CH.1 Earth's Structure and Plate Tectonics: A Comprehensive Study Guide

Earth's Layers: Composition and Properties

Compositional Layers

• Crust: The Earth's outermost layer, relatively thin compared to other layers. It's divided into oceanic and continental crust.

  • Oceanic Crust: Denser, thinner, and younger than continental crust. Composed mainly of basalt and gabbro (mafic rocks, rich in magnesium and iron). Typically 5-7 km thick, found beneath the ocean floor.

  • Continental Crust: Less dense, thicker, and older than oceanic crust. Composed mainly of granite (felsic rocks, rich in silica and aluminum). Typically 30-40 km thick, forms the continents. The oldest continental crust is over 4 billion years old.

• Mantle: The thickest layer, extending from the crust to the core. Composed primarily of peridotite (ultramafic rock, rich in magnesium and iron). The mantle is divided into the upper mantle and lower mantle.

  • Upper Mantle: Includes the lithosphere (rigid, strong) and asthenosphere (plastic, weak). The asthenosphere is partially molten, allowing for plate movement. A low-velocity zone (LVZ) exists within the upper mantle, characterized by slower seismic wave speeds, possibly due to partial melting. The transition zone separates the upper and lower mantle.

  • Lower Mantle (Mesosphere): More rigid than the upper mantle. Mineral composition changes due to high pressure and temperature. The D" layer is a complex zone near the core-mantle boundary, characterized by both slow and fast seismic velocities.

• Core: The Earth's innermost layer, composed primarily of iron and nickel.

  • Outer Core: Liquid, responsible for generating Earth's magnetic field. The movement of molten iron generates electric currents, creating the magnetic field.

  • Inner Core: Solid, due to immense pressure. Despite high temperatures, the pressure is so high that the iron and nickel are solid.

Mechanical Layers

• Lithosphere: The rigid outermost shell, encompassing the crust and the uppermost part of the mantle. It's broken into tectonic plates.

• Asthenosphere: A partially molten, plastic layer beneath the lithosphere. Its plasticity allows the lithospheric plates to move.

• Mesosphere: The lower mantle, a more rigid layer below the asthenosphere.

• Outer Core: Liquid layer, responsible for the Earth's magnetic field.

• Inner Core: Solid innermost layer, extremely dense.

⛰ Plate Tectonics: Driving Forces and Boundaries

Plate Movement

• Driving Forces: Plate movement is driven by several forces, including:

  • Mantle Convection: Heat from the Earth's core drives convection currents in the mantle, causing the plates to move.

  • Slab Pull: The weight of subducting (sinking) plates pulls the rest of the plate along.

  • Ridge Push: Magma rising at mid-ocean ridges pushes the plates apart.

Plate Boundaries

• Divergent Boundaries: Plates move apart, creating new crust. Mid-ocean ridges are examples of divergent boundaries. Seafloor spreading occurs at these boundaries.

• Convergent Boundaries: Plates collide, resulting in subduction or continental collision.

  • Subduction Zones: One plate slides beneath another, often forming volcanic arcs and trenches.

  • Continental Collisions: Two continental plates collide, forming mountain ranges (orogenic belts). The Himalayas are an example.

• Transform Boundaries: Plates slide past each other horizontally. The San Andreas Fault is a well-known example. Earthquakes are common along transform boundaries.

🌋 Hotspots and Mantle Plumes

• Hotspots: Long-lived areas of volcanic activity, not associated with plate boundaries. They are thought to be caused by mantle plumes.

• Mantle Plumes: Columns of hot material rising from deep within the mantle. They can create volcanic chains as plates move over them (e.g., Hawaiian Islands). The existence and origin of mantle plumes are still debated among scientists.

🌊 Oceanic and Continental Crust: A Comparison

Feature Oceanic Crust Continental Crust Composition Basalt, Gabbro (Mafic) Granite (Felsic) Density Higher (2.9-3.1 g/cm³) Lower (2.6-2.9 g/cm³) Thickness Thinner (5-7 km) Thicker (30-40 km) Age Younger (up to ~190 Ma) Older (up to >4.0 Ga) Elevation Lower (mostly submerged) Higher (mostly above sea level) Buoyancy Less buoyant More buoyant

🌎 The Wilson Cycle: A Model for Ocean Basin Formation and Destruction

The Wilson Cycle is a model that describes the cyclical opening and closing of ocean basins. It involves several stages:

1. Embryonic Stage: A rift valley forms on a continent.

2. Juvenile Stage: A narrow sea forms as the continent splits apart.

3. Mature Stage: A wide ocean basin develops with mid-ocean ridges and spreading centers.

4. Declining Stage: Subduction begins, closing the ocean basin.

5. Terminal Stage: Continental collision occurs, forming a mountain range.

6. Suturing Stage: The continents are joined together.

🪨 Rocks: Aggregates of Minerals

• Rocks: Aggregates of one or more minerals. They are classified based on their origin (igneous, sedimentary, metamorphic) and mineral composition.

• Igneous Rocks: Formed from the cooling and solidification of magma or lava. Examples include granite (intrusive) and basalt (extrusive).

• Sedimentary Rocks: Formed from the accumulation and cementation of sediments. Examples include sandstone and limestone.

• Metamorphic Rocks: Formed from the transformation of existing rocks due to heat, pressure, or chemical reactions. Examples include marble (from limestone) and slate (from shale).

Facts to Memorize

1. The Earth's layers are defined by both chemical composition and physical properties.

2. Oceanic crust is denser and thinner than continental crust.

3. The mantle is the thickest layer and is primarily composed of peridotite.

4. The asthenosphere is a partially molten layer that allows for plate movement.

5. Plate tectonics is driven by mantle convection, slab pull, and ridge push.

6. Divergent boundaries create new crust, while convergent boundaries destroy crust.

7. Transform boundaries are characterized by horizontal movement.

8. Hotspots are areas of volcanic activity not associated with plate boundaries.

9. Mantle plumes are thought to be the source of hotspots.

10. The Wilson Cycle describes the cyclical opening and closing of ocean basins.

11. Igneous rocks form from the cooling of magma or lava.

12. Sedimentary rocks form from the accumulation and cementation of sediments.

13. Metamorphic rocks form from the transformation of existing rocks.

14. The Moho discontinuity separates the crust from the mantle.

15. The Gutenberg discontinuity separates the mantle from the core.

16. The Lehman discontinuity separates the outer core from the inner core.

17. Subduction zones are characterized by inclined seismic zones (Wadati-Benioff zones).

18. Continental collisions form orogenic belts and mountain ranges.

19. Transform faults offset mid-ocean ridges.

20. The age of oceanic crust increases with distance from mid-ocean ridges.