismic Surveys
Findings
Sediment thicknesses were up to several thousands of meters thick near the continents, they were relatively thin—or even non-existent — in the ocean ridge areas
Same thing ocean drilling had discovered…
crust is relatively thin under the oceans (5 km to 6 km) compared to the continents (30 km to 60 km) and geologically very consistent, ocean crust composed almost entirely of basalt
Testing the Plate Tectonics Model
Evidence from Hot Spots and Mantle Plumes
mantle plume is a cylindrically shaped upwelling (i.e. blob) of molten rock (magma)
surface expression of a mantle plume is an area of volcanism called a hot spot
as a plate moves over a hot spot, a chain of volcanoes, known as a hot-spot track, forms
age of each volcano indicates how much time has elapsed since it was over the mantle plume
Examples: Hawaiian Islands, Yellowstone
Hotspots
A plume of hot material rises from deep within Earth’s interior
The plume impinges on the lithosphere, leading to the outpouring of of flood basalts on the ocean floor.
Waning volcanic activity produces individual volcanoes that are carried away by plate motion to form a chain of extinct volcanoes.
Plate Boundaries
Plates bounded by three distinct types of boundaries differentiated by the type of movement they exhibit
1. Divergent plate boundaries
where two plates move apart
results in upwelling of hot material from mantle to create new seafloor
2. Convergent plate boundaries
where two plates move together
results in either:
oceanic lithosphere descending beneath an overriding plate, eventually to be reabsorbed into the mantle
collision of two continental blocks to create a mountain belt
3. Transform plate boundaries
two plates grind past each other without the production or destruction of lithosphere
Plate boundaries
Divergent and convergent plate boundaries each account for about 40% of all plate boundaries.
Transform faults account for the remaining 20%
Oceanic-Oceanic Divergent Oceanic Ridge (mid-ocean ridge)
broad, linear swell (mid ocean ridge) along a divergent plate boundary
Longest topographic feature on Earth
Occupy elevated positions
Segments offset by transform faults (strike-slip motion)
Extensive faulting and earthquakes (shallow focus)
Rift valley (deep, down-faulted structure) exists on axis of most ridges
Volcanoes (new ocean crust created here)
Remember Seafloor Spreading?
Harry Hess, 1962
Seafloor spreading occurs along mid ocean ridges
Newly formed melt (from decompression melting of mantle) slowly rises toward surface
Most melt (magma) solidifies in lower crust, but some escapes to sea floor and erupts as lava
Volcanoes present here!
Why Are Ocean Ridges Elevated?
Newly created lithosphere is hot and less dense than surrounding rocks
As the newly formed crust moves away from the spreading center, it cools and increases in density
Divergent Plate Boundaries Continental Rifts (Continent-Continent)
Evolution of an Ocean Basin
Divergent plate margins first develop in continents
New ocean basin begins with formation of continental rift
elongated depression (rift valley) where lithosphere is stretched and thinned
When lithosphere is thick and cold, rifts are narrow:
East African Rift, Rio Grande Rift, Baikal Rift, Rhine Valley
When the lithosphere is thin and hot, rift can be very wide:
Basin and Range, USA
East African Rift Valley
Continental rift extending through eastern Africa
Consists of several interconnected rift valleys
Normal faulting led to grabens (down-faulted blocks)
Area has expensive basaltic flows and volcanic cones (volcanoes present here!)
lithosphere is thick and cold, rifts are narrow
Mount Kilimanjaro, Tanzania (Volcano!)
Red Sea Rift
Formed when Arabian Peninsula rifted from Africa beginning ~30 million years ago
Fault scarps surrounding Red Sea similar to structures seen in East African Rift
If spreading continues, Red Sea will grow wider and develop an elongated mid-ocean ridge
Atlantic Ocean
After tens of millions of years, the Red Sea may develop into a feature similar to the Atlantic Ocean
As new oceanic crust was added to the diverging plates, rifted margins moved further from region of upwelling
These margins cooled and subsided below sea level
New Madrid Seismic Zone (NMSZ)
prolific source of intraplate earthquakes (earthquakes within a tectonic plate) in the southern and midwestern US
responsible for the 1811–12 New Madrid, MO earthquakes
M 7-8 (?)
has the potential to produce large (M 7-8?) earthquakes in the future!
