Comprehensive Guide to Earth's Layers and Plate Tectonics

Analogy of the Crust: The Earth's crust is compared to the skin of an apple because it is extremely thin relative to the other three layers.
Percentage of Earth's Mass: The crust accounts for only $1\%$ of the Earth.
Plate Structure: The crust is not a single solid shell but is broken into numerous pieces known as plates.
Crustal Thickness Measurements:
- Beneath continents: Typically about $25\,\text{miles}$ thick.
- Beneath oceans: Typically about $6.5\,\text{miles}$ thick.
- General range: Between $5\,\text{km}$ and $100\,\text{km}$ thick.
Seismic Activity: Most earthquakes occur within the crust because it is relatively light and brittle.
Types of Crust:
- Oceanic Crust: Characterized as very dense and primarily composed of basalt.
- Continental Crust: Characterized as less dense and primarily composed of granite.

The Lithosphere: This layer is comprised of both the Crust and the Moho.
The Moho: This serves as the specific dividing line or boundary between the Earth's Crust and the Mantle.

Position and Size: The mantle is the middle layer located just below the crust and extends all the way down to the core. It is the largest layer of the Earth.
Physical Properties: Unlike the brittle crust, the mantle is relatively flexible. This flexibility allows it to flow rather than fracture.
Regional Divisions: The mantle is divided into two primary sections:
- Upper section.
- Lower section.
The Asthenosphere: This is the outer layer of the mantle. It is a liquid layer consisting of magma (melted rock) where convection currents occur.

Mass and Composition: The core makes up $\frac{1}{3}$ of the Earth's mass and is composed mostly of iron ( $Fe$ ) and nickel ( $Ni$ ). It is a region of intense heat.
Outer Core:
- State of Matter: Liquid/Fluid.
- Thickness: Approximately $1488\,\text{miles}$ thick.
- Composition: Dense liquid iron and nickel.
- Magnetic Field: The flow of the iron-rich fluid in the outer core generates the Earth's magnetic field.
Inner Core:
- State of Matter: Solid.
- Conditions: Temperatures and pressures are so extreme that the metallic components are squeezed together and cannot move, resulting in a solid state.
- Composition: Solid iron and nickel.

Continental Drift Theory: Proposed by Alfred Wegener, this theory suggests that the continents were once joined in a single supercontinent called "Pangaea," which translates to "all land."
Timeline of Continental Movement:
- Supercontinent Pangaea: Existed approximately $200\,\text{million years ago}$ .
- Breakup state: Significant separation by $135\,\text{million years ago}$ .
- Further drift: Position by $35\,\text{million years ago}$ .
- Present Day: The current configuration of continents.
Theory of Plate Tectonics: This theory states that the Earth's crust is broken into sections or pieces called tectonic plates that move on top of the liquid mantle (specifically the asthenosphere).

The Puzzle Fit: Continents, such as South America and Africa, appears to be able to fit together like puzzle pieces.
Fossil Evidence: Identical fossils have been discovered on the coastlines of different, now widely separated continents, including South America, Africa, India, Antarctica, and Australia.
Rock Types and Geologic Features: Mountain ranges that appear on one coastline are found to continue on another continent across the ocean.

Movement: There are approximately $7$ major plates that move extremely slowly but continuously.
Major Plates:
- Eurasian Plate.
- African Plate.
- Australian-Indian Plate.
- North American Plate.
- Pacific Plate.
- Antarctic Plate.
- South American Plate.
Intermediate Plates:
- Caribbean.
- Cocos.
- Nazca.
- Arabian.
- Philippine.
- Juan de Fuca.
- Scotia.
Plate Classifications:
- Oceanic: Plates located under the ocean.
- Continental: Plates made of land.

Convection Cells: These cells in the mantle are responsible for moving the tectonic plates.
The Convection Process:
1. Magma in the center is heated, becomes less dense, and rises toward the surface ( $\text{HOT-goes UP}$ ).
2. As magma reaches the surface, it cools, becomes denser, and sinks back down ( $\text{COOL-goes DOWN}$ ).
3. This creates a continuous circular pattern of movement.
Slab Pull: This occurs at trenches, contributing to plate movement.
Density Dynamics:
- Oceanic Plates: Heavy and dense; they sink or subduct underneath other plates.
- Continental Plates: Lighter and less dense.

General Definition: Plates moving apart from one another.
Seafloor Spreading (Oceanic-Oceanic):
- The process where two oceanic plates move apart, allowing magma to rise.
- As the magma cools, it forms new oceanic crust.
- Example: The Mid-Atlantic Ridge.
Paleomagnetism (Magnetic Reversals):
- Evidence for seafloor spreading.
- The Earth's magnetic poles reverse periodically from North to South.
- The youngest crust is located at the center of the ridge, with the crust becoming progressively older as you move outward.
Mid-Ocean Ridges: Underwater mountain ranges formed where oceanic plates continue to separate.
Rift Valleys (Continental-Continental):
- Formed when continental plates pull apart.
- This process results in volcanoes and the creation of new land.
- Example: The East African Rift Valley.

General Definition: Plates coming together.
Subduction Zones: Occurs when one plate goes under another. The denser (heavier) plate always subducts beneath the less dense plate.
Volcanic Arcs (Oceanic-Continental Subduction):
- The denser oceanic crust sinks under the less dense continental crust.
- The subducting rock melts due to heat and rises to the surface, creating volcanoes.
- Significant trenches are also created during this process.
Deep-Sea Trenches: These are depressions or "holes" in the ocean floor at subduction zones, which may contain sand deposits.
Volcanic Island Chains (Oceanic-Oceanic Subduction):
- Formed when two oceanic plates meet and one subducts under the other.
- Example: The Aleutian Islands.