SEED Science

The Earth — A Living Planet
Plate Borders & Mountain-Building

Plates move in relation to each other in three different ways: moving apart (divergent), pushing together (convergent), and rubbing sideways (transform).

Divergent plate movement

Divergent Plate Boundary (above) — Plates moving apart, as at the mid-Atlantic ridge. Click to see animation of different types of plate boundaries.

Subduction plate movement

Convergent Plate Boundaries — Plates moving towards each other can create either mountains (above) or a subduction zone (below). Click to see animation of different types of plate boundaries

Convergent plate movement

Transform plate movement (below) — Plates moving sideways and rubbing against each other can cause major earthquakes. Click either image to see animation of different types of plate boundaries.

Transform plate movement

Divergent Plate Movement

When plates move apart from each other, or diverge, it causes a weakness in the crust. This makes it possible for magma to come to the surface from below.

Imagine very hot rock coming up from below and arriving at the surface on the ocean floor. As it cools down, this lava creates new crust. This is what is happening at the mid-oceanic ridges. These ridges are actually chains of volcanoes running along the ocean floor. Sometimes the tops of the volcanoes rise above the surface of the ocean water to form an island such as Iceland.

Ocean floor volcanoes produce rocks from the mantle. These rocks, called basalts, create new oceanic floor. This explains why the rocks at the bottom of the ocean are young compared to other rocks. The spreading rate of new ridges varies from place to place around the world. In some locations as much as 18 centimeters may be added per year. In other places there may be almost no growth at all.

The lavas produced under water at these ridges have a special name because of their shape. Because they are formed in water, they cool quickly while flowing down the side of the submarine volcano. They are called pillow lavas because the cooling basalt forms pillow-shaped lumps.

Although the spreading rate of the ridges is not very fast, it has been going on for millions of years. Imagine what would happen if no crust were destroyed — the Earth would grow bigger every day! Thankfully, there is a way to destroy crust.


Convergent Plate Movement:
Subduction Zones

When plates move towards each other, or converge, crust is destroyed as one plate rides over the top of the other. Oceanic plates are heavier and thinner, so they are generally forced underneath the lighter, thicker continental plates. The area where such an event takes place is called a subduction zone.

Volcanoes are also found near subduction zones. When the cold oceanic crust is forced down under the edge of a continental plate, it is heated by the underlying hot mantle. The heating is hastened by friction as the plates move against each other, causing earthquakes.

Over millions of years, thick layers of sediment have been laid on top of the old basaltic ocean floor. These layers include sand and mud from rivers and fragments of shells from sea creatures. As the oceanic plates go beneath the continental plates, the sediments are dragged down to the mantle where they are heated. They melt to create a new kind of magma that forms rocks called andesite, named after the Andes Mountains in South America — an example of a subduction zone. Andesitic magma often escapes through cracks to the surface, causing volcanic eruptions. The melting sediments contain a lot of cold seawater, so these eruptions can be very dangerous. They are often associated with the release of superheated steam and other gases that can cause violent explosions.

Convergent Plate Movement:

When plates move towards each other, rather than one sinking below the other, they may crumple into each other. The result is mountain-building. This is how the Himalayas were formed when the Indian subcontinent collided with Asia.

Transform Plate Movement

Transform plate movement takes place when two plates move sideways and rub against each other. The result is likely to be an earthquake. Transform plate movement is not associated with volcanoes or mountain-building. The San Andreas Fault in California, USA, is a site of this type of plate movement.