Tectonic processes and activity

Constructive/divergent plate margin

Process

1) Two continental/oceanic plates diverge from each other (have to be the same density, hence two of the same plates are needed) which is controlled by convection currents in the upper mantle. As they move, the plates are disturbed as it is not a smooth movement so minor earthquakes are caused.

2) A gap forms between the plates, so pressure is reduced from the mantle. Therefore, the mantle’s viscosity decreases as the atoms are more free to move and mantle melts as its viscosity decreases because it has, cooled down to form a runny liquid which can easily travel to the surface through the cracks in the crust as lava.

3) New ocean floors and shield volcanoes created. When the lava spews out from various gaps, it has a low viscosity, so it travels white far before it solidifies. This creates layers of a shield volcano, which has a wide base and short height because of the runny lava. This will continue to happen, which mean volcanic eruptions will be frequent, but gentle. When the volcano is tall enough, it can create islands above water, like Iceland, with the volcano as the central point. As these activities occur often at plate boundaries, it would be a dangerous place to live as there is a high chance of more tectonic activity (e.g. Iceland has 130 volcanoes).

4) One example of this is the Eurasian and North American plate which formed Iceland

Landforms created

Shield volcanoes – Forms when there is a gap between two crusts. Magma oozes out as lava and because of high temperatures, it gently solidifies (1500 degrees Celsius). Because of its low viscosity, it travels far and results in the gentle sloping sides of the volcano, its wide base and short height. It is formed by layers of basaltic lava with little ash. It frequently erupts, but often they are gentle and end up forming islands above water like Iceland and Hawaii.

Mid-oceanic ridges - These are formed when two oceanic plates diverge. They leave a gap, but no volcano, and the picture on the left is of a ridge in Iceland. It tends to be very deep, narrow and has jagged edges. One example is the mid-Atlantic ridge in the middle of the Atlantic Ocean, on the equator. It is part of the Andes mountain range, which is the longest range in the world.

Rift valleys – Rift valleys are like mid oceanic ridges, only that they form when two continental plates diverge. An example of this is the great rift valley in Kenya. Eventually, it will form a river, once it connects to a water source. For example, when the Great Rift valley opens up and floods, then it will separate Somalia from Kenya, and will create a new channel/bay of water.

Destructive plate margin

Process

1) An oceanic plate and a continental plate converge towards each other and collide.

2) Due to the oceanic plate being denser, it sub ducts under the continental plate and melts into the magma. This builds up friction, and when the pressure is overcome, the plates jolt past each other, sending huge shock waves through the rock which is an earthquake. If the earthquake is over the strength of 7.5 on the Richter scale underneath the oceanic plate, then a tsunami may occur. This is caused by the jerking of the two plates, which sends a huge wave of water above, destroying the nearest coast and killing many people and homes.

3) As the oceanic plate is melting in the magma, it increases the pressure in the upper mantle, and the excessive viscous magma results in violent volcanic eruptions on the continental plate through cracks, which build up and forms composite volcanoes. Fold mountains are also formed.

4) One example is the Nazca and South American Plate near South America.

Landforms created

Fold mountains – Fold mountains are formed during the subduction process of the oceanic crust. As the crust is not exactly straight, the continental crust scrapes the remaining sedimentary rock, which crumples the rock up, creating folds (that’s why they are called fold mountains – as seen in the picture on the left). Due to the heat and pressure, metamorphosis takes place and the rock becomes metamorphic rock. But as this is not a smooth process, earthquakes tend to occur during this formation. Most fold mountains continue to grow as the oceanic plate continues sub ducting. When they are folded upwards, they are called anticlines and when downwards, then synclines, and severely folded are called nappes (the picture on the left has nappes). These mountains are tall, steep, rugged and usually snow capped due to high altitudes, so there are often lakes which are fed by glaciers there. Examples of these are the Alps, Rockies, Andes and the Himalayas.

Composite volcano – Composite volcanoes are volcanoes that are formed with alternating layers of lava and ash (ash from the eruptions – composite materials contains more than one element). Initially, when the oceanic crust contributes to the magma underneath, it causes it to overflow, and pushes magma through cracks in the continental crust. After several eruptions, it eventually becomes a volcano. Its eruptions may be pyroclastic flow (mass containing very hot ash, fragments of lava and gases which flow at great speed and are over 400°C). Sometimes the magma cools in the vent, which causes lots of gas to build up. If it can’t escape, then it forms a secondary cone, and it explodes quite violently. Examples of these are the Mount St Helens, Mount Fuji, Mount Etna and Mount Vesuvius.

Ocean Trenches – Theses are formed again during the subduction process. The part between the oceanic crust and continental crust creates a steep, long cut, which is called a trench. There are rare creature living their, like the Dumbo octopus. We do not have clear pictures of the trench as they are quite deep and remote, so it makes it hard for us to reach.

The deepest trench in the world in the Marianas trench, located in the east of the Pacific Ocean. It’s the deepest part of the world, and even taller that mount Everest itself.

Conservative plate boundary

Process

1) Two tectonic plates move in different/same directions at different speeds which depends on the convections currents below in the upper mantle.

2) The two plates rub past each other, causing friction. Because of their jagged edges, they sometimes get stuck and pressure builds up. The line in the middle is called the fault line because it had a geological fault, in this case, the two tectonic plates get stuck.

3) Eventually the pressure is overcome and the plates suddenly break free and jerk past each other, which causes minor earthquake. The earthquake focus (where the energy is building up and suddenly releases) sends out multiple shock waves through the rock which is released on the crust.

4) One example of this is the Pacific plate and the North American plate (which is also known as San Andreas Fault).

NO landforms created, only tectonic activity

Collision plate boundary

Process

1) Sediments are deposited in a geosyncline (a place where sediment is deposited) by rivers, and gradually accumulate.

2) Because they are the same density, both continental plates on either side move towards each other because of the convection currents in the upper mantle and collide. The sediments in the middle are compressed and due to excessive heat and pressure, turns into sedimentary rock. The water in the middle evaporates due to heat and pressure. The fold mountains that are formed have a series of anticlines and synclines because of the compression.

3) As this is not a smooth process, earthquakes are caused. As compression occurs, the rock gets crumpled upwards and sends vibrations in the rocks because of the two plates, so it can lead to powerful earthquakes. There is no volcanic activity because there is no gap of the magma to rise, or no volcanoes being formed.

4) One example of this is the Indo-Australian plate and the Eurasian plate which collide to form the Himalayas.

Landforms created

See ‘Fold mountains’ above

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Include process, landforms, and tectonic activity at four tectonic plate boundaries