Plate Tectonics Science I Amy Urling

Plate Tectonics Theory The Earth’s rigid lithosphere is broken into plates that move on the pliable asthenosphere. In 1912, Alfred Wegener, proposed this theory of continental drift (that the continents have moved over time). By the 1970s this theory was well-accepted and explains why volcanoes and earthquakes occur where they do.

How Do The Plates Move? Mantle convection: Magma which is hotter and less dense than its surroundings rises upward at mid-ocean ridges. The convection current then moves away from the ridge and draws the lithospheric plate with it. The cooler, denser rock then sinks down at a subduction boundary.

Types of Plate Boundaries Divergent Boundaries Convergent Boundaries Subduction Zones Collision Boundaries Transform Boundaries

Divergent Boundaries At these zones, two plates are moving away from each other. In the gap formed between the two plates, magma wells up from the mantle to form a chain of volcanoes creating long ridges. All of these zones are found under the ocean (except Iceland) and are called mid-ocean ridges.

Convergent Boundaries – Subduction Zones In subduction zones, heavier oceanic plates slide under lighter continental plates and melt into magma. Many volcanoes and earthquakes occur at these zones. An example is along the western coast of North America.

Convergent Boundaries – Collision Zones In collision zones, two continental plates collide and form folded mountain ranges. An example is where the Australian Plate collides with the Eurasian Plate to form the Himalayan Mountains. Mount Everest

Transform Boundaries These boundaries occur where two plates are sliding past each other. This forms transform faults such as the San Andreas Fault in CA where the Pacific Plate is moving past the North American Plate.

Summary of Plate Boundaries Boundary Type Surface Feature Earthquakes Divergent Mid Ocean Ridges Shallow depths only Convergent Subduction Collision Volcanoes/trenches High folded mountains Shallow to deep All depths Transform Earthquake faults Shallow to medium

Evidence for Plate Tectonics Shape of Continents The continents fit together like puzzle pieces. Around 400 million years ago, one super-continent Pangaea existed and slowly the continents have drifted apart. For example, the rock types, ages and fossil evidence on the west coast of Africa matches that on the east coast of South America.

Evidence for Plate Tectonics Earthquake and Volcano Patterns Earthquakes and volcanoes tend to occur along plate boundaries. For example, the “Ring of Fire” is a continuous line of volcanoes around the Pacific Ocean.

Hot Spot Volcanoes Some volcanoes are found in the middle of tectonic plates. These volcanoes form from mantle hot spots which melt the plate above. As the plate moves, chains of islands can form. The Hawaiian Islands have formed as the Pacific Plate moves across a hot spot.

Why is Plate Tectonics Important? Mineral Deposits: some minerals are found at certain plate boundaries. Ex: copper, lead and zinc are found in rocks at convergent plate boundaries. Oil: subduction zones are ideal for the formation of oil deposits. Natural Hazards: by understanding how volcanoes and earthquakes occur, many lives can be saved.