Describe the three condition under
which magma can form.
 Explain what volcanism is.
 Identify three tectonic settings where
volcanoes form.
 Describe how magma can form plutons.

 The
cause of many volcanic eruptions
is the movement of tectonic plates
› The movement of these plates is driven
by Earth’s internal heat
 Mantle
is mostly solid due to high pressure,
even though high temps.
› Solid mantle that melts and becomes liquid
rock is known as magma.
 Can
form under 3 conditions
 Temp of rock rises above melting pt.
 Pressure is reduce and melting pt. lowers
 Addition of fluids may decrease melting pt.
of some minerals in the rock, causing rock to
melt
 Fig 1, p. 319
 Any
activity that includes the movement of
magma toward or onto Earth’s surface
 Bodies of magma rise to the surface in 2
ways
› Magma is hot enough to melts surrounding
rock
› Magma rises, is forced into cracks, and causes
large blocks to break off and melt. This adds
to the magma body
 Lava
– magma that flows onto Earth’s
surface; the rock that forms when lava
cools and solidifies
› May flow out of a vent, cool, and build up a
cone of material that may become a
mountain.
 Volcano
– a vent or fissure in Earth’s
surface through which magma and gases
are expelled
 Fig
2 – map of active volcano locations
 Most occur at or near both convergent
and divergent plate boundaries
 Pacific Ocean – Ring of Fire
› Also a major earthquake zone
 Area
where one tectonic plate moves
under another
› Oceanic plates are more dense than
continental plates are usually subducted
› A deep trench forms on ocean floor along
continent boundary
› Continental plate forms Mtns as it is folded and
pushed back
› Subducted plate is melted to form magma
› May form volcano chain
› 2 oceanic plates – island arc may form
 Divergent
plate boundaries – plates
are pulling away
› area where largest amount of magma
rises to surface
› Pillow lava
› Fissures – cracks through which lava flows
to Earth’s surface
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A
volcanically active area of Earth’s
surface, commonly far from a tectonic
plate boundary

Mantle plumes – columns of hot, solid material
rise and reach the lithospere
› Appear to remain stationary, even though
lithospheric plates drifs above the plumes.
 Theory
that states hot spots result from
cracks in Earth’s crust.
 Magma
is not dense, therefore it rises
through or intrudes the overlaying layer of
rock.
› Magma may fracture or melt the surrounding
rock.
› Rock that is melted may cool again into
igneous rock.
 Magma
may cool and solidify within the
crust, resulting in many large plutons
› Dikes – small plutons that are tabular in shape
› Batholiths – large plutons that cover a lot of
area