Notes on Plate Tectonics and Natural Hazards
Introduction to Plate Tectonics
Earth's Lithosphere & Asthenosphere
The lithosphere is composed of rigid plates that float atop the asthenosphere, a hotter and more plastic region of the upper mantle.
These lithospheric plates are in constant motion due to underlying forces from the Earth's interior.
Basics of Plate Tectonics
The Earth's crust undergoes constant recycling through:
Sea-floor spreading: New lithospheric material forms at mid-ocean ridges and spreads outward.
Subduction: Old lithospheric material is destroyed as it is forced beneath another plate into the mantle, thereby recycling crust material.
Historical Context
Acceptance of Plate Tectonic Theory
The theory gained credibility in the late 1960s and early 1970s, influenced significantly by oceanographic research post-World War II.
Advances were made due to increased funding from the U.S. Navy, which facilitated scientific exploration of the ocean floor.
Notably, Marie Tharp contributed by mapping the North Atlantic mid-ocean ridge, lending support to plate tectonic theory.
Types of Plate Boundaries
Divergent Boundaries:
Plates move away from each other (e.g., Mid-Ocean Ridge or Gulf of California).
Rising asthenosphere causes sea-floor spreading, creating new basaltic crust.
Types:
Oceanic Ridge System: Characterized by shallow earthquakes, volcanic activity, and high heat flow (e.g., Mid-Atlantic Ridge).
Young Ocean Basins: Very young rift systems (e.g., Gulf of California).
On-land Rifting: Continental splits may form ocean basins with volcanic activity (e.g., East African Rift).
Convergent Boundaries:
Plates collide, leading to subduction scenarios.
Denser ocean plates subduct beneath lighter continental plates.
Types:
Ocean-Ocean Convergence: One oceanic plate subducts beneath another (e.g., Tonga, Aleutians).
Ocean-Continent Convergence: Dense oceanic plates subduct under continental plates (e.g., Andes).
Continent-Continent Convergence: Both plates are low-density, resulting in mountain building (e.g., Himalayas).
Transform Boundaries:
Plates slide parallel but in opposite directions, resulting in low heat flow and shallow earthquakes (e.g., San Andreas Fault).
Additional Concepts
Convergence and Earthquake Depths:
Depths of Earthquakes: Can vary from shallow (0-30 km) to deep (400-700 km), with heat flow being low at trenches and high at volcanic arcs.
Hotspots:
Fixed magma plumes create chains of volcanic islands as plates move across them, leading to features such as the Hawaiian Islands.
Example Calculation for Hotspot Motion:
Distance between oldest and youngest volcanoes = 2,000 km
Time = 20 million years
Moving Rate =