Download Chapter 18

Volcanic Activity chapter 18
18.1
Magma: mixture of molten rock, suspended mineral grains
and dissolved gases beneath the Earth’s surface
3 factors that control the formation of magma:
1) Temperature: Depending on composition, rocks melt
around 800-1200 degrees C.
2) Pressure: As pressure increases, so does the melting
point.
3) Water: the more water a rock has in it the lower the
Viscosity: A fluids resistance to flow. High viscosity =
Section
19.1
page
497
thick (traps gas and therefore is explosive)
Three types of magma
1) Rhyolitic: forms when molten material rises and mixes
with overlying silica and water rich continental crust and
has a high viscosity and is explosive
2) Andesitic: Found at ocean crust subduction zones with
an intermediate viscosity
3) Basaltic: Forms when rocks in the upper mantle melt,
has a low viscosity and erupts quietly
They are named after the igneous rocks which can be found
in the same row in your ESRT’s.
Factors that effect viscosity
1. Composition: The more silica, the more viscosity
2. Temperature: The hotter it is the lower the viscosity
18.2 Intrusive activity
Plutons: large intrusive igneous rock
bodies, most commonly made of granite.
Batholiths: The largest of plutons, can
cover at least 100 square Km
Stocks: Irregularly shaped pluton that is similar yet
smaller than a batholith
Laccoliths: A mushroom shaped pluton with a round top
and flat bottom
Sill: A pluton that forms when magma intrudes into
parallel layers of rock
Dike: A pluton that cuts across preexisting rocks and is
Section
19.1
page
497
perpendicular to the rock layers.
Plutons and
Tectonics
Most Plutons are formed
during a mountain
building process such as
the
continental/continental
convergent boundaries.
They can also form as a
result of ocean to ocean
convergence zones.
18.3 Volcanoes
Section 19.1
Anatomy of a volcano.
Vent: The opening
through which the lava
erupts.
Crater: The bowl shaped
depression around the
vent
page 498
Calderas: A
larger depression
than a 498
crater that can
Section
19.1 page
form as a result of the summit or side of a volcano
collapsing into the magma chamber.
This is a picture of a calderas called Crater Lake
Volcanoes are classified by two
Section
19.1
page
factors.
1. The type of material that forms
the volcano.
2. The type of eruption that takes
place.
There are three types of volcanoes.
1. Shield volcano: Is a mountain
with broad, gently sloping sides and
a nearly circular base. These form
when basaltic lava flows during non
explosive eruptions. These are the
largest of the volcanoes.
498
2. Cinder-Cone Volcanoes
Section 19.1 page
Forms when material ejected high
into the air falls back to Earth and
piles around a vent. These are the
smallest of the volcanoes often no
higher than 500 m.
498
3. Composite Volcanoes
Section 19.1 page
Form when layers of volcanic
fragments alternate with lava. As
with cinder cone volcanoes, there is
also a lot of water, silica and gases.
These are much larger than cinder
cone volcanoes and are a threat to
people and the environment.
498
Volcanic material
Section 19.1 page 498
Tephra: Rock fragments thrown into the air during a volcanic
eruption
Volcanic material
Section 19.1
Tephra is classified by size.
Dust < 0.25 mm
Ash < 2 mm
Lapilli < 64 mm
Blocks any thing larger than 64
mm and can be as large as
houses and can shoot 10 km into
the air.
page 498
Pyroclastic Flow: The clouds of ash, gas, and tephra
that can travel down slope at speeds at nearly 200
km/h. The temperature can exceed 700 degrees c and
kill on contact.
Volcanoes occur in one of three places.
1. Convergent
boundaries
2. Divergent
boundaries
3. Hot spots hot regions of
the Earth’s
mantle where
mantle material
reaches the
Earth’s surface.
Chains of volcanoes form from hot
spots as the plates continue to move.