Chapter 12: Earth's Internal Processes

Section 1: Evolution of Earth’s Crust

Continental Drift * In 1915, Alfred Wegener proposed a hypothesis that suggested that Earth’s continents once were part of a large super-continent called Pangaea * Then, about 200 million years ago, the super-continent broke into pieces that drifted over the surface of Earth like rafts on water. * This hypothesis of continental drift was not accepted by most other geologists. * Matching features on different continents provided evidence that the continents were once joined together where matches occurred. * Wegener’s opponents pointed out that the coastlines are constantly wearing away due to wave action. * Oceanographers were able to show, using sonar, that the edges of the continental shelves matched very well * Weathering of the continental edges does not affect the continental shelves * Large land animals provided better evidence because they could not have crossed oceans. * Animals that could fly or swim could appear in the fossil record in widely separated places due to their mobility, not because the places were necessarily joined. * Wegener chose fossils of animals that could not swim or fly to prove Pangaea’s existence. * Mountain ranges were shown to be continuous in Pangaea * Wegener’s hypothesis showed mountains on several continents were once part of the same range. * Wegener hypothesized that the continents were moving by pushing through the ocean floor.
Seafloor Spreading Hypothesis * Using sonar data, three-dimensional seafloor models were created in 1960. * Hess found a feature called a mid-ocean ridge, or MOR, which was part of all Earth’s ocean basins. * Rift Valley: long, linear, dropped-down valley between twin, parallel mountain ranges produced by faulting. * Several types of evidence supported the seafloor spreading hypothesis. * One type of evidence was the ages of sediments in cores extracted from the seafloor * More evidence supporting the seafloor spreading hypothesis comes from the study of the magnetic properties of seafloor rocks. * Studies show that Earth’s magnetic field has reversed direction many times. * On the seafloor, researchers have found bands of rock with alternating polarities, extending out from an MOR.
Theory of Plate Tectonics * In the 1960s geological data led to the development of the theory of plate tectonics. * According to the theory of plate tectonics, Earth's surface is made of separate slabs called plates

that move slowly over Earth’s upper layers. * There are three main kinds of plate motions. * Plates can move apart, move together, or slide past one another. * These three types of motion result in three types of plate boundaries— divergent plate boundaries, convergent plate boundaries * Different geological features are produced as plates interact at the different types of boundaries. * Divergent Boundary: The boundary between two plates that are moving apart * Convergent Boundaries: Where plates come together * Subduction: the oceanic side bending and being forced downward beneath the continental slab * The collision of two plates at a convergent boundary can also produce earthquakes that can cause tsunamis. * Along some convergent plate boundaries, two continental plates of low density collide and tend not to subduct. * Transform Boundaries: the horizontal motion of two plates past each other. * Transform faults are extremely important where they cut perpendicular to the MOR. * If you observe arrows that indicate net motion along these faults, you will notice that this net motion trends away from the MOR.

What drives the plates? * Plate motion is caused by a combination of forces. * One such force is called ridge push and occurs at an MOR. * When plate subduction occurs at a convergent boundary, a force called slab pull is thought to operate. * Friction between a plate and mantle material below the plate probably has a major effect on plate motion. * Internal convection of mantle material is the driving force for all mechanisms of plate motion. * The main source of thermal energy that keeps Earth materials convecting comes from the decay of radioactive elements in Earth.

Section 2: Earthquakes

Global Earthquake Distribution * The zones where earthquakes occur are the boundaries of Earth’s plates. * Most earthquakes occur along the edges of plates. * Depths at which earthquakes occur also provide information about plate boundaries. * The depth at which an earthquake occurs, indicated by the stars, depends on the type of plate boundary. Earthquakes tend to be shallower at a divergent boundary than at a convergent boundary.
Causes of Earthquakes * An earthquake is the sudden movement or vibration of the ground that occurs when rocks slip along enormous cracks in Earth’s crust. * The shaking of the ground that occurs during an earthquake can cause buildings to collapse * Earthquakes are caused by forces that act on rocks. * The strain that occurs when a stress is applied to an object is related to the amount of deformation that occurs. * Stresses can be of four types: * a compressive stress, in which an object is

squeezed or shortened * a tension stress, in which an object is stretched or lengthened * a shear stress, in which different parts of an object are moved in opposite directions along a plane * a torsion stress, in which an object is twisted. * Elastic deformation occurs when a material deforms as a stress is applied, but snaps back to its origin shape when the stress is removed. * Plastic deformation occurs when a material deforms, or changes shape, as a stress is applied and remains in the new shape when the stress is removed. * Deep inside Earth, where temperatures are high, rocks deform plastically. * When an object is deformed, a form of energy called strain energy can be stored in the object. * A fault is a crack in Earth’s crust along which rock has moved. * Elastic Rebound: The sudden release of strain energy when rock moves along a fault

