Unit 1: Major Geological Processes Associated with Plate Tectonics

This unit explores the scientific theory explaining how the Earth’s lithosphere is divided into plates that move over the mantle. It examines the internal forces that create landforms such as mountains, rift valleys, and oceanic trenches. Understanding these processes is essential for grasping the physical evolution of the Ethiopian landscape and the wider world.

Theories of Continental Drift and Plate Tectonics

• Continental Drift Theory: Proposed by Alfred Wegener in 1912, suggesting that all continents were once joined in a single supercontinent called Pangea.
• Evidence for Drift:
  1. Jigsaw Fit: The matching shapes of continental coastlines (e.g., South America and Africa).
  2. Fossil Correlation: Identical fossils found on widely separated landmasses.
  3. Geological Match: Matching rock types and mountain ranges across oceans.
  4. Paleoclimatic Data: Evidence of past glaciations in now-tropical regions.
• Plate Tectonics Theory: A modern theory stating that the Earth’s outer shell is divided into several lithospheric plates that glide over the plastic asthenosphere.

Types of Plate Boundaries

The movement of plates occurs at three primary types of boundaries:

1. Divergent Boundaries: Occur where plates move apart. This leads to seafloor spreading and the formation of rift valleys.
2. Convergent Boundaries: Occur where plates collide.
   • Subduction: One plate slides beneath another, often creating deep-sea trenches and volcanic arcs.
   • Continental Collision: Two continental plates crash, forming massive fold mountains (e.g., the Himalayas).
3. Transform Boundaries: Occur where plates slide horizontally past each other, often resulting in earthquakes along fault lines.

The Process of Folding and Faulting

When tectonic plates move, they subject crustal rocks to intense pressure, leading to two main types of deformation:

Folding

1. Occurs when rocks are compressed (pushed together).
2. The rock layers bend rather than break.
3. Anticlines are the upward folds (arches).
4. Synclines are the downward folds (troughs).

Faulting

1. Occurs when rocks break due to tension (pulling apart) or compression.
2. A fault is a fracture in the rock where movement has occurred.
3. Normal Faults: Caused by tension; one block slides down.
4. Reverse Faults: Caused by compression; one block is pushed up.
5. Graben (Rift Valley): A block of land that sinks between two parallel faults.
6. Horst (Block Mountain): A block of land pushed upward between faults.

Volcanism and Earthquakes

• Volcanism: The process by which magma (molten rock) rises from the Earth’s interior to the surface as lava.
• Intrusive Volcanism: Magma cools and solidifies underground (e.g., batholiths, sills, dykes).
• Extrusive Volcanism: Magma reaches the surface through vents or fissures.
• Earthquakes: Sudden vibrations in the Earth’s crust caused by the release of energy along faults.
  • Focus: The point inside the Earth where the earthquake starts.
  • Epicenter: The point on the surface directly above the focus.

Master Glossary & Key Terminology

• Anticline: An upward, arch-like fold in rock layers.
• Asthenosphere: The semi-fluid layer of the mantle below the lithosphere.
• Batholith: A very large mass of intrusive igneous rock.
• Continental Drift: The hypothesis that continents slowly move across the Earth’s surface.
• Convergent Boundary: A boundary where two tectonic plates move toward each other.
• Divergent Boundary: A boundary where two tectonic plates move away from each other.
• Dyke: An intrusive body of magma that cuts across rock layers vertically.
• Epicenter: The point on the Earth’s surface directly above an earthquake’s starting point.
• Faulting: The breaking and displacement of rock layers.
• Focus: The exact location inside the Earth where an earthquake originates.
• Folding: The bending of rock layers due to compressive forces.
• Graben: A depressed block of the crust bordered by parallel faults; a rift valley.
• Horst: An uplifted block of the crust bordered by parallel faults.
• Lithosphere: The rigid outer part of the earth, consisting of the crust and upper mantle.
• Magma: Molten rock located beneath the Earth’s surface.
• Pangea: The ancient supercontinent that existed before continental drift began.
• Plate Tectonics: The theory explaining the movement of the Earth’s lithospheric plates.
• Seafloor Spreading: The process by which new oceanic crust forms at divergent boundaries.
• Sill: A flat, horizontal sheet of intrusive magma between rock layers.
• Subduction: The process of one tectonic plate sinking beneath another.
• Syncline: A downward, U-shaped fold in rock layers.
• Transform Boundary: A boundary where plates slide past each other horizontally.