Plate Tectonics: Continental Drift, Plate Boundaries and Mountain Formation

1: Plate tectonics explains the movement of the continents and the formation of seas, mountain ranges, islands, volcanoes, earthquakes, and the formation of the oceanic crust and the rock cycle.

It affirms that the lithosphere is not a continuous rock mass but is divided into large blocks called plates. The plates move and displace the continents. At their borders, plates interact by separating, colliding, or rubbing against each other, and this is what causes tectonic activity.

At divergent boundaries, the lithosphere is created as magma rises and solidifies at the center of the spreading ridge. To compensate, an equal amount of oceanic lithosphere is destroyed in subduction zones, so the surface of the Earth remains constant.

2: Types of plates:

• Oceanic (the lithosphere is only oceanic)
• Continental (the lithosphere is only continental)
• Mixed (contains both continental and oceanic lithosphere)

3: The boundaries of the plates:

As the plates move, the boundaries are where the major tectonic activity of the planet is concentrated. There are three types of contact (or plate boundaries):

Constructive boundaries (divergent)

Two plates separate and a new crust is created. Magma rises through the rift, solidifies, and forms new oceanic crust. They are also called divergent boundaries, where the plates move in opposite directions. An example is the Mid-Atlantic Ridge, which increases the size of the Atlantic Ocean by about 3 cm per year.

Destructive boundaries (convergent)

Lithosphere is destroyed where two plates collide. These are also called convergent boundaries. Subduction can occur in three different situations depending on the type of lithosphere:

1. Collision between oceanic and continental lithosphere: The denser oceanic plate sinks beneath the continental plate (which is thicker, lighter, and less dense). This process is called subduction. The oceanic plate descends into the mantle at about 45° (Benioff zone) to a depth of roughly 100 km. The plate begins to melt due to high temperatures; the molten rock rises and crosses the lithosphere to the surface, forming a mountain range on the continent with associated volcanic activity. An example is the collision of the Nazca Plate with the South American Plate.
2. Collision between two oceanic plates: Subduction also occurs. The denser and older oceanic plate sinks beneath the other. Volcanic island arcs form on the surface, arranged in an arc parallel to the trench. Collision occurs in areas such as the western Pacific Ocean.
3. Collision between two continental plates: No subduction occurs, because the continental lithosphere is too light and thick to sink into the mantle. Sediments accumulate at the margins, deform, and are uplifted, forming high mountain ranges where the two continents meet. An example is the Himalayan range, formed when India collided with Asia.

Passive boundaries

Two plates slide past each other horizontally without creating or destroying lithosphere. Friction between the plates causes high seismicity. Examples include the western coast of North America and the San Andreas Fault.