knowt logo

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.

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.

robot