Chapter: Earth’s Internal Processes

Chapter: Earth’s Internal Processes

Table of Contents

Section 2: Earthquakes

Section 1: Evolution of Earth’s Crust

Section 3: Earth’s Interior

Section 4: Volcanoes

• In 1915, Alfred Wegener (VEG nur) proposed a hypothesis that suggested that Earth’s continents once were part of a large super-continent, Pangaea.

• About 200 million years ago, the super-continent broke into pieces that drifted over the surface of Earth like rafts on water.

Continental DriftEvolution of Earth’s Crust

1

• The most apparent match of continents is the eastern coastline of South America with the western coastline of Africa.

• Wegener argued that you could match rock types, fossils, erosion features, and mountain ranges.

Matching Coastlines1Evolution of Earth’s Crust

• If you found similar formations and structures on each continent then the continents could have been joined together in that place.

• Large land animals provided better evidence because they could not have crossed oceans.

Matching Fossils1Evolution of Earth’s Crust

• Mountain ranges were shown to be continuous in Pangaea.

• Once Pangaea broke apart, the mountain ranges became separated.

Matching Rocks and Mountains1Evolution of Earth’s Crust

• Wegener was able to show that continents that were joined shared unique rocks and minerals.

• Wegener’s hypothesis was not accepted by his contemporaries because he was unable to conceive of a force or mechanism that could drive continents apart.

Matching Rocks and Mountains1Evolution of Earth’s Crust

• A mid-ocean ridge system, or MOR, was continuous and wrapped around Earth.

• Dr. Harry Hess used sonar, intended to detect submarines, to obtain accurate maps of the seafloor.

Seafloor Spreading Hypothesis1Evolution of Earth’s Crust

• The faulting causes twin mountain ranges with a down-dropped rift valley between.

• Hess proposed a hypothesis of seafloor spreading, or divergence.

Seafloor Spreading Hypothesis1Evolution of Earth’s Crust

• Magma from the mantle is forced upward because of its low density.

• This causes the crust to crack (fault) and move apart.

• When the ages of rocks are measured, the continental rocks are billions of years old, while seafloor rocks are less than 200 million years of age.

Ages of Sediment and Rocks1Evolution of Earth’s Crust

• Studies show that Earth’s magnetic field repeatedly reverses itself.

Magnetic Polarity of Rocks1Evolution of Earth’s Crust

• Vine, Matthews, Wilson, et al discovered bands of reversed polarity in the seafloor rocks.

• As magma crystals form, they take on the polarity of Earth at the time they form.

• The pattern is identical on both sides of the MOR.

• This system consists of about a dozen major plates and many minor ones.

Theory of Plate Tectonics1Evolution of Earth’s Crust

• Plates are composed of a rigid layer of uppermost mantle and a layer of either oceanic or continental crust above.

Divergent Plate Boundaries1Evolution of Earth’s Crust

• At a mid-ocean ridge (MOR), magma rises along a faulted rift valley, spreads, and cools to form new oceanic crust. This spreading apart is what happens at divergent boundaries.

• A MOR represents divergence that is well-developed.

Divergent Plate Boundaries1Evolution of Earth’s Crust

• Divergent boundaries exist as rift valleys, where no mature ocean basins exist yet.

• Where plates collide, they come together to form convergent boundaries.

Convergent Plate Boundaries1Evolution of Earth’s Crust

• Less-dense, thick continental lithosphere moves toward denser thin oceanic lithosphere.

• The ocean side is forced downward beneath the continental slab in a process called subduction.

Convergent Plate Boundaries1Evolution of Earth’s Crust

• The region of collision also has a deep-sea trench that parallels the zone.

Convergent Plate Boundaries1Evolution of Earth’s Crust

• Magma erupted here produces chains of volcanic islands called island arcs.

• Convergent plate boundaries also exist between two slabs of oceanic lithosphere. In this case, theoceanic lithosphere that is colder, and therefore denser, subducts.

• Two continental slabs of low density collide and tend not to subduct.

Convergent Plate Boundaries1Evolution of Earth’s Crust

• The plates collide and buckle upward to form a high range of folded mountains.

• Volcanic activity is noticeably absent and there is no trench.

• The main result of transform boundaries is horizontal motion of lithosphere.

• No new lithosphere is forming, as along a divergent boundary.

Transform Plate Boundaries1Evolution of Earth’s Crust

• Old lithosphere is not being recycled, as along a subduction zone.

• Research indicates that plates are driven by a combination of forces. One such force is ridge push at a mid-ocean ridge.

What drives the plates?1Evolution of Earth’s Crust

• Divergent boundaries are higher at the center of the ridge, gravity forces material down the slopes of the MOR.

What drives the plates?1Evolution of Earth’s Crust

• When a plate subducts back into Earth at some convergent boundaries, the process of slab pull is thought to operate.

