Plate Tectonics Overview
Introduction
- The theory of plate tectonics extends Alfred Wegener's work.
- Developed in the mid-1900s, building on early 20th-century ideas.
Earth's Structure
- Lithosphere: Solid outer shell of the earth, akin to "Magic Shell" on ice cream.
- Composed of tectonic plates (lithospheric plates).
- Types of crust:
- Continental Crust: Thick, granite, less dense (~2.7 g/cm³).
- Oceanic Crust: Thinner, basalt, more dense (~3 g/cm³).
- Rigid Mantle: Combined with crust to form the lithosphere.
- Asthenosphere: Gooey, partially melted layer beneath the lithosphere.
- Convection occurs here, driving plate movement.
Plate Movement
- Convection Currents: Hot material rises, cools, and sinks, creating convection cells.
- Plates float on the asthenosphere and move due to these currents.
Plate Boundaries
- Types of Boundaries:
- Transform: Plates slide past one another.
- Characterized by earthquakes (e.g., San Andreas Fault).
- Convergent: Plates collide.
- Subduction zones (oceanic vs. continental crust) create trenches, volcanoes.
- Island arcs occur with oceanic-oceanic crust collisions.
- Collision zones (continental-continental) form mountain ranges (e.g., Himalayas).
- Divergent: Plates drift apart.
- Rift zones on land (e.g., East African Rift).
- Mid-ocean ridges form mountains and volcanoes.
- Magma creates new rock at ridges, which records Earth’s magnetism.
Special Features
- Hotspots:
- Areas where magma punches through the middle of a plate.
- Example: Hawaii, creates a chain of islands with one active volcano.
Key Observations
- Earthquakes, volcanoes, and mountain ranges tend to occur at plate boundaries.
- Some volcanic activity occurs at non-boundary hotspots.
- Plates move, indicated by the direction of extinct volcanoes in hotspot chains.
These notes give an overview of key concepts in plate tectonics, essential for understanding the geological processes shaping the Earth. Further details to be explored in class sessions.