The Dynamic Earth

What does dynamic mean? Ever changing in varying degrees of intensity.

The Sources of Energy for the Earth What is the source of the Earth’s dynamic nature? A constant flow of energy. External Sources of Energy Sunlight (Light Energy) Internal Sources of Energy Leftover thermal energy from the Earth’s formation Friction from the movement of the lithospheric plates Decay of radioactive elements (Uranium, for example)

The Earth’s Atmosphere Layers Composition Evolution

Atmospheric Layers Four Layers Thermosphere (outermost layer, receives intense solar radiation) Mesosphere (coldest layer) Stratosphere (contains the ozone layer, warmer due to absorption of ultra-violet light) Troposphere (the weather layer, the densest layer) Also, the Ionosphere (forms due to the interaction of cosmic radiation from the Sun with the faint nitrogen and oxygen concentration in the thermosphere)

Atmospheric Composition Nitrogen (N2) (78%) Oxygen (O2) (almost 21%) Argon (almost 1%) Small amounts of other gases (0.1%), including Water vapor Carbon Dioxide Methane Layer of Ozone (O3)

Atmospheric Evolution Primordial atmosphere contained mostly carbon dioxide and nitrogen, with some ammonia and hydrogen—volcanic gases. 2.5 billion years ago, a change occurred, plant cells algae, began to consume carbon dioxide and give off oxygen (photosynthesis) Atmosphere gradually filled with oxygen 350 million years ago…basically same level of oxygen as today.

Earth’s Protection System Ultra-violet light shield (ozone layer) Thermal insulating blanket (greenhouse gases in the atmosphere) Solar wind blocker and atmospheric erosion protector (Earth’s magnetic field or magnetosphere)

Without the Ozone Layer Eventually, no terrestrial life (DNA destroyed)

Without the Greenhouse Gases Earth would much colder and iced over (-27 degrees C)

Without the Magnetic Field Atmosphere would be eroded by the solar wind, and cosmic radiation (electrons and protons from the Sun) would harm living organisms.

Greenhouse Effect The surface of the Earth is heated by visible light from the Sun. The Earth then radiates thermal energy as infrared light. The presence of greenhouse gases, CO2 and H2O and methane (CH4) and a few other trace gases, serve to trap or absorb some of the infrared light radiated from the Earth’s surface. Visible light’s wavelength is too short to be absorbed by these greenhouse gases, but infrared light’s wavelength is longer and can be absorbed. This interaction increases the surface temperature of the Earth.

Global Warming The greenhouse effect is a good thing for life. However… …Any increase in the greenhouse gas concentration (in parts per million) will cause more infrared light radiated from the Earth’s surface to be trapped, causing the Earth to become warmer. The recent increase in the concentration of carbon dioxide is due to the combustion of hydrocarbon- rich fossil fuels, such as coal, petroleum and natural gas. (cause: see Industrial Revolution)

Ozone Layer Ozone absorbs the majority of incoming ultra-violet light from the Sun. Ozone is composed of three oxygen atoms, and is toxic to humans. Ozone holes or thinning is primarily caused by CFCs (chlorofluorocarbons)—used in aerosol sprays, now banned worldwide. Some evidence of the “healing” of the ozone layer is observed, however, it is estimated that it should recover in another 2-3 decades without CFCs.

Earth’s Magnetic Field The source of the Earth’s magnetic field--The Dynamo Theory: The rotation of the Earth causes the electrons in the outer liquid core (composed of molten iron and some nickel) to move. Moving electrons, as you recall, produce magnetic fields (that is, electromagnetism).

Atmosphere Pressure and Wind What causes the wind? Differences in atmospheric pressure. What causes differences in atmospheric pressure? Differences in the heating of the Earth’s surface. What causes differences in the heating of the Earth’s surface? Three reasons Angle of the sunlight striking the Earth’s surface Duration of the sunlight Ratio of sunlight being reflected vs. absorbed

Wind Belts and the Coriolis Effect Without the rotation of the Earth, two main convection cells would operate, from equator to poles. With the rotation, the winds are deflected either east or west, due to the Coriolis effect. Winds from the poles are deflected to the west, winds from the equator are deflected to the east. These are the prevailing winds (trade winds, westerlies and polar easterlies)

Ocean Currents Surface ocean currents circulate throughout the Earth. Surface ocean currents are produced by the prevailing wind belts.

Earth’s Thermal Energy Cycle Excess heat (thermal energy) accumulates in the vicinity of the Earth’s equator. This heats the atmosphere and the Earth’s oceans. Heat or thermal energy is moved away from the equator to the poles to equalize the temperature of the Earth—Earth strikes a balance.

Earth’s Thermal Energy Cycle How does the Earth correct for this imbalance in thermal energy (hot equator, cold poles)? Winds form a convection current from the equator to poles (warm air moves to the poles, is cooled and returns to be heated). Ocean currents carry warm water to the poles and, when cooled, the cooler water from the poles circulates to the equator to be heated again.