Reelfoot Rift formed during breakup of supercontinent Rodinia ~ 750 million years ago)
resulting rift system failed to split the continent, but has remained as zone of weakness deep underground
Charleston, South Carolina Earthquake
September 01 02:51 UTC (local August 31), 1886
Magnitude 7.3
Convergent Plate Boundaries: 3 types of convergent plate boundaries
ocean-continental
ocean-ocean
Continental-continental (mountains)
Ocean will subduct, older will always be the one subducting
Convergent Plate Boundaries Oceanic-Oceanic
two oceanic plates move toward one another
one oceanic plate bends and sinks down into the asthenosphere beneath the other plate
Geologists refer to the sinking process as subduction
These types of convergent boundaries are also known as subduction zones
EARTHQUAKES and VOLCANOES have the potential to cause TSUNAMIS at subduction zones
Major Features of Subduction Zones
Volcanic island arc
Deep-ocean trench
Forearc region
Back-arc region
Volcanic Island Arc
subducting slab partially melts the overlying mantle wedge
Melt migrates upward through the overlying oceanic lithosphere and forms a growth called a volcanic island arc or island arc
Volcanic Island Arc
The Aleutian Islands in Alaska are a volcanic island arc formed via subduction
Deep Ocean Trenches
Trenches created when oceanic lithosphere bends as it descends into the mantle
Trench depth is related to the age of the subducting lithosphere
Old lithosphere is cold and dense
Plates subduct at steep angle, producing a deep trench
Young lithosphere is warm and buoyant
Plates subduct at shallower angle and produce shallower trenches (if at all)
Forearc and Back-Arc Regions
forearc region
area between the trench and the volcanic island arc
back-arc region
located on the side of the volcanic island arc opposite the trench
both regions consist of pyroclastic material and eroded sediments
tensional forces prevalent in these regions, causing stretching
Convergent Plate Boundaries Oceanic-Continent
Convergent Plate Boundaries Ocean-Continent
Oceanic plate and continental plate move toward one another
oceanic plate bends and sinks down into the asthenosphere beneath the continental plate
Geologists refer to the sinking process as subduction
These types of convergent boundaries are also known as subduction zones
EARTHQUAKES and VOLCANOES have the potential to cause TSUNAMIS at subduction zones!
Major Features of Oceanic/Continent Subduction Zones
Continental volcanic arc
Deep-ocean trench
Convergent Plate Boundaries: Ocean-Continent
oceanic plate always denser than continental plate
oceanic plate subducts and melts, generating magma
magma rises
if magma reaches surface, volcanoes form
Convergent Plate Boundaries: Ocean-Continent
thick continental crust impedes the ascent of magma
most magma never reaches the surface and crystallizes underground as massive plutons called batholiths
uplift and erosion eventually expose the batholiths
typically range from granites to diorites
Convergent Plate Boundaries Continent-Continent
Two continental plates move toward one another
neither plate can be subducted due to their high buoyancy (continental crust=styrofoam)
The two continents become welded together as they are compressed together over time
The underthrusting of one continent thickens the crust under the other
EARTHQUAKES
Mountain Belts - no volcanoes!!
