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Earth Structure and Plate Tectonics
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Earth Structure and Plate Tectonics
Earth Structure and Plate Tectonics
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Question
What is the difference between Earth's crust and lithosphere?
Answer
The crust is Earth’s thin, brittle outer shell made of rock, whereas the lithosphere includes the crust plus the uppermost mantle, which is also brittle. Thus, lithosphere is a thicker mechanical layer than the crust alone.
Question
How do oceanic crust and continental crust differ in composition and thickness?
Answer
Oceanic crust is thin (5-12 km), made mainly of basalt lava and gabbro, and is denser (~3.0 g/cm³). Continental crust is thicker (~35 km), composed of various igneous, metamorphic, and sedimentary rocks, with an average composition similar to granite, and is less dense (~2.7 g/cm³).
Question
Why do continents stand higher than ocean basins?
Answer
Continental crust is thicker and less dense than oceanic crust, causing it to float higher on the mantle. Oceanic crust is denser and thinner, so it sinks deeper into the mantle, forming ocean basins.
Question
Explain how heat flows within Earth’s mantle.
Answer
Heat flows by conduction in solid materials, where energy transfers via atomic collisions, and by convection in materials that can move (like fluids or ductile solids). In the mantle, convection causes warm material to rise and cooler material to sink, driving mantle movement.
Question
What evidence indicates Earth’s core is metallic and has both liquid and solid parts?
Answer
Meteorites are metallic and thought to represent the core. Earth's overall density is higher than surface layers, implying a dense interior. Seismic waves show the outer core is liquid and the inner core solid. Earth's magnetic field requires a moving metal, supporting a liquid outer core with convection.
Question
What was Alfred Wegener's continental drift hypothesis?
Answer
Wegener proposed that continents were once joined as a single supercontinent called Pangaea and have since drifted apart to their current locations. He based this on the fitting shapes of continents and other evidence suggesting past connection and movement.
Question
How did World War II technology contribute to understanding seafloor spreading?
Answer
Echo sounders used in WWII to detect submarines allowed scientists to map ocean floor depths (bathymetry). This revealed mid-ocean ridges and deep trenches previously unknown, key features for the seafloor spreading hypothesis.
Question
Describe the process and significance of seafloor spreading.
Answer
Hot mantle material rises at mid-ocean ridges, creating buoyant, elevated ridges. Lava erupts, cools, and captures Earth's magnetic polarity as stripes. As new seafloor forms, it pushes older seafloor away, causing ocean floor to spread. This explains ocean crust creation and continental movement.
Question
What are plate boundaries, and what are the three types based on relative plate motions?
Answer
Plate boundaries are edges where tectonic plates interact, sites of geologic activity. The three types are divergent (plates move apart), convergent (plates move together), and transform (plates slide past each other).
Question
How does mountain building occur and what types of mountains result from tectonic processes?
Answer
Mountain building (orogeny) mainly occurs at convergent plate boundaries where crust deforms. Fold mountains form from crust buckling and folding (e.g., Himalayas), fault-block mountains form from crustal blocks tilting or moving between faults (e.g., Sierra Nevada), and volcanic mountains form from magma eruption building layers (e.g., Andes).