The Earth The layers of the Earth (Crust, Mantle and Core) Why layered? Differentiation (the settling of heavier elements, such as iron, towards the center of the Earth, less dense iron rich mantle rock “floats” on the dense iron core) Geochemistry (hot inner iron core under pressure can remain in solid state; should also see a lowering of pressure away from center of Earth where solid iron core changes to liquid molten iron core) Geophysics (s-waves or transverse seismic waves cannot travel through liquids, and s-waves cannot penetrate the outer core; thus, the outer core must be liquid)

Figure 1.10c: In this way, a differentiated Earth formed, consisting of a dense iron-nickel core, an iron-rich silicate mantle, and a silicate crust with continents and ocean basins. Fig. 1-10c, p. 14

Differentiation Fig. 1-10, p. 14 Figure 1.10: (a) Early Earth was probably of uniform composition and density throughout. (b) Heating of the early Earth reached the melting point of iron and nickel, which, being denser than silicate minerals, settled to Earth’s center. At the same time, the lighter silicates flowed upward to form the mantle and the crust. (c) In this way, a differentiated Earth formed, consisting of a dense iron-nickel core, an iron-rich silicate mantle, and a silicate crust with continents and ocean basins. Fig. 1-10, p. 14

Figure 1.11: A cross section of Earth, illustrating the core, mantle, and crust. The enlarged portion shows the relationship between the lithosphere (composed of the continental crust, oceanic crust, and solid upper mantle) and the underlying asthenosphere and lower mantle. Fig. 1-11, p. 15

Active Figure 9.21: (a) P-waves are refracted so that little P-wave energy reaches the surface in the P-wave shadow zone. (b) The presence of an S-wave shadow zone indicates that S-waves are being blocked within Earth. Fig. 9-21, p. 210

Why does the surface of the Earth Constantly Change? Water Cycle (weathering, erosion and transport of weathered rock sediments) Rock Cycle (melting of rock produces igneous rocks, weathering of igneous rock produces sedimentary rocks, and high pressure and temperature without melting produces a metamorphic rock.) Plate Tectonics (the Earth’s rigid lithosphere—crust + top part of the mantle is broken into plates that move into, away from or across each other)

Figure 1.15: The rock cycle showing the interrelationships among Earth’s internal and external processes and how each of the three major rock groups is related to the others. Fig. 1-15, p. 19

What is Plate Tectonics? The rigid lithospheric plates move over the plastic flowing part of the mantle called the asthenosphere. As heat (thermal energy) from the core moves towards the Earth’s surface (heat flows from hot to cold), this produces convection currents in the mantle, which move the asthenosphere and drag the rigid lithospheric plates across the Earth.

Figure 1.3: The atmosphere, biosphere, hydrosphere, lithosphere, mantle, and core are all subsystems of Earth. The interactions among these subsystems make Earth a dynamic planet, which has evolved and continues to change since its origin 4.6 billion years ago. Fig. 1-3, p. 5

Plate Boundaries Divergent Plate Boundary (oceanic ridges and undersea volcanoes—see the Atlantic Ocean) Submergent Plate Boundaries (trenches and volcanic mountain chains—see the Andes Mountains); also known as a convergent plate boundary. Transform plate boundaries (side-by-side plate motion—see the San Andreas Fault

Figure 1.13: Earth’s lithosphere is divided into rigid plates of various sizes that move over the asthenosphere. Fig. 1-13, p. 17

The Mechanism for Plate Motion is Convection in the Mantle Figure 1.12: Earth’s plates are thought to move as a result of underlying mantle convection cells in which warm material from deep within Earth rises toward the surface, cools, and then, on losing heat, descends back into the interior. The movement of these convection cells is thought to be the mechanism responsible for the movement of Earth’s plates, as shown in this diagrammatic cross section. The Mechanism for Plate Motion is Convection in the Mantle Fig. 1-12, p. 15

Figure 1. 17: Plate tectonics and the rock cycle Figure 1.17: Plate tectonics and the rock cycle. The cross section shows how the three major rock groups—igneous, metamorphic, and sedimentary—are recycled through both the continental and oceanic regions. Fig. 1-17, p. 20

Three types of plate boundaries Divergent plate boundary 2. Convergent Plate Boundary 3. Transform Plate boundary Figure 1.14: An idealized cross section illustrating the relationship between the lithosphere and the underlying asthenosphere and the three principal types of plate boundaries: divergent, convergent, and transform. Fig. 1-14, p. 18

Evidence for Plate Tectonics Geographic fit of continents Flora and fauna associations Paleomagnetism patterns associated with the iron in the spreading sea floor. Location pattern of volcanoes, earthquakes and mountains

