Plate Tectonics Theory
Isostasy: Vertical Crustal Movements
Dutton’s Isostasy
In 1892, Dutton named the isostatic adjustment mechanism Isostasy, explaining vertical crustal movements. Overloaded areas sink; unloaded areas rise.
Isostasy Mechanism
- In mountains, the crust is higher and thicker.
- Erosion removes material, activating isostatic recovery, raising mountain bases.
- Recovery is regionally distributed, preventing major side jumps.
Clarification of Isostasy
- Isostatic adjustments are very slow.
- Isostatic equilibrium is regional, not local. The lithosphere, despite rigidity, moves up and down like pieces of wood, arching under overload, distributing the effort gradually across a region.
- Though the sub-lithospheric mantle is solid, high pressures and temperatures mean that, on a geologic timescale, mantle materials behave like fluids. Convection currents are an example.
Two-Step Land Relief
Between the highest summits and deepest ocean trenches (≈20,000 m), a hypsometric curve shows two major steps in land relief: one for continental crust (sparse, coarse materials), and one for oceanic crust (denser, thinner materials). Each zone’s altitude reflects its isostatic equilibrium. Isostasy explains the existence of oceans and continents:
- Sparse areas are continental crust.
- Thin crust areas are denser oceanic crust.
- Thicker crust is both higher and deeper; any process increasing crustal thickness leads to higher altitudes.
Early Ideas on Continental Movement
Continents experience not only vertical (isostatic) but also horizontal movements. Theories supporting this are called mobilist; opposing theories are fixist. Alfred Wegener championed continental movement.
Precursors to Wegener’s Ideas
The complementarity of African and South American coastlines, noted since the 16th-19th centuries, hinted at geological continuity (Alexander von Humboldt). Frank Taylor’s work, focusing on Asian and European mountain ranges, provided a more direct precedent, proposing crustal shifts from north to south, causing collisions and ridge formation.
Continental Drift Theory
Wegener’s Pangea
Wegener proposed that all landmasses were once united in a supercontinent, Pangea. Present continents resulted from Pangea’s breakup and fragment displacement.
Wegener’s Arguments
- Geographical Arguments: The continents’ shapes fit together like puzzle pieces.
- Paleontological Arguments: The distribution of fossils like Mesosaurus (South Africa and South America) and Glossopteris (South America, Africa, India, Antarctica, Australia) supports the idea of past continental unity.
- Geological Arguments: Analysis of mountain ranges and geological formations on either side of the Atlantic.
- Paleoclimatic Arguments: Glacial deposits (tillites) of the same age in now widely separated locations indicate past climatic conditions inconsistent with their current positions.
Causes of Continental Movement (Wegener)
Wegener attributed continental movement to two forces: polar Earth rotation (displacing continents towards the equator) and tidal forces from the Sun and Moon (westward displacement).
Continental Drift and Plate Tectonics
Current Understanding
Wegener’s errors were: 1. Incorrectly identifying the causes of continental movement; 2. Assuming a fixed ocean floor (the seafloor also moves).
The Shift Towards Mobilist Theories
Arthur Holmes supported Wegener, suggesting mantle convection currents. In 1965, Tuzo Wilson introduced the term”plat” for large, uniformly moving lithospheric fragments. Three years later, the theory of plate tectonics was fully developed. This theory posits that the lithosphere is divided into moving plates, driven by Earth’s internal heat, causing volcanism, seismicity, mountain ranges, and changes in land and sea distribution.
Ocean Floor Dynamics
Mid-Ocean Ridges
The Atlantic Ocean features a mid-ocean ridge, rising 2-3 km above the surrounding plains. Iceland is part of this ridge, which extends 60,000 km, bifurcating into the Indian and Pacific Oceans. The Atlantic Ridge has a central rift valley, flanked by normal faults. Other ridges (e.g., Pacific) lack this feature. Transform faults interrupt the ridge periodically.
- Sediment Distribution: The scarcity of sediments on ridges and their relative scarcity elsewhere was a surprising finding.
- Youth of Oceanic Crust: Basalts on ridges are recent (<1 million years old). Basalt age increases with distance from the ridge, never exceeding 180 million years.
Seafloor Spreading
To explain magnetic striping, Vine and Matthews proposed seafloor spreading: new oceanic lithosphere is generated at ridges from interior material. This explains magnetic striping, volcanic activity at ridges, the increasing age of the ocean floor away from ridges, and sediment distribution in ocean basins.
Ocean Floor Subsidence
The gradual cooling and subsidence of the lithosphere (thermal subsidence) explains why ocean floor far from ridges is deeper.
Subduction
Subduction is the process of lithosphere descending into the Earth’s interior.
Subduction Zones
Subduction zones are convergent plate boundaries where one plate is forced beneath another.
Continental-Oceanic Convergence
Since continental lithosphere is lighter than oceanic lithosphere, in a continental-oceanic convergence, the oceanic plate subducts beneath the continental plate. Plate displacement is not continuous, resulting in earthquakes classified by depth: shallow (<70 km), intermediate (70-300 km), and deep (300-700 km).
Friction increases water temperature and lowers the melting point of minerals in the subducting oceanic lithosphere, causing partial melting of silica-rich minerals.
Ocean-Ocean Convergence
Older, denser oceanic lithosphere (around 100 million years old) can subduct spontaneously.
Continental-Continental Convergence
When an oceanic plate subducts, and only continental lithosphere remains, continental collision occurs. Continental lithosphere is too buoyant to subduct, resulting in collision and the formation of mountain ranges (e.g., Himalayas, Alps).
Transform Faults
Transform faults are conservative plate boundaries where lithosphere is neither created nor destroyed. There are two types:
- Faults that crosscut ridges, causing lateral displacement.
- Fractures connecting different plate boundaries (e.g., San Andreas Fault, Alpine Fault).
Transform faults lack volcanism but have frequent, shallow-focus earthquakes.
Lithospheric Plate Movement
Earth’s internal heat is crucial for lithospheric dynamics. The Earth is considered a heat engine. Gravitational energy also plays a key role.
- Mantle Convection: Earth’s internal heat drives mantle convection, causing plate movement.
- Mantle Plumes: Rising plumes of hot material from the lower mantle (layer D) create hotspots (e.g., Hawaii, Iceland).
- Gravity: The higher elevation of ridges facilitates downslope sliding of the ocean floor, and the cold, dense subducting lithosphere, further densified by mantle pressure, pulls the plate.
Theory of Plate Tectonics
- The lithosphere is divided into rigid plates.
- Plate boundaries are of three types: mid-ocean ridges (divergent), subduction zones (convergent), and transform faults (conservative).
- Lithospheric plates move over the plastic asthenosphere.
- Plate movement is driven by Earth’s internal heat and gravitational energy.
- Oceanic lithosphere is continually renewed; continental lithosphere is more permanent.
- Throughout Earth’s history, the position, shape, size, and number of plates have changed.