Continent-Continent Collisions: Collisional Mountain Belts
zone, where two continents collide, is called a suture
Typically contains slivers of oceanic lithosphere
deformation of a thick sequence of sedimentary rocks called a fold-andthrust belt
aka collisional mountain belts…
Collisional Mountain Belts: Himalayas
Prior to the collision, India’s northern margin consisted of a thick platform of continental shelf sediments
Asia’s southern margin was an active continental margin with a well-developed accretionary wedge and volcanic arc
Collisional Mountain Belts: Himalayas
ensuing continental collision folded and faulted the crustal rocks that lay across the margins of these continents to form the Himalayas
Mount Everest, Nepal and China (29,032′)
Collisional Mountain Belts: Appalachians
A volcanic arc (Taconic volcanic arc) develops to the east of North America
Taconic Orogeny
volcanic arc thrust over the continental block 450 million years ago
volcanic rocks and marine sedimentary rocks were metamorphosed and are exposed in New York
Acadian Orogeny
Continued closing of the ocean basin resulted in Avalonia microcontinent colliding with North America 350 ma ago
Thrust faults, metamorphism, and granite intrusions are associated with this event
Substantially added to the width of North America
Alleghanian Orogeny
Africa collided with North America 250–300 million years ago
Material was displaced 250 kilometers inland on North America
Pangaea began rifting apart ~200 million years ago (during the Early Jurassic Period)
Rift was eastward of the suture, leaving a remnant of Africa welded to North America
Transform boundaries
zones of shearing, where two plates slide horizontally past each other
Rocks in the shear zone are strongly deformed, BUT crust is neither produced nor destroyed as the plates slide horizontally past each other
EARTHQUAKES
NO VOLCANOES
Most transform faults are found on the ocean floor
commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes
Transform Boundaries
a few transform faults occur on land
Example: San Andreas fault zone
transform fault connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda-Juan de Fuca-Explorer Ridge, another divergent boundary to the north
Transform Plate Boundary M 7.0 Haiti earthquake - January 12, 2010
result of shallow strike-slip faulting in the boundary region separating Caribbean plate and North America plate
Enriquillo-Plantain Garden fault zone (EPGFZ)
220,000-300,000 deaths
estimates of death toll vary widely…
Hot Spots and Mantle Plumes
Hotspots are the surface expressions of mantle plumes
Mantle plumes are hot columns of partially molten rock anchored (at least relative to plate movements) in the deep mantle
origin of mantle plumes unclear…
geoscientists infer they arise from deep in the mantle, near the boundary between the core and the mantle (?)
Hot Spots and Mantle Plumes
At a hotspot, plumes of abnormally hot but solid rock rising within Earth’s mantle begin to melt as the rock pressure on them drops (pressure reduction melting)
Wherever peridotite of the asthenosphere partially melts, it releases basalt magma that fuels a volcano on the surface
If the hotspot is under the ocean floor, the basalt magma erupts as basalt lava
If the hot basalt magma rises under continental rocks, it partially melts continental rocks to form rhyolite magma
rhyolite magma often produces violent eruptions
Hot Spots and Mantle Plumes
Hotspots are the surface expressions of mantle plumes
Mantle plumes are hot columns of partially molten rock anchored (at least relative to plate movements) in the deep mantle
origin of mantle plumes unclear…
geoscientists infer they arise from deep in the mantle, near the boundary between the core and the mantle (?)
Hot Spots and Mantle Plumes
At a hotspot, plumes of abnormally hot but solid rock rising within Earth’s mantle begin to melt as the rock pressure on them drops (pressure reduction melting)
Wherever peridotite of the asthenosphere partially melts, it releases basalt magma that fuels a volcano on the surface
If the hotspot is under the ocean floor, the basalt magma erupts as basalt lava
If the hot basalt magma rises under continental rocks, it partially melts continental rocks to form rhyolite magma
rhyolite magma often produces violent eruptions
Hot Spots and Mantle Plumes
presence of hot spot inferred by anomalous volcanism (i.e. not at a plate boundary) • Hawaiian volcanoes located in middle of Pacific Plate
hot spots also develop beneath continents
Yellowstone hot spot
Hot Spots and Mantle Plumes
As oceanic volcanoes move away from the hot spot, they cool and subside, producing older islands, atolls, and seamounts
As continental volcanoes move away from the hot spot, they cool, subside, and become extinct
Earth is old… really, really, really old
~4,600,000,000 (~4.6 billion) Years old
4,600 million years (Ma) = 4.6 billion years
Precambrian time (boring!) represents the first 87 yards
Dinosaurs first appeared 5 yards from the goal line (exciting!)
