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.

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