Active Figure 2.4: The best fit among continents occurs along the continental slope, where erosion would be minimal. Fig. 2-4, p. 30

Figure 2.5: When continents are brought together, their mountain ranges form a single continuous range of the same age and style of deformation throughout. Such evidence indicates the continents were at one time joined together and were subsequently separated. Fig. 2-5, p. 31

Figure 2.6: (a) If the Gondwana continents are brought together so that South Africa is located at the South Pole, then the glacial movements indicated by the striations make sense. In this situation, the glacier, located in a polar climate, moved radially outward from a thick central area toward its periphery. (b) Permian-aged glacial striations in bedrock exposed at Hallet’s Cove, Australia, indicate the direction of glacial movement more than 200 million years ago. Fig. 2-6, p. 31

Figure 2.7: Some of the animals and plants whose fossils are found today on the widely separated continents of South America, Africa, India, Australia, and Antarctica. These continents were joined together during the Late Paleozoic to form Gondwana, the southern landmass of Pangaea. Glossopteris and similar plants are found in Pennsylvanian- and Permian-aged deposits on all five continents. Mesosaurus is a freshwater reptile whose fossils are found in Permian-aged rocks in Brazil and South Africa. Cynognathus and Lystrosaurus are land reptiles that lived during the Early Triassic Period. Fossils of Cynognathus are found in South America and Africa, and fossils of Lystrosaurus have been recovered from Africa, India, and Antarctica. Fig. 2-7, p. 32

Figure 2.8a: Earth’s magnetic field has lines of force just like those of a bar magnet. Fig. 2-8a, p. 34

Figure 2.8b: The strength of the magnetic field changes uniformly from the magnetic equator to the magnetic poles. This change in strength causes a dip needle to parallel Earth’s surface only at the magnetic equator, whereas its inclination with respect to the surface increases to 90 degrees at the magnetic poles. Notice the 11½-degree angle between the geographic and magnetic poles. Fig. 2-8b, p. 34

Figure 2.10: Magnetic reversals recorded in a succession of lava flows are shown diagrammatically by red arrows, and the record of normal polarity events is shown by black arrows. Fig. 2-10, p. 35

Active Figure 2.11: The sequence of magnetic anomalies preserved in the oceanic crust on both sides of an oceanic ridge is identical to the sequence of magnetic reversals already known from continental lava flows. Magnetic anomalies are formed when magma intrudes into oceanic ridges; when the magma cools below the Curie point, it records Earth’s magnetic polarity at the time. Seafloor spreading splits the previously formed crust in half, so that it moves laterally away from the oceanic ridge. Repeated intrusions record a symmetric series of magnetic anomalies that reflect periods of normal and reversed polarity. The magnetic anomalies are recorded by a magnetometer, which measures the strength of the magnetic field. Fig. 2-11, p. 36

Who came up with this idea of Plate Tectonics? Alfred Wegener first suggested moving continents in his Continental Drift theory. But he had no mechanism; he thought that perhaps the continents slowly plowed through the oceanic crust. Hess in 1960s began to observe age differences in sea floor core samples collected in the Atlantic Ocean. Youngest crustal rock was closest to the ridge and the oldest crustal rock was furthest away from the ridge (true for both sides of the ridge!) This became known as sea floor spreading.

Figure 2.3: Alfred Wegener proposed the continental drift hypothesis in 1912 based on a tremendous amount of geologic, paleontologic, and climatologic evidence. He is shown here waiting out the Arctic winter in an expedition hut. Fig. 2-3, p. 29

Figure 2.12: Artist’s view of what the Atlantic Ocean basin would look like without water. The major feature is the Mid-Atlantic Ridge. Fig. 2-12, p. 36

Figure 2.13: The age of the world’s ocean basins established from magnetic anomalies demonstrates that the youngest oceanic crust is adjacent to the spreading ridges and that its age increases away from the ridge axis. Fig. 2-13, p. 37

Figure 9.5: The relationship between earthquake epicenters and plate boundaries. Approximately 80% of earthquakes occur within the circum-Pacific belt, 15% within the Mediterranean-Asiatic belt, and the remaining 5% within plate interiors or along oceanic spreading ridges. Each dot represents a single earthquake epicenter. Fig. 9-5, p. 191

Figure 2.14: A map of the world showing the plates, their boundaries, relative motion and rates of movement in centimeters per year, and hot spots. Fig. 2-14, p. 38

Significance? Continental crust is less dense than oceanic crust, and literally floats in the oceanic crust. The lithospheric plate consists of continental and/or oceanic crust and the very top of the mantle. Continents are carried with the oceanic crust and top part of the mantle. This can lead to the formation of trenches, ridges, and mountain chains.