– 1 yard line dinosaurs go extinct in MASS EXTINCTION
The glacial epoch (ICE AGE) occurred in the last inch
Historic time is so short that it cannot be represented on this figure
Geologic Time: Rocks as Clocks
The Importance of a Time Scale
Rocks record geologic and evolutionary changes throughout Earth’s existance
Without a time perspective, these events have very little meaning
Principle of Cross-Cutting Relations (Lyell)
Younger features cut across older features
An event that cuts across existing rock is younger than the disturbed rocks (the rocks it cuts)
Principle of Cross-Cutting Relations (Lyell)
Which is older? Granite (a)
– Igneous Dike
– Granite Hint
what cuts what? dike(a)
Principle of Cross-Cutting Relations (Lyell)
Which is older? Rock (a)
-Fault?
– Rock?
Correlation of Rock Layers: Fossil Assemblages
fossil assemblage
–group of fossils used to determine a rock’s age
Geologic Time: How old is the Earth? …and how do we know?
Archbishop James Ussher (1581-1656)
Volume 4 of Annals of the World (1650)
– “…beginning of the night which preceded the 23rd of October in the year 710 of the Julian period”
Earth created in 4004 B.C.E. – 4004 + 2023 = 6027 (Earth currently 6027 years old)
“Salt Clock” or Chemical Denudation
Hypothesis - Streams continually bring dissolved salts into the oceans, so over time, the oceans should get saltier
How much time it would take for an initially fresh-water ocean to achieve its current level of salinity?
– Earth cannot be very young (few thousand years) -- oceans would still be mostly freshwater
– Earth could not be infinitely old -- oceans would be thoroughly saturated with salt, like the Dead Sea
“Salt Clock” or Chemical Denudation
Edmond Halley (1656–1742) – Idea only; didn’t actually do the math…
These guys actually do the math…
– T. Mellard Reade (1876): 25 Ma
– John Joly (1899): 99.4 Ma and (1909) 80-150 Ma
– George F. Becker (1910): 50-70 Ma
Rivers dissolve rocks, which makes the water hold ions. Makes the sea saltier.
George-Louis Leclerc, Comte de Buffon (1707-1788)
Epochs of Nature (1774)
Estimated the time it would take for molten Earth to cool to its present state
– Actually performed experiments designed to help him make the best possible estimates
Experimented with heating iron spheres and scaling their cooling to an Earth-sized mass
– His conclusion: Earth ~75,000 years old
William Thomson, 1st Baron Kelvin (1824-1907)
Refined Buffon's calculations
Assumed that Earth had formed as a completely molten object, and determined the amount of time it would take for the near-surface temperature gradient to decrease to its present value
– Kelvin conceded that his calculation would only be accurate if no other source of heat existed within the Earth
His latest (and most confident) answer, reached in 1897 after more than 50 years of study, was in the range of around 24 million years Oops!!
Turns out there is another source of heat – radioactive decay!
Atomic Number
All atoms of a given element have the same number of protons in their nucleus—we call this number the atomic number
–Atomic number = number of protons in the nucleus of an atom
Determines the atom’s chemical nature
Mass Number
Number of protons + number of neutrons
Atoms of a given element always have the same atomic number, but they can have different mass numbers
Isotopes
Carbon always has atomic number 6, but its mass number can be 12, 13, or 14
12C = 6 protons, 6 neutrons
13C = 6 protons, 7 neutrons
14C = 6protons, 8 neutrons
Potassium-40 to Argon-40
One of the isotope pairs widely used in geology is the decay of K-40 to Ar-40 (potassium-40 to argon-40)
K-40 is a radioactive isotope of potassium that is present in very small amounts in all minerals that have potassium in them
It has a half-life of 1.3 billion years (Ga)
– Over a period of 1.3 Ga one half of the 40K atoms in a mineral or rock will decay to 40Ar, and over the next 1.3 Ga one-half of the remaining atoms will decay, and so on
Potassium-40 to Argon-40
In order to use the K-Ar dating technique, we need to have a rock that includes a potassiumbearing mineral
One good example is the igneous rock granite, which normally contains the mineral potassium feldspar
Dating with Radioactivity
A complex process!