Earthquake Waves * Focus: point of origin of an earthquake * Epicenter: The point on Earth’s surface directly above the focus * The movement of rock along the fault causes an earthquake at the focus. Earthquake waves travel out from the focus in all directions. * Seismic waves can be sorted broadly into two major types. * Body waves travel through Earth. * Surface waves travel across Earth’s surface. * Primary waves, which are also called P-waves, are like the waves that travel along a coiled spring. * P-waves cause rock to be compressed and expanded as the wave passes, just like a wave on a spring compresses and expands the coils as it travels. * S-waves are like the waves moving along a rope. * S-waves can cause rock to move up and down perpendicular to the direction in which the wave travels. * Surface waves move in a more complex manner, often causing a rolling motion much like ocean waves. * Surface waves produce an up-and-down rolling motion similar to the motion caused by ocean waves. At the same time, the surface can shift from side to side.
Earthquake Measurement * Two measurement schemes that have been used to characterize earthquakes are the Modified Mercalli intensity scale and the Richter magnitude scale. * The Modified Mercalli scale ranks earthquakes according to the amount of damage they cause. * The Richter magnitude scale, which is also called the Richter scale, measures the amount of energy released during the earthquake. * A device called a seismograph records the vibrations produced by an earthquake. * The level of destruction by earthquakes is extremely variable. * In countries where there are poorly constructed buildings, it is not uncommon for tens of thousands of people to die in a single earthquake event. * Active earthquake zones are well established, but predicting precise times for earthquakes in those zones is not yet possible. * Although no building can be made entirely earthquake proof, scientists and engineers are finding ways to reduce the damage to structures during mild or moderate earthquakes

Section 3: Earth’s Interior

What’s Inside? * To study Earth’s interior, geologists use seismic waves. * By studying how seismic waves are affected as they travel in Earth, geologists can infer the structure of Earth’s interior. * The direction in which seismic waves travel can change when the waves travel from one material into another. * The refraction of seismic waves as they pass through Earth provides information about Earth’s structure.
Earthquake Observations * The speed and direction of seismic waves change when the properties of the materials in which they move change. * Discontinuity: The boundary between two layers of material that have different densities. * The Mohorovicic discontinuity separates Earth's crust from the denser upper mantle.
When an earthquake occurs, seismic waves spread out and travel through Earth.
Seismographs record the arrival times and shapes of the seismic waves at different places all over Earth.
S-waves cannot travel in Earth's liquid outer core, but P-waves pass through the outer core and the solid inner core.
For each earthquake there is a shadow zone
The effects of the inner core on the movement of P-waves show that the inner core is solid.
Composition of Earth’s Layers * Layering of Earth is caused by heat and pressure. The most dense materials are at the center and the less dense materials are near the crust. * Asthenosphere: weaker, plasticlike layer upon which Earth’s lithospheric plates move. * Astronomers hypothesize that early Earth may have formed from meteorite-like material that was forced together by gravity and heated to melting.

Section 4: Volcanoes

Origin of Magma * Inside Earth, temperatures are about 1,000°C at depths of around 100 km below the surface and can reach 7,000°C in the inner core. * Melted rock inside Earth is called magma. * Liquid magma is less dense than the surrounding rock and is forced upward. Magma reaches the surface through cracks in Earth's crust, forming a volcano * A volcano is a feature that forms when magma reaches the surface. * Magma that has erupted onto Earth’s surface is called lava. * Eruptions of magma commonly occur at subduction zones of convergent plate boundaries, at rifts where plates are separating, and at hot spots. * At a diverging plate boundary, or rift, magma can be forced upward between the separating plates.
Eruptive Products * Volcanic eruptions can expel a variety of materials. * All solid materials expelled by a volcano are collectively called pyroclasts. * Often, lava is ejected into the air as globules. * These globules cool and solidify as they fall to Earth. * Volcanoes release a variety of gases, including water vapor, carbon dioxide, and sulfur dioxide. * While in the atmosphere, the droplets reduce the amount of solar radiation reaching Earth’s surface. * Magma from a volcano or fissure may remain a liquid, at least initially, and flow across the Earth’s surface as lava. * Viscosity: a measure of the resistance of a fluid to flow. * Liquids with low viscosity flow more easily than liquids with high viscosity. * The viscosity of a liquid, such as magma, decreases as its temperature increases. * The more dissolved gas the magma contains, the lower its viscosity.
Eruptive Styles * Volcanoes can erupt in different ways, depending on the magma viscosity. * Thick, sticky, high-silica magmas are so viscous that they tend to erupt less easily, causing the pressure within a volcano to rise. * The runny, low-silica, high-temperature basaltic lavas are so low in viscosity that they erupt quite easily and often produce quiet eruptions of freely flowing lava. * Many volcanoes occur on Earth along plate boundaries, over hot spots, or in rift valleys. * Large earthquakes and violent volcanic eruptions often occur along these ocean-continent and ocean-ocean convergent boundaries. * Divergent plate boundaries also are volcanically active, but most of the activity is underwater, along the mid-ocean ridge, and goes unnoticed. * Hot spots are volcanically active sites that occur in places where large quantities of magma move to the surface in large, column-like plumes. * Hot spot volcanic eruptions produce lava somewhat similar to that formed along divergent boundaries.
Types of Volcanoes * Volcanoes are classified according to their size, shape, and the materials that compose them. * The temperature, composition, and gas content of magma are important controls on the type of volcanic structure that forms during an eruption. * Cinder Cone Volcanoes: When the primary eruptive products are large fragments of solid material; tend to be small, with most cones having heights in the hundreds of meters range. * Shield Volcanoes: form from high-temperature, fluid, basaltic lava and erupt with abundant lava flows that can move for kilometers over Earth’s surface before stopping; broad, flat structures made up of layer upon layer of lava. * Composite Volcanoes: formed from alternating highly explosive events that form pyroclastic materials, and lava flows; composed of alternating layers, are large, often thousands of meters high and tens of kilometers across the base.