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Major forms of extrusive activity – types of volcanoes and Intrusions. Find out more
Landforms and Features of volcanic activity
There are many features of volcanic activity.  EXTRUSIVE features are those that extrude onto the surface and are hence surface landforms.  The major types are all volcanoes of various shapes and forms, but there are much smaller types too.  INTRUSIVE volcanic features are intruded into the lithosphere or rock, there they cool and solidify into rocks and are later exposed at the land surface as erosion and weathering DENUDE the land downwards. 

Shield Volcanoes- Mauna Loa

A very wide volcano, of 3 or 4 miles and heights of 1,500 to 2,000m.  Mauna Loa has heights of 28,000 feet. They are built up slowly by the accretion of thousands of flows of highly fluid basaltic (from basalt, a hard, dense dark volcanic rock) lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone.  The magma has very low gas content and is low in silica, allowing it to flow large distances.



Caldera - Yellowstone Volcano

A huge depression in the earth’s crust that is surrounded by a caldera rim, a marked ridge of higher relief.  It is cauldron shaped and can be many 10s of kms in diameter. Formed after massive super eruptions, where a huge magma chamber erupts upwards and the land above collapses downwards into the void left by the erupting magma.  If the magma is rich in silica (Andesitic or Rhyolitic), the caldera is often filled in with ignimbrite, tuff, rhyolite, and other igneous rocks. Silica-rich magma has a high viscosity, and therefore does not flow easily like basalt. As a result, gases tend to become trapped at high pressure within the magma. When the magma approaches the surface of the Earth, the rapid off-loading of overlying material causes the trapped gases to decompress rapidly, thus triggering explosive destruction of the magma and spreading volcanic ash over wide areas.



Acid or Dome Volcano -Puy de Domes, France

Acid or dome volcanoes tend to have craggy, steep-sloped sides covered with rock debris. These volcanoes are typically found near large composite volcanoes. They are made of layer upon layer of lava. They are formed by repeated violent eruptions and slow moving lava flows, which gives the layered structure. The magma is made of higher silica content and high gas pressure, making the magma slow moving and explosive


Composite or strato volcano - Pinatubo in the Philippines, Chaiten in Chile

A strato volcano is a tall, conical volcano built up by many layers (strata) of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, strato volcanoes are characterized by a steep profile and periodic, explosive eruptions and quiet eruptions. The lava that flows from strato volcanoes typically cools and hardens before spreading far due to high viscosity. Strato volcanoes are sometimes called "composite volcanoes" because of their composite layered structure built up from sequential outpourings of eruptive materials. Its eruptions are characterized by viscous lava explosions, which allow its lava to flow for great distances and to spill over and around its vent. The increase in temperature causes the dome to expand while its outer lava cools. This growth causes the newly hardened surface to splinter, causing loose debris to fall from its sides, the reason for its appearance. GCSE Pinatubo case study here!


Lava Plateaux - Columbia Plateau, western USA


A wide area of solidified lava that can be up to 1km thick covering large surface areas.  They are multi layered because of repeated lava flows and occur where lava (basaltic with low viscosity, lower silica content and low explosivity) pours out of long fissures rather than a central vent, covering much larger areas with thick layers of magma.

Intrusive Volcanic Features

INTRUSIVE volcanic features are intruded into the lithosphere or rock, there they cool and solidify into rocks and are later exposed at the land surface as erosion and weathering DENUDE the land downwards.  The majority of volcanic features are actually intrusive, most volcanic materials never actually make it to the ground surface in their molten state.  Batholiths are huge scale features where magma is intruded into the ground and cool very slowly, giving coarse grained rocks with large crystals.  The outer edge of the magma is in contact with existing rocks, and the heat and pressure changes the rock through metamorphosis creating a metamorphic aureole.  If magma is squeezed HORIZONTALLY along bedding planes through layers of existing rock a layer of rock can cool which is known as a Sill.  If magma is squeezed VERTICALLY upwards cutting across bedding planes of sedimentary rocks a Dyke is created.  The Isle or Arran in Western Scotland is essentially an exposed Batholith, which has dykes and sills, including the Drumadoon Sill.  Excellent notes on Arran can be found here.