• Portions of descending plates are pulling the rest of a plate down with them.

• Internal convection of mantle material is the driving force for all mechanisms of plate motion.

Thermal Energy1Evolution of Earth’s Crust

• The main source of thermal energy comes from the decay of radioactive elements in Earth.

Section Check

1Question 1

__________ is the hypothesis that continents have slowly moved to their current locations.

A. Continental driftB. Mid-ocean shiftingC. PangaeaD. Seafloor spreading

Section Check

1 Answer

The answer is A. Continental drift is the theory that the continents have slowly moved. Seafloor spreading is a process that would help explain how the continental drift might occur.

Section Check

1Question 2

Who proposed the hypothesis of continental drift?

A. EskerB. GagarinC. HessD. Wegener

1Section Check

AnswerThe answer is D. Wegener proposed the hypothesis of continental drift. Hess theorized that the seafloor is spreading.

Section Check

1Question 3

What is Pangaea?

Section Check

1Answer

Pangaea means “all land” and is the name that Wegener used to refer to the one large landmass that he believed existed before it broke apart into continents.

Earthquakes

2

• Earthquakes occur in well-defined zones. Global Earthquake Distribution

• These zones coincide with the edges of lithospheric plates.

Earthquakes

2

• Divergent boundaries are associated plates that move in opposite directions.

Depth of Focus

• This faulting creates a narrow band of numerous, shallow earthquakes.

Earthquakes

2 Depth of Focus• Convergent boundaries have broad zones of

earthquakes with the shallowest foci near the surface at the point of convergence, and the deepest foci located under volcanoes or mountains created in the collision area.

Earthquakes

2

• An earthquake is any seismic vibration of Earth caused by the rapid release of energy.

Causes of Earthquakes

• A strain is deformation in response to a stress.

Deformation

Earthquakes

2

• Stress is the force per unit area that acts on a material.(1) compressive stress(2) a tension stress(3) a shear stress

Deformation

(4) torsion stress

Earthquakes

2

• Elastic deformation occurs when a material deforms as a stress is applied, but returns to its original shape when the stress is removed.

Elastic Deformation

• 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 released.

• The sudden energy release that goes with fault movement is called elastic rebound.

• A fault is a crack along which movement has taken place.

Earthquakes

2

• When this strain energy is released suddenly, it causes rock to lurch to a new position.

Energy Release

• The point on Earth’s surface directly above the focus is the epicenter.

Earthquakes

2

• Earthquake waves travel out in all directions from a point where strain energy is released. This point is the focus.

Earthquake Waves

Earthquakes

2

• Primary waves, also called P-waves, cause particles in a material to undergo a push-pull type motion.

Body Waves

• The particles do not permanently change location.

• P-waves travel through all kinds of matter.

• Particles can bump into each other, then primary waves can move through it.

Earthquakes

2

• Secondary waves (S-waves) are sometimes called shear waves, because of the relative motion of particles as energy is transferred.

Body Waves

• S-waves cause particles to move perpendicular to the direction of wave travel.

• S-waves can only travel through solids.

Earthquakes

2

• Surface waves move in a more complex manner.

Surface Waves

• They can exhibit an up and down rolling motion, and also a side-to-side motion that parallels Earth’s surface.

Earthquakes

2 Surface Waves

Earthquakes

2 Earthquake Measurement• The Modified

Mercalli scale ranks earthquakes in a range from I-XII, XII being the worst and uses eyewitness observation and post-earthquake assessments to assign an intensity value.

Earthquakes

2 Earthquake Measurement

• Richter magnitude is intended to give a measure of the energy released during the earthquake.

• The Richter magnitude scale uses the amplitude of thelargest earthquake wave.

Earthquakes

2 Earthquake Measurement• The table shows

the global frequency of different magnitude earthquakes.

Earthquakes

2 Levels of Destruction• Research has shown that poor building

methods are the largest contributors to earthquake damage and loss of life.

• 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.

Earthquake Proofing

2Question 1

Which of the following is NOT a type of stress in rock?

A. compressionB. epicenterC. shearingD. tension

Section Check

2Answer

The answer is B. The epicenter is the point on Earth’s surface located directly above the earthquake’s center.

Section Check

2 Question 2Where do P- and S-waves occur in relation to surface waves?

AnswerSeismic waves travel away from the epicenter in all directions. P-waves travel the fastest through rock material. S-waves move through the rock and cause particles to vibrate. Both P- and S-waves travel through the Earth’s interior while surface waves move along Earth’s surface.

Section Check

2Question 3

Why is it difficult to predict earthquakes?

Section Check

AnswerGeologists can monitor changes in Earth that are associated with earthquakes. Measuring devices have been developed to assess changes in groundwater level and rock layers; however, no single change in Earth occurs for all earthquakes.

