Download Chapter 18 Volcanic Activity

Chapter 18
Volcanic
Activity
Section 18-1
Magma
Objectives:
• Describe factors that can affect the
formation of magma
• Compare and contrast the different
types of magma
Volcanic Eruptions
• In the last 10,000 years more than 1500
major volcanic eruptions have been
recorded…where and why do these eruptions
occur?
How Magma Forms
• Recall that magma is a mixture of
molten/melted rock, suspended mineral
grains, and dissolved gasses found deep
beneath the Earth’s surface.
How Magma Forms
• Magma usually forms at temperatures
between 800 and 1200 C (1600 to 2400 F)
• The temperature, and the pressure on the
rock along with the amount of water in the
rock all determine at what temperature the
rock will melt.
The Inner Earth
1. Crust
(lithosphere)
2. Upper Mantle
3. Lower Mantle
4. Outer Core
5. Inner Core
Rock Review
• Sedimentary (sediments)
• Igneous (cooling magma/lava)
• Metamorphic (heated solid rock)
Pressure and Temperature
• As pressure increases on the rock inside the
Earth, the temperature of the rock increases.
• So, the deeper down into the Earth that you
go, the higher the temperature.
Water and Magma
• If rock contains spaces that hold water, the
rock will melt at a lower than normal
temperature because the water found inside
the rock helps it liquefy as it heats up.
Types Of Magma
• Basaltic
• Andesitic
• Rhyolitic
Basaltic Magma
• Typically forms when rocks in the upper
mantle begin to melt, it tends to be fast
moving and relatively quiet as it erupts due
to its low gas content.
Andesitic Magma
• Andesitic magma is made in subduction
zones and is typically about 60% silica, it
tends to move and erupt at a medium pace.
Rhyolitic Magma
• Rhyolitic magma is thick and slow moving…it
is filled with gas and water and tends to be
very explosive because pressure builds within
it.
Viscosity
• Viscosity refers to the rate or speed at which
something will flow.
• If something has a high viscosity, the thicker
and slower it will flow
• If something is said to have a low viscosity,
the thinner and faster it will flow.
Section 18-2
Intrusive Activity
Objectives:
• Explain how magma affects overlying crustal
rocks
• Compare and contrast intrusive igneous rock
bodies
Density and Movement
• Because molten rock is nearly liquid
compared to the solid rock around it, it is less
dense and wants to move upward.
• What happens as the magma flows upward
into cooler crust? The process is called
“intrusion”.
Magma Intrusions
Intrusions
• The magma can force the solid rock to push
apart and create fissures
• The magma can contact the upper solid rock
and cause pieces of it to melt and fall into the
magma pool
• The magma can immediately melt the rock
into which it flows
Plutons
As the flowing magma cools inside of the solid
rock, it crystalizes and forms blocks, ribbons or
veins of new “intrusive” igneous rock.
Batholiths
Batholiths are the specific name given largest
formations of plutons. They are usually found
in large mountain ranges. They cut across rock
layers.
Laccoliths
Laccoliths tend to form near the Earth’s surface
where they cause the rock above to push
upward in a dome shape.
Sills
Sills are intrusions that form parallel
to the existing rock, they tend to stay
underground and flow like a “spill”.
Plate Movement
• Underground plutons can be brought to the
surface as tectonic plates converge and push
upward exposing the rock within it.
• Most igneous rock on the surface of the Earth
is not from volcanic activity but from the
slow gradual process of plate tectonics.
Section 18-3
Volcanoes
Objectives:
• Describe the major parts of a volcano
• Compare and contrast shield, cinder-cone,
and composite volcanoes
• Contrast the volcanism that occurs at plate
boundaries
Anatomy of a Volcano
• The magma that erupts to the Earth’s surface
is then called lava.
• The lava erupts through an opening in the
crust called a vent.
• The lava will cool and solidify around the
vent forming a mountain that is called a
volcano.
Anatomy of a Volcano
At the top of a volcano around the vent is a
bowl-shaped depression called a crater
Types of Volcanoes
• The appearance of a volcano depends
on two factors:
1. The type of material that forms the
volcano
2. The type of eruptions that occur
Types of Volcanoes
• Shield
• Cinder-cone
• Composite
Shield Volcanoes
• A wide volcano/mountain with gently sloping
sides and a nearly circular base
• It results from the slow eruption of basaltic
lava that builds up in layers
• Hawaiian Islands
Shield Volcano
Cinder-Cone Volcanoes
• Very steep mountain/volcano that results
from material being ejected straight up into
the air and falling back down around the vent
• Magma/lava contains some gases that make
the eruptions explosive
Cinder Cone
Composite Volcanoes
• Larger versions of cinder-cone that have been
made of layers of lava and solid material. The
magma/lava that makes them up is full of gas
that builds up until it finally explodes
violently.
• Mt. St. Helens
Composite
Volcanic Material
Rock fragments thrown into the air during an
eruption are called tephra.
• Classified by size
– Dust
– Ash
– Lapilli
– Volcanic blocks (angular)
– Volcanic bombs (rounded)
Pyroclastic Flows
• Violent eruptions that send out a wave of
gas, ash, and tephra that can travel up to 400
mph.
Where do Volcanoes Occur?
The distribution of volcanoes around the world
is not random, most occur at plate boundaries.
• 80% convergent
• 15% divergent
• 5% non-boundary “Hot Spots”
Convergent Volcanism
Plates coming together, this forms the Pacific
Ocean “Ring of Fire” and the “Mediterranean
Ring” in the Atlantic.
Divergent Volcanism
• Plates spreading apart also create a way for
magma to escape…these are called rift zones.
• Mid-Atlantic Ridge
Hot Spots
• The Hawaiian Islands are not formed along a
plate boundary but instead are found in an
area of the mantle that is usually hot called a
“hot-spot”
• As the plates move over the hot spot
volcanoes form.