Download Guided Notes on Volcanoes

Guided Notes on
Volcanoes
Section 18.1
Magma
1. In the last 10,000 years, more
than 1500 different volcanoes
have erupted, providing
evidence that the Earth is
indeed geologically active.
2. All volcanoes are fueled by
magma deep beneath the
Earth’s surface.
3. What is the
asthenosphere?
• The asthenosphere is the
plasticlike portion of the mantle
directly beneath the
lithosphere. It is hot enough to
contain molten magma.
Basaltic Magma
•
•
•
•
•
50% silica
Low viscosity
Least violent eruptions
Low amounts of trapped gases
Occur in Hawaii, Iceland, and
areas with oceanic crust
Andesitic Magma
• 60% silica
• Intermediate viscosity
• Some violent eruptions, some
quiet ones
• Are found along subduction
zones
• An example is Mt. St. Helens, in
Washington
Rhyolitic Magma
•
•
•
•
70% silica
High viscosity
Usually have violent eruptions
Occur in areas with continental
crust
• An example is Yellowstone Park
5. The viscosity of magma
depends upon its temperature
and composition. The hotter
the magma, the lower the
viscosity. Magmas high in silica
have higher viscosities than
magmas low in silica.
Section 18.2
Intrusive Activity
1. Why does magma come
into contact with overlying
rock?
• Magma is less dense, so it
moves upward where it
eventually comes in contact
with overlying rock.
3 Ways That Intruding
Magma Can Affect the
Crust
• It can force the rock apart and
enter the newly formed fissures
• It can cause blocks of rock to
break off and sink into the
magma, where they melt
• It can melt the rock into which
it intrudes
Define Pluton
• A pluton is an intrusive igneous
rock body formed through
mountain-building processes
and oceanic-oceanic collisions.
It can be exposed at the Earth’s
surface due to uplift and
erosion.
Types of Plutons
Sills
• When magma intrudes into
parallel rock layers
• Between a few centimeters and
hundreds of meters thick
• An example is the Palisades Sill
in New York
Types of Plutons
Dikes
• When magma cuts across preexisting rock layers, magma
invades the cracks in rock
• Usually 10 km. long and a few
meters wide
• An example is the Great Dike in
Zimbabwe
Types of Plutons
Laccoliths
• Mushroom-shaped plutons with
rounded tops and flat bottoms
• Less than 16 km. wide
• An example is the Black Hills of
South Dakota
Types of Plutons
Batholiths
• Large plutons that are at least
100 km2
• Irregularly shaped
• Coarse-grained
• An example is the Coast Range
Batholith in British Columbia
Types of Plutons
Stocks
• Similar to batholiths, but
smaller than 100 km2
5. What texture do most
plutons have, and why?
• Most plutons are coarse-grained
because they have cooled
slowly under the earth’s
surface.
6. How are plutons
usually formed?
• Plutons are usually formed as a
result of mountain-building
processes.
2 Types of Convergence
that Create Batholiths
• Continental-Continental:
creates chains of mountains
• Oceanic-Oceanic: create
batholiths that are later uplifted
and eroded
Section 18.3
Guided Notes about
Volcanoes
How Volcanoes Form
Step 1: magma chambers deep
within the earth fuel volcanoes.
How Volcanoes Form
Step 2: magma that reaches the
surface is called lava. Lava
erupts through an opening
called a vent.
How Volcanoes Form
Step 3: Lava cools and solidifies
around the vent
How Volcanoes Form
Step 4: over time, lava
accumulates to form a mountain
called a volcano
How Volcanoes Form
Step 5: a crater forms at the top
of the volcano near the vent.
2. What is a caldera?
• A caldera is a large volcanic
crater that forms when the
summit or side of a volcano
collapses into the magma
chamber of the volcano
3. The appearance of a
volcano depends upon:
• The type of material that forms
the volcano
• The type of eruptions that occur
The 3 Types of
Volcanoes
Cinder-Cone
• Forms when tephra is ejected
high in the air, falls back to
earth, and piles up around the
vent
• They are usually small, with
steep sides
• They have viscous magma, high
in silica and trapped gases.
The 3 Types of
Volcanoes
Composite
• Forms when layers of volcanic
fragments alternate with lava
• The magma contains high
amounts of water, with
moderate levels of silica and
trapped gases
• Are larger than cinder-cone
volcanoes
The 3 Types of
Volcanoes
Shield
• They have broad, gently sloping
sides and a nearly circular base
• They are made when basaltic
lava accumulates during quiet
eruptions
• They are the largest volcanoes
5. What is tephra?
• Tephra are rock fragments
thrown into the air during a
volcanic eruption.
• Tephra can be newly cooled and
hardened lava, mineral grains
that have crystallized, or pieces
of the volcanic cone.
The 5 Classifications for
Tephra
• Dust: less than .25 mm in
diameter
• Ash: between .25 and 2 mm in
diameter
• Lapilli: between 2 and 64 mm in
diameter
• Blocks: angular, greater than 64
mm.
7. Differences between
blocks and bombs
• Blocks are angular fragments of
lava. Bombs are blobs of lava
that are squeezed out of a vent
and form a rounded,
streamlined shape.
8. Describe a
pyroclastic flow.
• Clouds of gas, ash, and other
tephra that move down a
volcanic slope at tremendous
speeds is a pyroclastic flow.
• Speeds can be 200 km. per hour
and temperatures can be 700
degrees Celcius.
9. Worldwide
Distribution of
Volcanoes
• 80% of volcanoes are found at
convergent boundaries
• 15% are found at divergent
boundaries
• Only 5% are found away from
plate boundaries
10. How does convergence
lead to the formation of a
volcano?
• Convergence causes the crust
to descend into the mantle and
melt. The magma generated is
forced upward and forms
volcanoes when it reaches the
surface.
11. How does
divergence lead to the
formation of a volcano?
• Magma rises into the fractures
and cracks formed when two
plates separate. Usually this
occurs at ocean ridges.
12. What is a hot spot?
• A hot spot is an unusually hot
region of the mantle, where
plumes of magma rise to the
surface, creating volcanoes
• An example is Hawaii
13. How do hot spots
create volcanoes?
• The intense heat of the plumes
melt crustal rock, which is
forced through a vent to form
volcanoes
14.How do the chains of
volcanoes form over hot
spots?
• As the earth’s plates move
across hot spots, they create
chains of volcanoes
• The rate and direction of motion
can be calculated from the
positions of volcanoes formed.