Earth Structure and Plate Tectonics

Key Concepts Earth’s interior is layered, and the layers are arranged by density Continents rise above sea level because they float on a dense, deformable layer beneath them The brittle surface of Earth is fractured into about a dozen tile like “plates” Table 2-1 Principles of the Cell Theory

Key Concepts (cont’d.) Movement of the subterranean material on which these plates float moves them relative to one another Continents and oceans are formed and destroyed where the plates collide, flex, and sink Compelling evidence for plate movement is recorded in magnetic fields within the ocean floor

The Theory of Continental Drift Theory proposed by Alfred Wegener Pangaea – all continents in one landmass Fossil evidence across continents Proposed a centrifugal force mechanism Dismissed as a “crank”

Continents That Were Once Joined Formed Continuous Chains Figure 3.3 Mountain ranges in Scandinavia, Scotland, and North America are now separated by the Atlantic but are remarkably similar in age and composition. Fossils of the reptile Mesosaurus were found in Argentina and Africa, but nowhere else. The seed fern Glossopteris was found in all the southern landmasses. If the continents were once joined, as shown here, these mountain ranges and fossil bands would have formed continuous chains.

3.2 Earth’s Interior Is Layered Density stratified, with denser material towards the center Density: mass per unit volume Figure 3.5 A cross section through Earth showing the internal layers

3.3 The Study of Earthquakes Provides Evidence for Layering Earthquakes generate seismic waves Surface waves – travel along Earth’s surface Body waves – travel through Earth P wave – compressional wave S wave – shear wave Seismograph – detects and records seismic waves

Earthquake Wave Shadow Zones Confirmed the Presence of Earth’s Core Predicted by Richard Oldham S waves cannot pass through liquids P waves are refracted at density boundaries Data from an earthquake, a “natural experiment,” confirmed theories of Earth’s layering

How Earthquakes Contributed to Our Model of the Layered Earth Figure 3.7 How earthquakes contributed to our model of the layered Earth.

3.4 Earth’s Inner Structure Was Gradually Revealed Earth’s internal layers are based on chemical composition Crusts, mantle, and cores Earth’s internal layers are also based on physical properties Lithosphere Asthenosphere Lower mantle Core

Radioactive Decay and Isostatic Equilibrium Earth’s interior is heated by radioactive decay Conduction Convection Isostatic equilibrium supports continents above sea level Buoyancy – displacement of fluid as an object floats

Mountains Continental crust Low-density Mantle mountain root Transport Erosion Deposition Subsidence Uplift Figure 3.11 Erosion and isostatic readjustment can cause continental crust to become thinner in mountainous regions. As mountains are eroded over time (a–c), isostatic uplift causes their roots to rise. (The same thing happens when a ship is unloaded or an iceberg melts.) Further erosion exposes rocks that were once embedded deep within the peaks, sometimes exposing once-buried structures like Half-Dome in Yosemite Valley (d). Subsidence Stepped Art Figure 3-10 p79

Erosion and Isostatic Readjustment in Continental Crust Figure 3.11 Erosion and isostatic readjustment can cause continental crust to become thinner in mountainous regions. As mountains are eroded over time (a–c), isostatic uplift causes their roots to rise. (The same thing happens when a ship is unloaded or an iceberg melts.) Further erosion exposes rocks that were once embedded deep within the peaks, sometimes exposing once-buried structures like Half-Dome in Yosemite Valley (d).

3.5 The New Understanding of Earth Evolved Slowly The age of Earth has been controversial James Hutton – uniformitarianism Controversy with biblical school of thought – catastrophism Darwin and Wallace – biological evolution Evidence from exploration has convinced most researchers that Earth is of great age

3.6 Wegener’s Idea Is Transformed Pacific Ring of Fire Tectonic activity surrounding Pacific Ocean Oceanic crust dated and found to be young compared to the age of Earth Echo sounding used to reveal seafloor topography

3.7 The Breakthrough: From Seafloor Spreading to Plate Tectonics Seafloor spreading – new hypothesis Mid-ocean ridges are spreading centers Subduction zones – areas where oceanic crust plunges down into the mantle Plate tectonic theory – John Tuzo Wilson Lithospheric plates “float” on asthenosphere Plate movement Form at mid-ocean ridges Pulled downward into the mantle by leading edge

