Types of Tectonic Plate
The Earth’s lithosphere, which includes the crust and upper mantle, is made up of a series of
pieces, or tectonic plates, that move slowly over time.
A divergent boundary occurs when two tectonic plates move away from each other. Along
these boundaries, earthquakes are common and magma (molten rock) rises from the Earth’s
mantle to the surface, solidifying to create new oceanic crust.
When two plates come together, it is known as a convergent boundary. The impact of the
colliding plates can cause the edges of one or both plates to buckle up into a mountain ranges
or one of the plates may bend down into a deep seafloor trench. A chain of volcanoes often
forms parallel to convergent plate boundaries and powerful earthquakes are common along
these boundaries.
At convergent plate boundaries, oceanic crust is often forced down into the mantle where it
begins to melt. Magma rises into and through the other plate, solidifying into granite, the rock
that makes up the continents. Thus, at convergent boundaries, continental crust is created and
oceanic crust is destroyed.
Two plates sliding past each other forms a transform plate boundary. Natural or human-made
structures that cross a transform boundary are offset—split into pieces and carried in opposite
directions. Rocks that line the boundary are pulverized as the plates grind along, creating a
linear fault valley or undersea canyon. Earthquakes are common along these faults. In contrast
to convergent and divergent boundaries, crust is cracked and broken at transform margins, but
is not created or destroyed.
There are three main types of plate boundaries:
1. Convergent boundaries: where two plates are colliding.
Subduction zones occur when one or both of the tectonic plates are composed of oceanic crust.
The denser plate is subducted underneath the less dense plate. The plate being forced under is
eventually melted and destroyed.
i. Where oceanic crust meets ocean crust
Island arcs and oceanic trenches occur when both of the plates are made of oceanic crust.
Zones of active seafloor spreading can also occur behind the island arc, known as back-arc
basins. These are often associated with submarine volcanoes.
ii. Where oceanic crust meets continental crust
The denser oceanic plate is subducted, often forming a mountain range on the continent. The
Andes is an example of this type of collision.
iii. Where continental crust meets continental crust
Both continental crusts are too light to subduct so a continent-continent collision occurs,
creating especially large mountain ranges. The most spectacular example of this is the
Himalayas.
2. Divergent boundaries – where two plates are moving apart.
The space created can also fill with new crustal material sourced from molten magma that forms
below. Divergent boundaries can form within continents but will eventually open up and become
ocean basins.
i. On land
Divergent boundaries within continents initially produce rifts, which produce rift valleys.
ii. Under the sea
The most active divergent plate boundaries are between oceanic plates and are often called
mid-oceanic ridges.
3. Transform boundaries – where plates slide passed each other.
The relative motion of the plates is horizontal. They can occur underwater or on land, and crust
is neither destroyed nor created.
Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in
both plates and when it exceeds the threshold of the rocks, the energy is released – causing
earthquakes.
