Chapter 4 – 5 The Theory of Plate Tectonics

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The Earth’s lithosphere is broken into separate sections called plates

These plates move around on top of the asthenosphere like rafts

The Earth is generally considered to have 15 major tectonic plates, seven or eight of which are primary plates, and the others are smaller, secondary plates. The primary plates are the African Plate, the Antarctic Plate, the Eurasian Plate, the Indo-Australian Plate, the North American Plate, the Pacific Plate and the South American Plate, with the Nazca Plate sometimes considered a primary plate instead of a secondary plate. The secondary plates are the Arabian Plate, the Caribbean Plate, the Cocos Plate, the Indian Plate, the Juan de Fuca Plate, the Philippine Sea Plate, the Scotia Plate and sometimes the Nazca Plate. In addition, there are dozens of minor, tertiary plates.

Remember, the lithosphere is less dense than the material below it so it “floats” on the

asthenosphere.

1965, Canadian scientist, J.T. Wilson proposes the scientific theory of Plate Tectonics by which pieces of

the lithosphere are in constant slow motion driven by convection currents within the asthenosphere

The theory of Plate Tectonics explains the formation, movement and subduction of Earth’s plates.

Plate Boundaries = Faults

Transform Boundary is a place where two plates slip past each other. Earthquakes often occur at these

boundaries.

Divergent Boundary is a place where two plates move apart. Mid-Ocean Ridge, (sea-floor

spreading), and Great Rift Valley.

Convergent Boundaries occur where two plates collide or “converge.” Collisions can involve oceanic to oceanic crusts,

oceanic to continental crusts or continental to continental crusts.

When two plates collide, the density of the plates determines which one comes out on top.

When two oceanic plates collide, subduction occurs and the more dense plate slides underthe other plate, returning to the mantle.

When an oceanic plate collides with a continental plate, the oceanic plate sinks below the less dense

continental plate.

When two continental plates collide, MIGHTY MOUNTAIN RANGES are formed!

Plates move slowly! Pangaea broke apart about 225,000,000 years ago!