Determining the quantities of parent and daughter isotopes must be precise
Some radioactive materials do not decay directly into stable daughter isotopes
Radiocarbon dating
Radioactive isotope = carbon-14
Half-life of carbon-14 is 5730 years
Can be used to date events from the historic past to events as old as ~70,000 years
Only useful in dating organic matter (doesn’t work for rocks)
Radiometric dating of Earth rocks
Earth is geologically active; surface rocks typically get recycled
Must find old rocks!
– Jack Hills, Australia (4.4 Gyr)
– Acasta Gniess, Canada (4.03 Gyr)
– Isua Greenstone Belt, Greenland (3.8 Gyr)
Radiometric dating of Lunar (Moon) rocks
Radiometric dating indicates Lunar (Moon) rocks 4.360 Ga
Apollo 17 mission, astronaut and geologist Harrison H. "Jack" Schmitt
Radiometric Dating of Meteorites
Meteorites are rocks that has fallen to Earth from outer space
4.568 billion years old
– Oldest age obtained for any Solar System object so far NWA 2364 meteorite (NW Africa)
Structure of the Geologic Time Scale: Eons
Geological time divided into four eons:
Hadean (4567 to 4000 Ma)- oldest
Archean (4000 to 2500 Ma)
Proterozoic (2500 to 538.8 Ma)
Phanerozoic (538.8 Ma to present)-youngest
Structure of the Geologic Time Scale: Eons
Proterozoic Eon
“Before Life”
2500 Ma – 538.8 Ma
Archean Eon
“Ancient”
4000 Ma – 2500 Ma
Hadean Eon (oldest)
4567 Ma to ~4000 Ma
isotope - same number of protons, different number of neutrons, different atomic mass
Phanerozoic eon
“visible life”
538.8 Ma to present
We live in the Phanerozoic eon
Structure of the Geologic Time Scale: Eras
Eons are divided into eras
The Phanerozoic eon is divided into three eras:
Paleozoic era (oldest) »538.8 Ma (“early or ancient life”)
Mesozoic era »252 Ma (“middle life”)
Cenozoic era »66 Ma (“new or recent life”)
We live in the Cenozoic era
Phanerozoic eon— the past 538.8 Ma of Earth’s history —is divided into three eras:
Paleozoic (“early or ancient life”)
Mesozoic (“middle life”)
Cenozoic (“new or recent life”)
Put time boundaries in areas of mass extinction
EX: paleozoic era ended with the extinction of dinos
Structure of the Geologic Time Scale: Period
Eras are divided into periods
Cenozoic era, which represents the past 66 Ma, is divided into three periods
1. Paleogene (oldest)
2. Neogene
3. Quaternary (present)
EON ->ERA-> PERIOD ->EPOCH
-Four eons
Hadean
Archean
Proterozoic
Phanerozoic
-Phanerozoic eon divided into three eras
Paleozoic
Mesozoic
Cenozoic
-Eras are further divided into periods
The cenozoic era is divided into three periods
Paleogene
Neogene
Quaternary
- Periods are divided into epochs
Element
Most fundamental substance into which matter can be separated by chemical means
An element is a pure substance that cannot be separated into other materials
~92 naturally occurring elements
Can also be synthesized (made) in a lab
Organized in a periodic table so that those with similar properties line up
Atom
Smallest unit of an element that possesses the properties of the element
Composed of:
Protons: charge of +1
Nucleus
Neutrons: charge of 0
Electrons: charge of –1
Electrons exist as a cloud of negative charges surrounding the nucleus
Atomic Number
Atomic number = number of protons in the nucleus of an atom
Determines the atom’s chemical nature
Fun Terminology
Mass Number/Atomic Mass Number
the number of protons plus neutrons in an atom of an element
atoms of a given element always have the same atomic number, but they can have different mass numbers
Isotopes
Iron has four naturally-occurring stable isotopes, 54Fe, 56Fe, 57Fe and 58Fe
Iron always has atomic number 26, but its mass number can be 54, 56, 57, or 58
54Fe = 26 protons, 28 neutrons
56Fe = 26 protons, 30 neutrons
57Fe = 26 protons, 31 neutrons
58Fe = 26 protons, 32 neutrons
Ions
Atoms that have as many electrons (-1 charge) as protons (+1 charge) are electrically neutral
Atoms can gain or lose electrons in their outermost shells
Atom that loses or gains electron(s) has a net electric charge and is called an ion
An ion is atom that is not neutral
Ions form in solution
Ex: saltwater
An ion with an excess negative charge (it has more electrons than protons) is an anion
example: Cl– has a single excess electron so has -1 charge
An ion with an excess positive charge (it has more protons than electrons) is a cation
Example: Fe2+ is missing two electrons so has +2 charge
Importance: attraction between cations and anions is the bonding force that often holds matter together
cations are positive- Grumpy Cat always has a positive attitude!