Mt. Everest is still rising? Himalayan Mountains represent a subduction plate boundary, where an ocean separated two continents. As the ocean closed, the two continents collided, which produced the highest mountain chain in the world today, and is still pushing the continental crust upward.

Active Figure 2. 15: History of a divergent plate boundary Active Figure 2.15: History of a divergent plate boundary. (a) Heat from rising magma beneath a continent causes it to bulge, producing numerous cracks and fractures. (b) As the crust is stretched and thinned, rift valleys develop and lava flows onto the valley floors. (c) Continued spreading further separates the continent until a narrow seaway develops. (d) As spreading continues, an oceanic ridge system forms and an ocean basin develops and grows. Fig. 2-15, p. 40

Active Figure 2. 17: Oceanic–oceanic plate boundary Active Figure 2.17: Oceanic–oceanic plate boundary. (a) An oceanic trench forms where one oceanic plate is subducted beneath another. On the nonsubducted plate, a volcanic island arc forms from the rising magma generated from the subducting plate. (b) Satellite image of Japan. The Japanese Islands are a volcanic island arc resulting from the subduction of one oceanic plate beneath another oceanic plate. Fig. 2-17, p. 42

Active Figure 2. 18: Oceanic–continental plate boundary Active Figure 2.18: Oceanic–continental plate boundary. (a) When an oceanic plate is subducted beneath a continental plate, an andesitic volcanic mountain range is formed on the continental plate as a result of rising magma. (b) Aerial view of the Andes Mountains in Peru. The Andes are one of the best examples of continuing mountain building at an oceanic–continental plate boundary. Fig. 2-18, p. 42

Seismic Waves Seismic waves are produced by earthquakes when stresses build up by moving plates are suddenly released. Interior waves produced by this disturbance include longitudinal waves or p-waves and transverse waves or s-waves. P-waves are faster than s-waves, and can travel through solids or liquids. S-waves cannot travel through liquids. The epicenter and focus of an earthquake can be calculated using seismic data from at least three seismic stations.

Active Figure 9.21: (a) P-waves are refracted so that little P-wave energy reaches the surface in the P-wave shadow zone. (b) The presence of an S-wave shadow zone indicates that S-waves are being blocked within Earth. Fig. 9-21, p. 210

Active Figure 9.4: The focus of an earthquake is the location where rupture begins and energy is released. The place on the surface vertically above the focus is the epicenter. Seismic wave fronts move out in all directions from their source, the focus of an earthquake. Fig. 9-4, p. 191

Active Figure 9. 8: Seismic waves Active Figure 9.8: Seismic waves. (a) Undisturbed material for reference. (b) and (c) show how body waves travel through Earth. (b) Primary waves (P-waves) compress and expand material in the same direction they travel. (c) Secondary waves (S-waves) move material perpendicular to the direction of wave movement. (d) P- and S-waves and their effect on surface structures. Fig. 9-8, p. 194

Figure 9.9: (a) A schematic seismogram showing the arrival order and pattern produced by P-, S-, and L (surface)-waves. When an earthquake occurs, body and surface waves radiate out from the focus at the same time. Because P-waves are the fastest, they arrive at a seismograph first, followed by S-waves, and then by surface waves, which are the slowest waves. The difference between the arrival times of the P- and S-waves is the P–S time interval; it is a function of the distance of the seismograph station from the focus. (b) Seismogram for the 1906 San Francisco earthquake, recorded 14,668 km away in Göttingen, Germany. The total record represents about 26 minutes, so considerable time passed between the arrival of the P-waves and the slower-moving S-waves. The arrival of surface waves, not shown here, caused the instrument to go off the scale. (c) A time–distance graph showing the average travel times for P- and S-waves. The farther away a seismograph station is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S-waves, and hence the greater the distance between the curves on the time–distance graph as indicated by the P–S time interval. Fig. 9-9, p. 195

Active Figure 9.10: Three seismograph stations are needed to locate the epicenter of an earthquake. The P–S time interval is plotted on a time–distance graph for each seismograph station to determine the distance that station is from the epicenter. A circle with that radius is drawn from each station, and the intersection of the three circles is the epicenter of the earthquake. Fig. 9-10, p. 196

Active Figure 2. 19: Continental–continental plate boundary Active Figure 2.19: Continental–continental plate boundary. (a) When two continental plates converge, neither is subducted because of their great thickness and low and equal densities. As the two continental plates collide, a mountain range is formed in the interior of a new and larger continent. (b) Vertical view of the Himalayas, the youngest and highest mountain system in the world. The Himalayas began forming when India collided with Asia 40 to 50 million years ago. Fig. 2-19, p. 43

Active Figure 2.23: The Emperor Seamount–Hawaiian Island chain formed as a result of movement of the Pacific plate over a hot spot. The line of the volcanic islands traces the direction of plate movement. The numbers indicate the ages of the islands in millions of years. Fig. 2-23, p. 46