The Tectonic System Is Powered by Heat Figure 3.14 The tectonic system is powered by heat. Some parts of the mantle are warmer than others, and convection currents form when warm mantle material rises and cool material falls. Above the mantle floats the cool, rigid lithosphere, which is fragmented into plates. Plate movement is powered by gravity: The plates slide down the ridges at the places of their formation; their dense, cool leading edges are pulled back into the mantle. Plates may move away from one another (along the ocean ridges), toward one another (at subduction zones or areas of mountain building), or past one another (as at California’s San Andreas Fault). Smaller localized convection currents form cylindrical plumes that rise to the surface to form hot spots (like the Hawai’ian Islands). Note that the whole mantle appears to be involved in thermal convection currents.

3.8 Plates Interact at Plate Boundaries Divergent plate boundaries Plates moving apart Rift valley forms as crust is pulled/pushed apart Figure 3.18 A model for the formation of a new plate boundary: the breakup of Pangaea and the formation of the Atlantic.

Convergent and Transform Plate Boundaries Convergent plate boundaries Plates coming together Ocean to ocean convergence Ocean to continent convergence Continent to continent convergence Transform plate boundaries Plates shear laterally past one another

3.9 A Summary of Plate Interactions Divergences Divergent oceanic crust (Mid-Atlantic) Divergent continental crust (rift valleys) Convergences Oceanic-continental (South America) Oceanic-oceanic (northern Pacific) Continental-continental (Himalayas)

3.10 The Confirmation of Plate Tectonics The history of plate movement is captured in residual magnetic fields Paleomagnetism – recording of Earth’s past magnetic field Iron-bearing minerals in ocean-floor basalts align with the magnetic field and remain when rock solidifies Magnetometer – records residual magnetism

The Age of the Ocean Floors Figure 3.29 The age of the ocean floors. The colors represent an expression of seafloor spreading over the last 200 million years as revealed by paleomagnetic patterns. Note especially the relative symmetry of the Atlantic basin in contrast with the asymmetrical Pacific, where the spreading center is located close to the eastern margin and intersects the coast of California.

Plumes, Hot Spots, and Terranes Provide Evidence of Plate Tectonics Plate movement above mantle plumes and hot spots Mantle plumes and hot spots remain stationary under moving plates Sediment age and distribution, oceanic ridges, and terranes Terranes – small pieces of different crust that gets sutured onto a continent at a subduction zone

Terrane Formation Figure 3.32 Terrane formation. Oceanic plateaus usually composed of relatively low-density rock are not subducted into the trench with the oceanic plate. Instead, they are “scraped off,” causing uplifting and mountain building as they strike a continent (a–d). Though rare, assemblages of subducting oceanic lithosphere can also be scraped off (obducted) onto the edges of continents. Rich ore deposits are sometimes found in them.

3.11 Scientists Still Have Much to Learn about the Tectonic Process Many questions remain unanswered Why should long lines of asthenosphere be any warmer than adjacent areas? Why do mantle plumes form? What causes a superplume? How long do they last? How far do most plates descend? (Recent evidence suggests that much of the material spans the entire mantle, reaching the edge of the outer core)

Chapter In Perspective Earth is composed of concentric spherical layers that are density sorted Layers are confirmed based on seismic evidence Plate tectonics suggests Earth’s surface is made up of lithospheric plates that interact in different ways at plate boundaries

Some Questions from Students What’s the potential for serious loss of life and property damage due to tectonic plate movement? Is plate movement a new feature of Earth? Relatively great. About 40% of the world’s largest cities lie within 160 kilometers (100 miles) of a plate boundary. By the year 2015, about 350 million people will be living in high-risk areas. About 80% of those at risk live in developing nations, where seismic safety is not a high priority in building design. Another 210 million people are also threatened by active volcanoes; most live along the subduction zones of the Pacific Ring of Fire, where 75% of Earth’s 850 active volcanoes are located. Since 1600, there have been approximately 262,000 deaths from volcanic eruptions, about 76,000 in the last century. No. Multiple lines of evidence suggest we may be in the middle of the sixth or seventh major tectonic cycle since tectonic movement began about 3 billion years ago. Megacontinents like Pangaea appear to have formed, split, moved, and rejoined many times since Earth’s crust solidified.