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Volcanoes & Volcanic
Mini Quiz To Begin Volcanoes
1. What is the difference between magma
and lava ?
2. What kind (type) of rocks are formed
from the cooling of this magma or lava?
3. Why does magma force its way up to
the surface of the earth ? (hint: what
does it want to do?)
Volcano Background
Volcano… a term borrowed from the
Romans… Vulcan was the Roman God
of fire that forged metal… the vent (or
chimney) of his furnace was an island
named Volcano… this was a simple
attempt by people to try to understand
the complex processes that cause
volcanic activity
Volcano: A Definition
Volcano is the modern term used to
describe vents in the earth’s surface
through which molten rock, heated rock
debris, gases and water vapor are
expelled from beneath the earth’s
surface in igneous processes
Volcanic Materials
The stuff that results from volcanic
activity can be in the form of solids,
liquids and gases… some of these
materials include
 water vapor (the major gas expelled)
 rock (lava or magma)
 pyroclastic materials
Pyroclastic Material
Anything that is ejected from a volcano
is termed Pyroclastic...
Pyro = fire & Clastic = particles
 Pyroclastic materials that are ejected
and go airborne are termed Tephra
Classified according to size:
– ASH : less than two millimeters in size
– LAPILLI : 2 mm to 64 mm in size
– BLOCKS : >64mm ejected as solid
– BOMBS : >64mm ejected as liquid
Although pyroclastic materials are born
of fire (igneous), as they fall to the
ground and settle they become
Igneous Rocks
Igneous rocks fall into two categories:
Intrusive (plutonic) that cool slowly
Extrusive (volcanic) that cool quicker
Magma Compositions
There are three general types of
volcanic magma compositions :
– Basaltic
– Andesitic
– Rhyolitic
Magma Compositions
These magmatic compositions are
classified according to the amount of
silica present…
the less silica present, the more fluid
the magma/lava will be
 the more silica present, the more
viscous the magma/lava will be
Magma Compositions
Magma compositions classified by the
percent silica present turn out like this:
– basaltic : 50 % silica
– andesitic : 60 % silica
– rhyolitic : 70 % silica
Explositivity of Magmas
The more silica that is present, the
more viscous (stickier) that magma is…
Basaltic : 50% Si, fluid, not explosive
 Andesitic : 60% Si, “regular” explosions
 Rhyolitic : 70% Si, really big bangs !
Land Shapes via Magma Type
The magmatic composition not only
controls the color and explositivity, but
it also controls the external volcanic
land features… those magmas that are
fluid when they erupt tend to run like
molasses while the really sticky
magmas don’t really erupt at all… they
just blow up.
Land Shapes via Magma Type
Basaltic : 50% Si , fluid.
 Basaltic eruptions tend to flow out on
the earth’s surface like molasses…
initially it will mound up but will settle
out under the influence of gravity
forming a broad, low shield-like volcano
 You guessed it - a shield volcano
Land Shapes via Magma Type
Andesitic : 60% Si, kind of viscous
 Andesitic eruptions are what most
people typically associate with volcanic
eruptions… there are lava flows, ash
clouds, and nuee ardentes. Andesitic
eruptions can form two volcanic
 composite cones and cinder cones
Land Shapes via Magma Type
Composite volcanoes are volcanoes
built of alternating flows of lava and
pyroclastic material… composite cones
are larger than cinder cones but aren’t
as steep… the composition of these
volcanoes is mainly andesitic… most
large active volcanoes around the world
are composite volcanoes…
Land Shapes via Magma Type
Cinder cones are volcanoes that have
steep slopes (~30 degrees) and a large
crater at the top… these volcanoes are
composed entirely of fragments of
volcanic rock and material… these
fragments are generally basaltic or
andesitic in composition…
Land Shapes via Magma Type
Rhyolitic : 70% Si, very viscous!
 Rhyolitic eruptions generally don’t
produce any distinctive landform at all…
the stuff is so viscous (sticky) that it
pressurizes until it just blows up
creating a big hole in the ground… all
that remains are usually basins that are
hotbeds of volcanic activity
So, where are these
The low-slung broad landforms that
characterize basaltic shield volcanoes
are nicely demonstrated by some wellknown island chains:
 Hawaiian Islands
 Galapagos Islands
So, where are these
The steep sloped sides of the cinder
cone volcanoes can be found
throughout Central and South America
and many other places world-wide…
just take, for example, Paracutin in
So, where are these
The classic volcano shape that we all
think of is demonstrated well by Mt
Ranier in Washington State, or the most
photographed volcano in the world… Mt
Fuji in Japan
So, where are these
Rhyolitic landforms are noted by their
absence… there is usually nothing left
after a rhyolitic eruption but a hole in
the ground that is active with
geothermal energy…just like Iceland or
Yellowstone National Park...