Destructive Plate Margins
Plates that move together form destructive plate margin. They are forced
together by the convection currents in the mantle. When the plates collide the
denser and heavier oceanic plate is forced below the lighter continental plate.
Here the Nazca plate which is made of oceanic crust is forced to sink below the
continental crust of the South American plate.
The oceanic crust is forced down below the continental plate, this creates a deep
ocean trench. The zone where the plates meet and are being forced downwards is
known as the subduction zone. At the subduction zone there is huge amounts of
friction and pressure– at certain times this may be realised causing an
earthquake.
The oceanic plate sinks into the mantle where it melts. This creates pressure in
the mantle and sometimes magma may rise upwards, causing violent volcanic
eruptions and leading to the creation of composite volcanoes. E.g Nervado Del
Ruiz, Columbia.
The continental crust can not be forced into the mantle. Instead when it collides
with the oceanic plate it starts to crumple and create fold mountains. A good
example of fold mountains is the Andes.
Oceanic trench
Subduction Zone
Task
 Using a diagram to help you, explain in detail why earthquakes and volcanoes
happen at a Destructive Plate Margins.
 What is an oceanic trench?
1
Constructive Plate Margin
Some plates like the North America and Eurasian plates are moving in opposite
directions, away from each other. This is called constructive plate margin. This
happens because the conventional currents in the mantle pull the plates apart.
This type of movement mostly happens under the oceans. The best example is
the Mid-Atlantic Ridge, which is widening at a rate of 3cm a year. As the plate
moves apart the gap is filled by the magma rising up from the mantle below. The
rising magma creates less violent shield volcanoes. If these become higher
enough, they form volcanic islands, a good example is Iceland. Small earthquakes
may also be felt along this plate boundary as the plates move apart in a jolting
fashion.
Oceanic Ridge
Task
 Using a diagram to help you, explain how the mid Atlantic ridge forms
 Are volcanic eruptions on Iceland violent? If not why?
2
Conservative (Slipping) Plate Margin
At other plate margins such as the San Andreas fault in California, the North
American Plate and the Pacific Plate are sliding past each other. Here the crust is
neither being destroyed nor created. At this plate boundary, the plates are
moving in the same direction but the North American plate is moving slightly
faster. The plates do not pass by each other smoothly, but create vast amounts
of friction. Frequently the plates become stuck and pressure builds up along the
fault until one plate suddenly jerks past the other, causing a potential violent
earthquake. The movement has also caused the land along the plate boundary to
become ridged and crumpled. Over 40,000 earth tremors are recorded in
California every year.
Task
Using a diagram to help you, explain in detail why California experiences so many
earthquakes.
3
Collision Plate Margin
Collision plate boundaries are where two plates that are both made of continental
crust crash into each other. Because continental crust cannot be pushed down
into the mantle it is forced upwards to form fold mountains. This creates some of
the highest mountain ranges in the world. A good example is where the IndoAustralian plate collides with the Eurasian plate forming the Himalayas. (The
mountain range with Mount Everest). Violent earthquakes can occur along this
plate margin, unlike the other margins, earthquakes can occur up to 1000km
from where the two plates meet, this makes it difficult to predict the location of
an earthquake.
Fold Mountains, like the Himalayas, are made from sedimentary rock. To create
them, you first of all need a large area of low land that is called a called a
geosyncline. Rivers flow into this low-lying land and deposit material onto the
seabed. Further layers are deposited and when they are
Syncline
compressed they form sedimentary rock. The plates then
move together and the sedimentary rock between the
plates is crumpled forming Fold Mountains. Where the
sedimentary rock is crumpled upwards it is called an
anticline and where is is crumpled downwards it is known
as a syncline.
Young Sedimentary Fold Mountains
Anticline
Task
 Why there are no volcanoes in the Himalayas but there is the risk of
earthquakes?
 How do young fold mountains like the Himalayas form?
4