Chemical Bonds
A chemical bond is an attractive force that holds two or more atoms together
There are several types of chemical bonds:
ionic bonds form when a cation and an anion (ions with opposite charges) get close together and attract each other
covalent bonds form when atoms share electrons
In materials with metallic bonds, some of the electrons can move freely
Chemical Bonds: Ionic Bonding (transfer)
One atom (metal) transfers an electron to another atom (nonmetal)
Attractive force is set up that creates an ionic bond
Halite (NaCl)—Example of Ionic Bonding
The transfer of an electron from a sodium(NA) to (CL) chlorine atom leads to the formation of a Na+ ion and a Cl- ion
Covalent Bonding (share)
Electrons from different nonmetal atoms “pair-up”
No electrons are lost/gained
No ions are formed
Each atom shares the electrons in order to fill the outer shell
Two hydrogen atoms combine to form a hydrogen molecule, held together by the attraction of oppositely charged particles- positively charged protons in each nuclei and negatively charged electrons that surround these nuclei.
Metallic Bonding (free flow)
Positive metal ions attract conducting electrons
Atoms are so tightly packed that electrons can be shared among several atoms
Valence electrons are free to migrate among atoms
This mobility accounts for the high electrical conductivity and ductile behavior of metals
Van der Waals Bonding (stick together)
Weak attraction between electrically neutral molecules that have asymmetrical charge distribution
Minerals
Minerals are all around us!
the graphite in your pencil
the salt on your fries
the drywall on your walls
the trace amounts of gold in your computer
everything made of metal is derived from minerals
minerals can be found in a wide variety of consumer products including paper, medicine, processed foods, cosmetics, electronic devices, and many more
Minerals are there, even if you can’t see them with your eyes
Minerals: Building Blocks of Rocks
Definition of a Mineral:
Naturally occurring
(Generally) inorganic
Solid substance
Crystalline material - specific, orderly crystalline structure
Definite chemical composition
Definition of a Rock:
Naturally occurring aggregate of minerals
Never, ever confuse rocks and minerals! minerals are to rocks as letters are to words.
Naturally Occurring
true minerals grow in nature, not in factories
only naturally occurring inorganic solids are minerals
chemists can manufacture materials that have characteristics virtually identical to those of real minerals
Such materials can be referred to as synthetic minerals
Synthetic products are not minerals!
(generally) Inorganic
distinction between inorganic and organic compounds not always clear
living (organic) vs nonliving (inorganic)
biogenic CaCO3 becomes problematic…
carbon (organic) vs no carbon (inorganic
calcite, graphite, diamond, etc. become problematic…
define an organic compound as one containing both carbon and hydrogen
Coal (not a mineral)
Solid
A mineral, like any matter in the solid state, can maintain its shape indefinitely, so it will not conform to the shape of its container
Minerals cannot be liquids (such as oil or water) or gases (such as air)
Liquid mercury is not a mineral (because it’s a liquid!)
