Chapter: Earth’s Internal Processes

Table of Contents Chapter: Earth’s Internal Processes Section 1: Evolution of Earth’s Crust Section 2: Earthquakes Section 3: Earth’s Interior Section 4: Volcanoes

Evolution of Earth’s Crust 1 Continental Drift • 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.

Evolution of Earth’s Crust 1 Matching Coastlines • 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. • If you found similar formations and structures on each continent then the continents could have been joined together in that place.

Evolution of Earth’s Crust 1 Matching Fossils • Large land animals provided better evidence because they could not have crossed oceans.

Evolution of Earth’s Crust 1 Matching Rocks and Mountains • Mountain ranges were shown to be continuous in Pangaea. • Once Pangaea broke apart, the mountain ranges became separated. • Wegener was able to show that continents that were joined shared unique rocks and minerals.

Evolution of Earth’s Crust 1 Matching Rocks and Mountains • 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.

Evolution of Earth’s Crust 1 Seafloor Spreading Hypothesis • Dr. Harry Hess used sonar, intended to detect submarines, to obtain accurate maps of the seafloor. • A mid-ocean ridge system, or MOR, was continuous and wrapped around Earth.

Evolution of Earth’s Crust 1 Seafloor Spreading Hypothesis • Hess proposed a hypothesis of seafloor spreading, or divergence. • Magma from the mantle is forced upward because of its low density. • This causes the crust to crack (fault) and move apart. • The faulting causes twin mountain ranges with a down-dropped rift valley between.

Evolution of Earth’s Crust 1 Ages of Sediment and Rocks • 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.

Evolution of Earth’s Crust 1 Magnetic Polarity of Rocks • Studies show that Earth’s magnetic field repeatedly reverses itself. • 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.

Evolution of Earth’s Crust 1 Theory of Plate Tectonics • This system consists of about a dozen major plates and many minor ones. • Plates are composed of a rigid layer of uppermost mantle and a layer of either oceanic or continental crust above.

Evolution of Earth’s Crust 1 Divergent Plate Boundaries • 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.

Evolution of Earth’s Crust 1 Divergent Plate Boundaries • A MOR represents divergence that is well-developed. • Divergent boundaries exist as rift valleys, where no mature ocean basins exist yet.

Evolution of Earth’s Crust 1 Convergent Plate Boundaries • Where plates collide, they come together to form convergent boundaries. • Less-dense, thick continental lithosphere moves toward denser thin oceanic lithosphere.

Evolution of Earth’s Crust 1 Convergent Plate Boundaries • The ocean side is forced downward beneath the continental slab in a process called subduction. • The region of collision also has a deep-sea trench that parallels the zone.

Evolution of Earth’s Crust • Convergent plate boundaries also exist between two slabs of oceanic lithosphere. In this case, the oceanic lithosphere that is colder, and therefore denser, subducts. 1 Convergent Plate Boundaries • Magma erupted here produces chains of volcanic islands called island arcs.

Evolution of Earth’s Crust 1 Convergent Plate Boundaries • Two continental slabs of low density collide and tend not to subduct. • The plates collide and buckle upward to form a high range of folded mountains. • Volcanic activity is noticeably absent and there is no trench.

Evolution of Earth’s Crust 1 Transform Plate Boundaries • No new lithosphere is forming, as along a divergent boundary. • Old lithosphere is not being recycled, as along a subduction zone. • The main result of transform boundaries is horizontal motion of lithosphere.

Evolution of Earth’s Crust 1 What drives the plates? • Research indicates that plates are driven by a combination of forces. One such force is ridge push at a mid-ocean ridge. • Divergent boundaries are higher at the center of the ridge, gravity forces material down the slopes of the MOR.

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

Evolution of Earth’s Crust 1 Thermal Energy • Internal convection of mantle material is the driving force for all mechanisms of plate motion. • The main source of thermal energy comes from the decay of radioactive elements in Earth.

Section Check 1 Question 1 __________ is the hypothesis that continents have slowly moved to their current locations. A. Continental drift B. Mid-ocean shifting C. Pangaea D. 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 1 Question 2 Who proposed the hypothesis of continental drift? A. Esker B. Gagarin C. Hess D. Wegener

Section Check 1 Answer The answer is D. Wegener proposed the hypothesis of continental drift. Hess theorized that the seafloor is spreading.

Section Check 1 Question 3 What is Pangaea?

Section Check 1 Answer 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 Global Earthquake Distribution • Earthquakes occur in well-defined zones. • These zones coincide with the edges of lithospheric plates.

Earthquakes 2 Depth of Focus • Divergent boundaries are associated plates that move in opposite directions. • 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 Causes of Earthquakes • An earthquake is any seismic vibration of Earth caused by the rapid release of energy. Deformation • A strain is deformation in response to a stress.

Earthquakes 2 Deformation • Stress is the force per unit area that acts on a material. (1) compressive stress (2) a tension stress (3) a shear stress (4) torsion stress

Earthquakes 2 Elastic Deformation • Elastic deformation occurs when a material deforms as a stress is applied, but returns to its original 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 released.

Earthquakes 2 Energy Release • When this strain energy is released suddenly, it causes rock to lurch to a new position. • A fault is a crack along which movement has taken place. • The sudden energy release that goes with fault movement is called elastic rebound.

Earthquakes 2 Earthquake Waves • Earthquake waves travel out in all directions from a point where strain energy is released. This point is the focus. • The point on Earth’s surface directly above the focus is the epicenter.

Earthquakes 2 Body Waves • Primary waves, also called P-waves, cause particles in a material to undergo a push-pull type motion. • The particles do not permanently change location. • Particles can bump into each other, then primary waves can move through it. • P-waves travel through all kinds of matter.

Earthquakes 2 Body Waves • Secondary waves (S-waves) are sometimes called shear waves, because of the relative motion of particles as energy is transferred. • S-waves cause particles to move perpendicular to the direction of wave travel. • S-waves can only travel through solids.

Earthquakes 2 Surface Waves • Surface waves move in a more complex manner. • 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 • 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. 2 Earthquake Measurement

Earthquakes • The Richter magnitude scale uses the amplitude of the largest earthquake wave. 2 Earthquake Measurement • Richter magnitude is intended to give a measure of the energy released during the earthquake.

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. Earthquake Proofing • 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 Check 2 Question 1 Which of the following is NOT a type of stress in rock? A. compression B. epicenter C. shearing D. tension

Section Check 2 Answer The answer is B. The epicenter is the point on Earth’s surface located directly above the earthquake’s center.

Section Check 2 Question 2 Where do P- and S-waves occur in relation to surface waves? Answer Seismic 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 2 Question 3 Why is it difficult to predict earthquakes? Answer Geologists 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.

Earth’s Interior 3 Earthquake Observations • A boundary that marks a density change between layers is called a discontinuity.

Earth’s Interior • One such discontinuity separates the crust from uppermost mantle, and is known as the Mohorovicic (moh huh ROH vee chihch) discontinuity, or Moho. 3 Earthquake Observations

Chapter: Earth’s Internal Processes