• A boundary that marks a density change between layers is called a discontinuity.

Earthquake Observations

Earth’s Interior

3

Earthquake Observations

Earth’s Interior

3

• One such discontinuity separates the crust from uppermost mantle, and is known as the Mohorovicic(moh huh ROH vee chihch) discontinuity, or Moho.

• This “dead zone” is termed the shadow zone.

Shadow Zones• P-waves and S-waves travel through Earth

for 105 degrees of arc in all directions.

Earth’s Interior

3

• Between 105 and 140 degrees from the epicenter, nothing is recorded.

Shadow Zones

Earth’s Interior

3

• If temperatures are high enough, atoms move apart enough to exist in the liquid state, even at extreme pressures.

Solid Inner Core• The fact that P-waves pass through the

core, but are refracted along the way, indicates that the inner core is denser than the outer core and solid.

Earth’s Interior

3

• When pressure dominates, atoms are squeezed together tightly and exist in the solid state.

• The asthenosphere is a weaker, plasticlike layer upon which Earth’s lithospheric plates move.

• Mantle below the asthenosphere also is composed of silicates.

Composition of Earth’s Layers• The crust and uppermost mantle, which

together form the lithosphere, are made of rocky material—mostly silicates.

Earth’s Interior

3

• The cores are made mostly of metallic material.

Section Check

Question 1

What is Earth’s core made of?

3

Answer

Earth’s core is primarily made of metallic material such as iron and nickel.

Section Check

Question 2

Earth’s internal layers become _______ with depth.

A. coolerB. darkerC. denserD. lighter

3

Section Check

Answer

The answer is C.

3

Section Check

Question 3

What can’t S-waves penetrate the liquid outer core?

3

Answer

S-wave only travel through solids. This suggests that the outer core is in a liquid state.

Origin of Magma

• A buoyant force acts on magma that forms from rock surrounding it.

Volcanoes

4

• Hot, nearly molten rock in Earth’s asthenosphere canchange to a liquid by decompression melting.

Origin of Magma

Volcanoes

4

• Rising magma may reach Earth’s surface if pressure conditions allow and the rock has conduits through which it can flow.

Eruptive Products

• All solid materials expelled by a volcano are collectively called pyroclasts.

• Volcanoes release a broad variety of superheated gases, the most common of which is water vapor.

Volcanoes

4

• In addition carbon dioxide and gases composed of sulfur compounds are expelled.

Solids

Gases

• Viscosity is a measure of the resistance of a fluid to flow.

Liquids• Lavas can vary considerably in

composition, which in turn affects their physical properties.

Volcanoes

4

Eruptive Styles• Eruptive style is strongly linked to

temperature and composition and can be linked to the type of plate boundary associated with it.

Volcanoes

4

Plate Boundary Setting• Most of Earth’s volcanoes lie in subduction

zones where continental and oceanic materials are being mixed and partially melted.

Volcanoes

4

Hot Spots• Hot spots are volcanically active sites

that arise in places where large quantities of magma move to the surface in large, column-like plumes.

Volcanoes

4

• A hot spot under an oceanic plate forms volcanic island chains, such as the Hawaiian islands.

• Yellowstone National Park is an example of a hot spot under a continental plate.

• When the primary eruptive products are large fragments of solid material, cinder cone volcanoes form.

Types of Volcanoes• Volcanoes are classified according to their

size, shape, and the materials that compose them.

Volcanoes

4

Cinder Cone Volcanoes

• Shield volcanoes erupt with abundant lava flows that can move for kilometers over Earth’s surface before stopping.

Shield Volcanoes

Volcanoes

4

• Shield volcanoes are broad, flat structures made up of layer upon layer of lava.

• Volcanoes formed from alternating explosive events that produce pyroclastic materials, and lava flows are called composite volcanoes.

Composite Volcanoes

Volcanoes

4

Section Check

Question 1

Where do most volcanoes occur?

4

Answer

Most volcanoes occur at plate boundaries where huge pieces of the crust pull apart or push together. As a result, the crust often fractures, allowing magma to reach the surface.

Section Check

Question 2

What type of volcano is formed by an explosive eruption followed by a quiet eruption?

4

A. cinder cone volcanoB. composite volcanoC. fissure eruptionD. shield volcano

Section Check

Answer

The correct answer is B. Composite volcanoes erupt explosively releasing large quantities of gas and ash. They are followed by quieter eruptions that form a lava layer over the ash.

4

Section Check

Question 3

How does a hot spot volcano form?

Answer

A volcano forms above a hot spot when magma erupts through the crust and reaches the surface. Hot spot volcanoes may lie in the middle of plates far from any plate boundaries or near or on plate boundaries.

4

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