Crystalline Material: Specific, Orderly Crystalline Structure
In a crystalline material, the atoms reside in an orderly, fixed pattern, locked in place by chemical bonds
The three-dimensional geometric arrangement of atoms or ions that defines that pattern is called a crystal (crystalline) structure
Minerals have a specific, orderly crystalline structure
René-Just Haüy (1743-1822)
Opal is a mineraloid (not a mineral!) because although it has all of the other properties of a mineral, it does not have a specific, orderly crystalline structure
crystalline structure of a mineral controls its physical properties
diamond and graphite are minerals
diamond and graphite have the same chemical composition [(carbon (C)], but the carbon atoms form different crystalline structures
Definite Chemical Composition
minerals have a specific (definite) chemical formula or composition
Can be expressed by a specific chemical formula
Quartz is SiO2 …always
Calcite is CaCO3… always
Halite is NaCl… always
How do minerals form?
New mineral crystals can form in several ways:
Solidification (freezing) of a melt
Precipitation from aqueous solution (water)
Solid-state diffusion
Biomineralization
Chemical Weathering
Precipitation from gaseous emanations (i.e. precipitation directly from a gas)
Metamorphism
Solidification (freezing) of a melt
Happens when a liquid (magma/lava) cools and turns into a solid
Similar to water freezing
When the magma/lava is hot, atoms are mobile
When the magma/lava cools, atoms slow and begin to chemically combine
water crystallizes to form ice (a mineral)
magma/lava crystallizes to form minerals
Precipitation from aqueous solution (water)
ions dissolved in an aqueous solution reach saturation and start forming crystalline solids
drop in temperature or water loss through evaporation can cause ions to reach saturation
Solid-state diffusion
results from the movement of atoms or ions through a solid to arrange into a new crystal structure
Garnets, for example, grow by diffusion in solid rock
During this process, they replace pre-existing minerals
Biomineralization
takes place when minerals grow at the interface between the physical and biological components of the Earth System
This process can happen because metabolic processes of some living organisms can cause minerals to precipitate either within their bodies, on their bodies, or immediately adjacent to their bodies
Shells (composed of the mineral aragonite or calcite) produced by marine organisms grow when these organisms extract ions from the water they live in
Chemical weathering
minerals unstable at Earth’s surface may be altered to other minerals
Precipitation directly from a gas can occur around volcanic vents or around geysers
volcanic gases or steam enter the atmosphere and cool, so some gas molecules of some elements are able to bind together
some of the bright yellow sulfur deposits found in volcanic regions form in this way
Metamorphism
formation of new minerals directly from the elements within existing minerals under conditions of elevated temperature and pressure
No melting occurs!
EX: blue kyanite
Graphite and diamond are both made entirely out of carbon
If we put graphite under a huge amount of pressure, the carbon atoms will be squeezed together and will rearrange themselves into the more compact crystal structure of diamonds
Minerals
~3800 minerals (!)
Rock-Forming Minerals
common minerals that make up most of the rocks of Earth’s crust
less than 2 dozen
A much more manageable number!!!
composed mainly of the 8 elements that make up most of Earth’s crust
Most minerals are made up of a cation (a positively charged ion) or several cations and an anion (a negatively charged ion (e.g., S2–)) or an anion complex (e.g., SO4 2–)
For example, in the mineral hematite (Fe2O3 ):
the cation is Fe3 + (iron)
the anion is O2– (oxygen)
8 most common elements in Earth’s crust
Oxygen
Silicon
Aluminum
Iron
Calcium
Sodium
Potassium
Magnesium
Mineral Classification
We classify minerals according to the anion part of the mineral formula
recall, an anion is a negatively charged ion such as Cl- or CO 3 -2
mineral formulas are always written with the anion on the right
pyrite (FeS2 )
Fe 2 + is the cation
S – is the anion
Carbonates
carbon and oxygen bonded with another element
Oxides
oxygen bonded with a metal
Halides
chlorine or fluorine, typically bonded with a metal from left side of table
