Extrusive Volcanic Features
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.
Extrusive Volcanic Features
When lava cools and solidifies on the earth’s surface it forms extrusive volcanic features.
Examples include, composite volcanoes, shield volcanoes , caldera and lava plateau. These are
all landforms are features above the earth’s surface.
Shield Volcanoes- Mauna Loa
Shield volcanoes are also known as Basic Lava Cones. Some volcanoes erupt a type of lava
which flows very easily and tends to flow quite some distance before solidifying. This is known
as basic lava. This type of lava tends to form broad volcanoes with very gentle slopes. They are
built up slowly by the accretion (deposits) 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. Lava also commonly erupt from vents along fractures (rift
zones) that develop on the flanks of the cone
Shield volcanoes are usually 3 or 4 miles wide. Mauna Loa has heights of 28,000 feet. The
magma has very low gas content and is low in silica, allowing it to flow large distances.
Composite or Strato volcano
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.
Composite cones are found near convergent plate boundaries. The Soufriere Hills volcano in
Montserrat is an example of a composite cone.
Caldera
This 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 (the whole top of the volcano
sinks into the magma below). 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.
After the super explosion, a huge crater is left which may by many kilometres in diameter. Later
eruptions may form new cones inside a caldera. Lakes may form in the lowest parts of a caldera.
An example is Krakatoa in Indonesia. The Qualibou caldera in St. Lucia is 3.5 km x 5 km in size
and was formed more than 30,000 years ago. The town of Soufriere is located inside of it.
Lava Plateau - Columbia Plateau, western USA
This speaks to a wide area of solidified lava that can be up to 1km thick covering large surface
areas. Lava does not always reach the surface in a spectacular way. Sometimes large quantities
of lava ooze out slowly onto the surface from fissures (surface cracks). These fissure eruptions
can cause lava to spread out over a very wide area. 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.
Over time, a number of fissure eruptions in the same area can build up a high plateau. Basalt
plateaux are very large features, covering hundreds of thousands of square kilometers and they
usually “drown” the pre-existing landscape. In India the Deccan is a lava plateau which covers
almost 650 000 km2