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Vulcanism
Vulcanism refers to all the movements of molten rock (magma) inside and outside the
earth’s crust. This includes:
A) Extrusive or volcanic activity – when magma reaches the surface through
some crack or vent. The molten rock is then known as lava.
B) Intrusive or plutonic activity – when the magma intrudes and cools in rock
layers under the earth’s surface.
Volcanic activity results in two types of features:
1. Lava Flows: these generally result when lava quietly flows from a crack or fissure in
the earth’s surface and spreads out in a flat sheet-like mass.
There are two types of lava flows:
 pahoehoe (rope-like or stringy appearance)
 aa (blocky or chunky).
2. Volcanic Cone: this is the most common landform created by volcanic activity.
There are three types of cones:
 Shield cones – generally flat dome-like broad cones formed mainly by gentle
lava flows
 Cinder cones – steep-sided cones composed of ash and cinders
 Composite cones – large, fairly steep cones made of alternating layers of
lava and ash/cinders. Many major mountain peaks are this type.
Volcanoes tend to be found in two major belts around the world:
1. The Circum-Pacific Belt (“Ring of Fire”) located around all of the margins of
the Pacific Ocean
2. The Mediterranean – Caribbean Belt
The process of lava extruding onto the earth’s crust is usually in one of the following
situations:
A) Fissure eruptions – occur when lava flows from cracks or fissures, and is the most
common source of volcanic materials (North Mountain basalt). The basaltic lava is
very fluid and thin, and has been known to spread up to 150 km from the original
fissure. The Columbia Plateau in the United States is an excellent example of this, as
well as areas on Iceland. The type of landform known as “flood basalt” produces
large plains of layered basalt by this process.
B) Sea-floor spreading – as the oceanic plates spread apart, they produce the greatest
volume of basalt. This process continues because the divergent plates allow lava to
flow out from the pressure of the crust. New “plate” material is then formed.
C) Subduction zones – as one plate is subducted and eventually melted, the magma
rises and can form into volcanoes. If this happens in the ocean, island arcs are
formed, such as the Aleutians, Japan islands etc.
D) “Hot Spots” – thought to be areas under the crust that are particularly hot, causing a
vent to form in the plate as it moves over the area. In oceanic plates, a series of
islands may form from the volcanic activity of the vent (ie: Hawaii). On the continental
plates, the lava may erupt from the vent as a fissure eruption, or flood basalt.
Sketch a diagram to illustrate each of the above four processes. Label clearly!
A)
B)
C)
D)
©TWebb