Sulfates (Sulphates)
sulfur combined with oxygen and bonded to a metal
Sulfides (Sulphides)
sulfur bonded with a metal
Native minerals
contain a single element
Silicate Structures: Independent (or Isolated) Tetrahedra
In this group, the silica tetrahedra do not share any oxygen atoms
Silica tetrahedra bond to other elements, not other silica tetrahedra
The attraction between silica tetrahedra and positive ions (cations) holds these minerals together
Examples: Olivine, Garnet, Zircon, Kyanite
Olivine
Olivine composed of individual (independent) silica tetrahedra
in olivine, the –4 charge of the silica tetrahedron is balanced by two divalent (i.e., +2) iron or magnesium cations
Olivine can be either Mg2SiO4 or Fe2SiO4 , or some combination of the two (Mg,Fe)2SiO4
Silicate Structures: Single Chains
In a single-chain silicate, the tetrahedra link to form a chain by sharing two oxygen atoms
strongly bonded so cleavage cuts parallel to the chains
two planes of cleavage at 90 degrees
Single Chain Silicate Minerals: Pyroxenes
The most common of the many different types of single-chain silicates are pyroxenes
Pyroxenes important components of dark-colored igneous rocks
Augite most common mineral in the pyroxene group
Black in color
Two distinctive cleavages at nearly 90 degrees
Dominant mineral in basalt
Silicate Structures: Double Chains
In a double-chain silicate, the tetrahedra link by sharing two or three oxygen atoms
minerals cleave parallel to the double chains
two planes of cleavage at 60 and 120 degrees
Double Chain Silicate Structures: Amphiboles
Amphiboles are the most common type of double chain silicates
Hornblende most common mineral in this group
Two perfect cleavages exhibiting angles of 120 and 60 degrees
Silicate Structures: 2D Sheet Silicates
All the tetrahedra in this group share three oxygen atoms and therefore link to form two-dimensional sheets
Other ions, and in some cases water molecules, fit between the sheets in some sheet silicates
Because of their structure, sheet silicates have cleavage in one direction and occur in books of very thin sheets
bonds between sheets are weak
This group includes micas and clays (which occur only in extremely tiny flakes)
2D Sheet Silicate Structures: Muscovite
Common member of the mica family
Excellent cleavage in one direction
Clear, thin sheets
Produces the “glimmering” brilliance often seen in beach sand
Silicate Structures: 3D Framework Silicates
In a framework silicate, each tetrahedron shares all four oxygen atoms with its neighbors, so the tetrahedra are configured in a threedimensional structure
Examples:Feldspars, Quartz
All oxygen ions are “shared” between tetrahedra
Feldspars
Most common mineral group
Forms under a wide range of temperatures and pressures
Exhibit two directions of perfect cleavage at 90 degrees
Quartz
Only common silicate mineral composed entirely of oxygen and silicon
Hard (7) and resistant to weathering
Breaks with conchoidal fracture
Often forms hexagonal crystals
Colored by impurities (various ions)
Nonsilicate Minerals: Halides
minerals that make up the halide group include those in which the halogen elements of fluorine, chlorine, bromine, and iodine are combined with one or more metals
includes halite, sylvite, fluorite
halogen elements are the anion
Nonsilicate Minerals: Oxides
Oxide minerals have oxygen (O2–) as their anion
but we exclude those minerals with oxygen complexes such as carbonate (CO3 2–), sulphate (SO4 2–), and silicate (SiO4 4–) from the oxide group
several minerals of great economic importance including the chief ores of iron, chromium, manganese, tin, and aluminum
Hematite (iron oxide Fe2O3 )
Magnetite (Fe3O4 )
Corundum (aluminum oxide Al2O3 )
Nonsilicate Minerals: Sulfates/Sulphates
basic unit of the sulfate minerals is the sulfate anion, SO4 2-
sulfate anion combines with metal cation to form the sulfate minerals
the metal cation has a +2 charge, which balances the –2 charge on the sulfate ion
many sulfates form by precipitation out of water at or near the Earth’s surface
Common sulfate minerals:
Barite (BaSO4 )
Gypsum(CaSO4 .2H2O)
Anhydrite(CaSO4)
Nonsilicate Minerals: Native Elements
Minerals that are composed of atoms from a single element are referred to as native elements
Native metals consist of pure masses of a single metal
The metal atoms are bonded by metallic bonds
Ex: gold, copper, diamond, sulfur,graphite