(4) Plate Tectonics

Alfred Wegener

Proposed the theory of continental drift. He hypothesized that continents move freely across Earth’s surface.

Evidence for the theory of continental drift…

Fit of coastlines, fossil evidence, and glacial evidence.

The theory of seafloor spreading was proposed by

Harry Hess. He hypothesized that new oceanic crust is formed at mid-ocean ridges and moves away from the ridge crests. Old oceanic crust is destroyed at oceanic trenches (subduction zones).

Evidence for the theory of seafloor spreading…

Mid-ocean ridges, marine magnetic anomalies, and the age of seafloor rocks.

Mid-Ocean Ridges

Underwater mountain ranges where new crust is formed.

Marine Magnetic Anomalies

Symmetric bands of magnetic “strips” parallel to mid-ocean ridges, reflecting the Earth’s magnetic field reversals recorded in the cooling basaltic crust.

Age of Seafloor Rocks

Seafloor rocks are youngest at the mid-ocean ridges and get progressively older with distance away from the ridge.

Fit of coastlines

Continents like South America and Africa appear to fit together like puzzle pieces.

Fossil Evidence

Identical fossils found on widley separated continents.

Glacial evidence

Distribution of ancient glacial deposits suggest continents were once joined in a supercontinent,

Paleomagnetism

The study of changes in Earth’s magnetic field recorded in rocks.

Mantle Plumes

Columns of hot mantle rock that rise from deep within the Earth.

Hotspots

Stationary mantle plumes that can create volcanic activity on the overlying plate as it moves. This results in volcanic chains (e.g., Hawaiian Islands), with the age of volcanoes increasing in the direction of plate movement.

Divergent Boundaries

Plates move apart.

Oceanic-Oceanic Divergence

Forms mid-ocean ridges and seafloor spreading.

Continental-Continental Divergence

Forms rift valleys that can evolve into new ocean basins.

Convergent Boundaries

Plates move toward eachother.

Ocean-Ocean Convergence

Forms deep-ocean trenches and volcanic island arcs.

Ocean-Continent Convergence

Forms deep-ocean trenches, volcanic mountain ranges (volcanic arcs) on the continent.

Continent-Continent Convergence

Forms large mountain ranges (e.g., Himalayas).

Transform Boundaries

Plates slide horizontally past each other. Characterized by transform faults. Often connect segments of divergent or convergent boundaries.

The Wilson Cycle

A model describing the opening and closing of ocean basins over geological time, involving stages from continental rifting to collision and eventual supercontinent formation.