Earth's Structure and the Dynamics of Plate Tectonics

The Origin of the Solar System

Nebular Hypothesis Stages

• Initial Nebula
• Formation of the Accretion Disk
• Formation of Planetesimals
• Formation of Protoplanets

The Origin of the Earth

Stages of Formation

• Collision of planetesimals
• Differentiation of materials by density
• Formation of the crust
• Formation of the atmosphere and hydrosphere

Earth's Internal Heat

• Initial Heat: From the planet's formation and accretion.
• Radioactive Heat: From the decay of radioactive elements.
• Thermal Flow: The process of heat transfer to the surface.

Seismic Waves

• Primary waves (P-waves)
• Secondary waves (S-waves)
• Surface waves

Wave Behavior

As seismic waves travel towards the surface, they are refracted and reflected when they encounter discontinuities between layers.

Earth's Internal Structure

Model Based on Seismic Discontinuities (Compositional Layers)

• Crust: The outermost layer, separated from the mantle by the Moho discontinuity.
• Mantle:
  • Upper Mantle: Rich in peridotite (olivines and pyroxenes).
  • Lower Mantle: Composed of ultrabasic rocks.
  • D" Layer: The boundary layer between the lower mantle and the outer core.
• Core: Divided into a liquid outer core and a solid inner core.

Model Based on Dynamics (Mechanical Layers)

• Lithosphere: The rigid outer part of the earth, consisting of the crust and upper mantle.
• Asthenosphere: The highly viscous, mechanically weak region of the upper mantle.
• Mesosphere: The part of the Earth's mantle below the lithosphere and the asthenosphere.
• D" Layer
• Endosphere: The core of the Earth.

Techniques for Studying the Oceanic Lithosphere

• Ocean floor drilling and core sampling
• Ship-based sonar mapping
• Geophysical research campaigns
• Directed submersibles (ROVs and AUVs)

Structure of Undersea Features

• Continental shelf
• Continental slope
• Abyssal plain
• Oceanic ridges
• Volcanic islands, seamounts, and guyots
• Ocean trenches

Continental Margins and Oceanic Lithosphere

Types of Continental Margins

• Passive Margins: Not located on a plate boundary, characterized by a continental rise.
• Active Margins: Located on a plate boundary, featuring island arcs and back-arc basins.

Composition of the Oceanic Lithosphere

• Layer 1: Marine sediments
• Layer 2: Basement of igneous rocks (basalt)
• Layer 3: Gabbros

Characteristics of Oceanic Ridges

• Elongated, swollen, and distended zones of the lithosphere.
• Characterized by magmatic material, not by folding.
• Features a central axis (rift valley) and transform faults.
• Formed by the partial fusion of ascending mantle materials.
• Site of numerous shallow-focus earthquakes.

Evidence for Seafloor Spreading

• Increasing sediment thickness away from the ridge.
• Symmetric, alternating magnetic bands (paleomagnetism).
• Increasing age of basaltic lavas away from the ridge.
• Concentration of marine earthquakes and volcanoes at the ridge.
• High thermal flow at the ridge.
• Hotspots: Anomalous, long-term heat sources originating from ascending mantle material.

Continental Structures

• Craton: The old, stable part of the continental lithosphere.
  • Continental shields: Exposed areas of the craton.
  • Stable platforms: Areas where the craton is covered by sediment.
• Orogen: A region of deformed rocks from mountain-building events.

Continental Drift: The Pangea Hypothesis

Evidence for Continental Drift

• Correspondence between the shapes of continents (jigsaw fit).
• Analogies in fossilized fauna and flora across continents.
• Evidence of the direction of ancient ice movement (glaciation).
• Alignment of ancient mountain ranges when continents are reassembled.

The Theory of Plate Tectonics

• The lithosphere is split into tectonic plates that move and interact.
• Plates move over the plastic asthenosphere (upper mantle).
• Seismic activity, volcanoes, and mountain building are concentrated at plate boundaries.
• New lithosphere is created at oceanic ridges, and old lithosphere is recycled at subduction zones.
• Plate movement is driven by mantle convection currents (ridge push and slab pull).

The Wilson Cycle

Stages of Ocean Basin Evolution

• Embryonic Stage: Uplift and rifting of a continent.
• Juvenile Stage: Divergence leads to a narrow sea (e.g., Red Sea).
• Mature Stage: A wide ocean basin forms with a mid-ocean ridge.
• Declining Stage: Subduction begins, closing the ocean basin.
• Terminal Stage: Collision of continents and mountain building.
• Suturing Stage: The supercontinent is reformed.