Volcanic Activity

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Intrusive Volcanic Activity

The high pressure inside the Earth keeps rocks in parts of the mantle semi-molten.

When pressure is released e.g. At constructive plate boundaries, the rocks become molten and is called magma.

Magma is less dense than the rocks around it so it rises up towards the Earth's surface.

When magma rises but does not reach the surface then it is called intrusive volcanic activity.

This takes place beneath the Earth's surface and includes the formation of large magma chambers and magma being forced into the crust (through cracks in the rock).

Intrusive volcanic activity forms dykes, sills and batholiths.

Batholiths - when large chambers of magma cool underground they form domes of igneous rock called batholiths e.g. Sierra Nevada, USA
Sills - when magma has flowed into gaps in the surrounding rock and cooled it forms horizontal sills between the layers of rock with vertical cooling cracks.
Dykes - when magma has flowed into gaps in the surrounding rock and cooled it forms vertical dykes across the layers of rock with horizontal cooling cracks.

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Different types of lava

Basaltic lava - made at CONSTRUCTIVE plate margins
Andesitic lava - made at DESTRUCTIVE plate margins
Rhylotic lava is made at DESTRUCTIVE plate margins

Basaltic lava has low silica content and low viscosity with a very high temperature (950☀+)
Basaltic eruptions aren't violent and erupt frequently for long periods of time.

Andesitic lava has medium silica content and medium viscosity.

Rhylotic lava has high silica conteng, high viscosity and low temperature (750☀-)
Rhylotic eruptions cause blockages in the volcanoes vents and pressure builds up, until the pressure is relieved by a violent eruption.

Andesetic and rhylotic lavas usually erupt intermittently and eruptions are short-lived.

Pyroclastic flow - material comprised of volcanic fragments of ash, rock and gas (known as tephra). Characteristic of explosive gaseous eruptions they are fast moving (450mph) and incredibly hot (1000ᵒC)

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Types of Volcano

Fissure - Elongated cracks in the crust that allows lava to spill out over a large area; found at spreading ridges where tension pulls the crust apart. Basaltic lava can flow a considerable distance over the gentle slopes
Rock: Basaltic
Location: rift/early constructive boundaries
Eruption: gentle and persistent

Shield -Gently sloping cones of less viscous lava, the gently sloping sides are made up from numerous basaltic lava flows
Rock: Basaltic
Location: hot spots and oceanic/oceanic destructive
Eruption: gentle and predictable

Composite - Found on land with slightly steeper sides often featuring parasitic cones, created by layers of ash formed from initial explosions and subsequent flows of lava
Rock: Andesitic
Location: destructive
Eruption: explosive and unpredictable

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Different types of volcano

Acid or Dome - Steep sided volcano formed from very viscous lava, as the lava does not travel far it forms convex cone shaped volcanoes
Rock: Rhyolitic
Location: continental
Eruption: explosive and unpredictable

Caldera - Formed when gases have built up beneath a blocked vent resulting in catastrophic eruption that destroys the summit, leaving a large crater that may have smaller cones within it from later eruptions
Rock: Andesitic
Location: destructive
Eruption: unpredictable

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Minor extrusive volcanic activity

Hot springs -
Springs are places where groundwater emerges at the surface.
If the groundwater source of a spring flows close to an area of recent intrusive volcanic activity, the water is heated and becomes a hot spring. The temperature varies from 20☀C to 90☀C.
Hot spring water has high mineral content because hot water can hold a lot of dissolved solids.
Hot springs are found all over the world and popular with tourists (North Island, New Zealand).

Geysers -
Geysers are a type of hot spring where hot water and steam are ejected to the surface in a fountain. They form in areas of intense volcanic activity.
Groundwater is heated above boiling point by magma deep in the crust. The hot water becomes pressurised and forces its way to the surface along cracks in the rocks. Finally, the hot water and steam spray out from a vent.
Geysers erupt periodically. This is because they only erupt when the pressure has built up enough to force the water out of the ground. Example - Old Faithful, Yellowstone National Park

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Minor extrusive volcanic activity

Boiling mud pools -
A boiling mud pool is a type of hot spring, formed in areas with very fine-grained soil (soil rich in in clay or volcanic ash) - the hot spring water mixes with the soil to create a hot muddy pool.
The pools can be brightly coloured because of the minerals deposited by the hot water. Sulfur can create yellow colours.
Boiling mud pools can be found all over the world e.g. Iceland and Yellowstone, Wyoming.

Volcanoes occur away from plate margins:
A hot spot is caused by a magma plume - a vertical column of magma that rises up from the mantle.
Volcanoes form above magma plumes.
The magma plume remains stationary but the crust moves above it over time.
Volcanic activity in the part of the crust that was above the hot spot decreases as it moves away.
New volcanoes form in the part of the crust that is now above the hot spot.
As the crust continues to move, a chain of volcanoes are formed.
There's a hot spot in Hawaii.

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