Plate Tectonics

The Earth is a restless planet The Earth is a restless planet. The ground beneath your feet is constantly moving. Great slabs of rock called plates make up the Earth’s crust.

Early 20th century geologist Alfred Wegener realized that the puzzle-like fit of the continents were more than a coincidence, but he couldn't correctly explain what powered their movement. Geologists now know that the Earth's outermost layer, the lithosphere, is divided into independently moving plates into which the continents are embedded. The plates "float" on a layer called the asthenosphere.

Plate tectonics is an important theory that explains the movement of the Earth’s lithosphere (surface).

Plate tectonic theory explains how the lighter, solid plates that form the Earth’s crust (surface) float on denser material just below the crust. Types of plate movement we will be looking at in further detail: Divergent Convergent Transform

Looking at the Earth’s layers will help us understand how the Earth’s plates move.

The Lithosphere can be divided up into layers

The lithosphere is solid rock. Beneath it is a softer, hotter layer of solid rock called the asthenosphere that extends down to around 220 kilometers depth. Temperatures of the asthenosphere are extremely hot, and as a result, the rock is weak ("astheno-" means weak in Greek). It will be effected by stress and bends in a plastic way.

Tectonic plates are able to move because the Earth's lithosphere has a higher strength and lower density than the underlying asthenosphere. The lateral relative movement of the plates typically varies from zero to 100 mm annually.  

Where plates meet, their relative motion determines the type of boundary: convergent Divergent transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries.

Convergent Boundaries   2 plates are running into each other Where plates serving landmasses collide, the crust crumples and buckles into mountain ranges. India and Asia crashed about 55 million years ago, slowly giving rise to the Himalaya, the highest mountain system on Earth. As the mash-up continues, the mountains get higher.

Convergent boundaries also occur where an ocean plate and continental plate meet. This process is called subduction. Ocean plate density > Continental plate. Thus dives beneath the continental plate, creating an ocean trench. As the overlying plate lifts up, it also forms mountain ranges. In addition, the diving plate melts and is often spewed out in volcanic eruptions such as those that formed some of the mountains in the Andes of South America.

Two ocean plates can collide as well Two ocean plates can collide as well. One plate will have a slight density difference and subduct . This creates an ocean trench and can create chains of composite volcanoes. Example: Philippines.

Divergent Boundaries Two plates are moving away from each other. The process renews the ocean floor and widens the giant basins. In the oceans, magma from deep in the Earth's mantle rises toward the surface and pushes apart two or more plates. Mountains and volcanoes rise along the seam. On land, giant troughs such as the Great Rift Valley in Africa form where plates are tugged apart. If the plates there continue to diverge, millions of years from now eastern Africa will split from the continent to form a new landmass. A mid-ocean ridge would then mark the boundary between the plates.

Transform Boundaries Two plates grind past each other along what are called strike-slip faults. Example: San Andreas Fault in California These boundaries don't produce spectacular features like mountains or oceans, but the halting motion often triggers large earthquakes, such as the 1906 one that devastated San Francisco.