Earth's Layers, Continental Drift, and Plate Boundaries

Study Methods of Earth's Interior

Several methods are used to study the Earth's interior:

• Probes and Mines: Direct observation is limited by depth (radius ~6371 km, probes go as deep as 13km).
• Study of Rocks: Natural phenomena like erosion and volcanic eruptions expose deeper rocks.
• Meteorites: Provide information about the primordial materials of the solar system, including Earth.
• Seismic Methods: The most effective method. Analysis of seismic waves from earthquakes that travel through the Earth's interior, providing data via seismographs.

Inner Layers of the Earth

The Earth's internal layers are distributed in order of increasing density, separated by seismic discontinuities.

Compositional Layers

• Crust and Mantle: Composed of rocks, with the less dense crust rich in Silicon (Si) and Magnesium (Mg).
• Core: Composed of metal, primarily Iron (Fe).

Mechanical Layers

• Lithosphere: A rigid surface layer covering the entire crust and a portion of the upper mantle.
• Mesosphere: Below the lithosphere, it is more plastic and ductile.
• Asthenosphere: The molten part of the mantle.
• Outer Core: Molten.
• Inner Core: Solid.

Evidence for Continental Drift

• Paleontological: Presence of fossils of similar land organisms on different continents.
• Geological: Continental coastlines fit together like puzzle pieces.
• Paleoclimatic: Continents show evidence of having had the same type of rock formations and climates in the past.

Types of Tectonic Plates

• By Size: There are 8 large plates and a series of microplates.
• By Lithosphere Type: Continental, oceanic, and mixed plates are distinguished.

Continental Drift and Plate Tectonics

• Subduction: The seafloor bends and sinks into the mantle. The melting material feeds volcanoes with magma.
• The lithospheric plate is divided into large blocks (plates) covering the surface.
• Internal geological activity is concentrated at the boundaries between plates.
• The ocean floor is continuously generated and destroyed by subduction.
• The dragging plates and continents interact. When two plates move apart, new oceans are generated. When they move closer, mountain ranges arise.

Types of Boundaries Between Plates

• Constructive (Divergent): Zones where plates move apart, causing the creation of new oceanic lithosphere.
• Destructive (Convergent): Two continents collide; subduction zones, where the seafloor is introduced into the mantle.
• Passive (Transform): Fractures known as transform faults, where two plates rub or brush laterally, causing earthquakes.

Modalities of Subduction

• Under Continental Lithosphere: A trench is located under the edge of a continent, causing compression and the rise of a continental edge volcanic ridge.
• Under Oceanic Lithosphere: Causes an arc of volcanic islands on the overriding plate, pointing towards the subducting plate.

The Wilson Cycle

Wilson was the first to propose the cyclical breaking and reunification of continents throughout Earth's history.

1. Expansion of Ocean and Continental Breakup: The Rift Valley is a deep groove through which magmas rise. If the process continues, the continent breaks into two.
2. Closure of the Ocean: Away from the ridge, the oceanic lithosphere becomes progressively colder, denser, and older. It sinks into the mantle, causing subduction. The ocean begins to shrink, and continents begin to close.
3. Continental Collision: The continental lithosphere, thicker and lighter than the oceanic lithosphere, does not subduct. After the collision, the speed of approach of the plates is reduced until it stops. Sediments are folded and deformed, forming an intercontinental mountain range.