Scientific Theory of Plate Tectonics

If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle.

Alfred Wegener Plate tectonic theory had its beginnings in 1915 when Alfred Wegener proposed his theory of "continental drift.“ Wegener proposed that the continents plowed through crust of ocean basins, which would explain why the outlines of many coastlines (like South America and Africa) look like they fit together like a puzzle. Wegener was not the first to notice this puzzle-like fit of the continents (Magellan and other early explorers also noticed this on their maps), but he was one of the first to realize that the Earth's surface has changed through time, and that continents that are separated now may have been joined together at one point in the past.

Pangaea What is Pangaea? Pangaea was a super continent at one time. Scientists use the similarity of rock types and fossil types that date to the same age to support their theory that the continents were connected to form a super continent. The map below give just one example of areas on different continents that show the same fossils and rock types.

The break up of Pangea

Fossil Evidence Paleontologists found that there were fossils of similar species found on continents that are now separated by great geographic distance. Paleoclimate studies, which concerns examining the climate in Earth's past, revealed that glaciers covered large areas of the world which also are now separated by great geographic distances. These observations seemed to indicate that the Earth's lithosphere had been moving over geologic time.

Tectonic Plate Movement

Plate Tectonics The Earth’s crust is divided into 12 major plates which are moved in various directions. This plate motion causes them to collide, pull apart, or scrape against each other. Each type of interaction causes a characteristic set of Earth structures or “tectonic” features. The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.

World Plates

What are tectonic plates made of? Plates are made of rigid lithosphere. The lithosphere is made up of the crust and the upper part of the mantle.

What lies beneath the tectonic plates? Below the lithosphere (which makes up the tectonic plates) is the asthenosphere.

Plate Movement “Plates” of lithosphere are moved around by the underlying hot mantle convection cells

What happens at tectonic plate boundaries? Three types of plate boundary

Divergent Boundaries Spreading ridges As plates move apart new material is erupted to fill the gap

Features of Divergent Boundaries Mid-ocean ridges rift valleys fissure volcanoes

Iceland: An example of continental rifting

Convergent Boundaries There are three styles of convergent plate boundaries Continent-continent collision Continent-oceanic crust collision Ocean-ocean collision

Continent-Continent Collision Forms mountains, e.g. European Alps, Himalayas

Himalayas

Continent-Oceanic Crust Collision Ocean plate colliding with a less dense continental plate Subduction Zone: where the less dense plate slides under the more dense plate VOLCANOES occur at subduction zones

Subduction Oceanic lithosphere subducts underneath the continental lithosphere Oceanic lithosphere heats and dehydrates as it subsides The melt rises forming volcanism E.g. The Andes

Ocean-Ocean Plate Collision When two oceanic plates collide, The less dense plate slides under the more dense plate . This causes it to sink into the mantle forming a subduction zone. The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. The worlds deepest parts of the ocean are found along trenches. E.g. The Mariana Trench is 11 km deep!

Transform Boundaries Where plates slide past each other Above: View of the San Andreas transform fault

Why do the Plates Move? No single idea explains everything but we can identify several forces that contribute to the movement of the plates. Slab pull The sinking of the cooled dense oceanic plates pulls on the rest of the plate Ridge rises The material deposited on the top of the ridge slides downs from the rise pushing on the plate Convection Movement within the mantle could be part of the driving force behind the motion of the plates.