VOLCANIC ACTIVITY
Chapter 18
MAGMA
Magma is a mixture of
molten rock,
suspended minerals,
and gases.
All volcanoes are
fueled by magma.
Formation of
Magma
Magma forms from three factors:
1.
Composition of Rocks (those
that need to melt to become
magma)
• Forms when temperatures are
high enough to melt rocks
• Most rocks begin melting
between 8000 and 12000 C
• These temperatures exist
between the lithosphere and
the asthenosphere
Pressure
• Pressure increases with depth
due to the weight of overlying
rocks
• As pressure increases,
temperature at which
substance melts also increases
• The effect of pressure explains
why most of the rocks in the
Earth’s lower crust and upper
mantle do not melt to form
magma, although temperatures
are high enough
Water
• The presence of water
influences whether or not a
rock will melt
• A wet mineral or rock will
melt at a lower temperature
than the same mineral or rock
under dry conditions
Viscosity of Magma
Viscosity is the internal resistance to flow; meaning,
the ability to flow rapidly (like smooth liquid) or
flow slowly (like a milkshake)
• Hotter magma has lower viscosity
• Cool magma flows less quickly than hot magma
• If magma is low in silica, the viscosity is low
Types of Magma
Magmas are named after extrusive rocks.
There are three types of magma, each forming from
different areas of the Earth.
The three types are:
• Basaltic
• Andesitic
• Rhyolitic
Magma Chart
Basaltic
Form from
rocks in the
upper mantle
Low silica
amounts
Low Viscosity
Quiet eruptions
(flowing)
Andesitic
Form from
oceanic crust
and sediments
About 60%
silica amount
(intermediate)
Intermediate
Viscosity
Intermediate
eruptions
Rhyolitic
Form deep
beneath
continental crust
Highest amount
of silica within
this type of
magma
High Viscosity
Explosive
eruptions (most
dangerous)
INTRUSIVE ACTIVITY
Plutons are intrusive igneous rock bodies that represent
most of the igneous activity on Earth
Mountain building is responsible for the formation of many
plutons.
There are five types of plutons:
•
Batholith
•
Laccolith
•
Sill
•
Stock
•
Dike
Batholiths
•
•
•
•
•
•
Largest plutons
Irregularly shaped masses
Coarse-grained igneous rocks
Cover at least 100km
Take millions of years to form
Common in interiors of major
mountain chains
Pluton
Classification
Stocks
• Similar to batholiths but
smaller in size
• Cuts across older rocks
• Forms 10-30km beneath
Earth’s surface
El Capitan – large granite
batholith in America…
Yosemite National Park
Pluton Classification
Laccoliths
• Mushroom shaped pluton with
a round top and flat bottom
• Forms when magma intrudes
parallel rock layers close to
Earth’s surface and rocks
“bow” upward due to heat and
pressure of magma body
• Relatively small compared to
batholiths and stocks
• At most, up to 16km wide
• Common in Black Hills of
South Dakota
Sills
• Forms when magma intrudes
parallel to layers of rock
• Can range from a few
centimeters to hundreds of
meters in thickness
Pluton Classification
Dikes
• Pluton that cuts across
pre-existing rocks
• Form when magma
invades cracks in
surrounding rock bodies
• Few cm to several m
wide and up to tens of
km long
• Most are coarse-grained
VOLCANOES
Volcanism produces various features that alter the Earth’s landscape
Common Parts of a Volcano
Within a volcano, there is a portion that fuels the eruptions. This is
referred to as the magma chamber.
Once the magma chamber is fueled, it erupts through an opening in the
crust. This opening is called the vent. Over time, the lava will
solidify and accumulate to form a mountain known as a volcano. At
the top of the volcano, sits the vent.
The vent is surrounded by a bowl-shaped depression that is referred to
as a crater. The crater connects to the magma chamber by way of the
vent.
In some instances, the pressure of the volcano becomes too great to
support its walls and the side wall (summit) will collapse into the
magma chamber, resulting in a caldera.
Anatomy of a Volcano
Volcanic Necks
Volcanic necks are also a type of
intrusive igneous rock activity,
created when magma hardens in the
vent of a volcano
Through erosion of the walls, the neck
becomes exposed
Volcanic necks are often the most
resistant part of the volcano
Example: Devils Tower in Wyoming
Crater Lakes
Sometimes, a crater lake will
form as result of too many
eruptions.
The top of the magma
chamber will become partially
empty and collapse, forming
a caldera.
The caldera will eventually fill
up with water and become a
lake, such as the one shown in
the picture of Crater Lake
National Park (Oregon)
Types of Volcanoes
The appearance of a volcano depends on two factors:
1.
2.
The type of material that forms the volcano
The type of eruptions that occur
Based on this, three major types of volcanoes have been
identified:
1.
2.
3.
Shield
Cinder Cone
(Strato) Composite
Shield Volcano
• Broad, gently sloping
sides
• Nearly circular base
• Layered basaltic lava
• Non-explosive
eruptions
• Low viscosity
• Low amounts of gases
and silica
• Make up the Hawaiian
Islands
• Mauna Loa – famous
example
Cinder Cone Volcano
• Steep sided volcano
• Material is ejected high into
the air and falls back to Earth,
around the vent
• Generally small (less than
500 m high)
• More water and silica than
shield volcanoes
• Large volume of gases within
the magma
• More explosive than shield
volcanoes
• Tephra thrown into the air
(rock fragments)
• Famous example includes the
Izalco Volcano in El
Salvador
(Strato)
Composite Volcano
•
•
•
•
• Largest (in height) of all volcanoes
• Highly explosive, except when
basaltic magma is erupting
• Most dangerous to humans
Form from alternating layers of
volcanic fragments / lava over
• Of the 1500 active volcanoes studied
hundreds of years
over the past 10,000 years, 699 are
stratovolcanoes
Most have several vents, some of
cinder cone appearance
• Famous examples include Mount St.
Helens, which is the youngest and the
Can erupt all magma types, at same
most active stratovolcano in the
time
Cascades
Large amounts of silica, water and
gases
Difference in Scale of Volcanoes
Volcanic Eruptions
1.
2.
3.
4.
5.
6.
Tephra
Pyroclastic Flow
Plinian Eruptions
Eruption Clouds
Ash Fall
Lahars
Tephra
As noted, tephra refers to rock
fragments thrown into the air
during volcanic eruptions.
Tephra can be one of the
following:
– Newly cooled / hardened lava
– Mineral grains that have
crystallized prior to eruption
– Pieces of the volcanic cone
Tephra Classification by Size
• DUST – less than .25mm in
diameter
• ASH – less than 2mm in
diameter (but larger than
dust)
• LAPILLI (little stones) –
less than 64mm in diameter
(but larger than ash)
• VOLCANIC ROCKS / BOMBS
– can be very large. Documented
sizes are comparable to a small
car or house. When angular, they
are called rocks and when
rounded, they are called bombs.
Pyroclastic Flow
Pyroclastic Flow refers to rapidly moving
volcanic material.
– Can include hot ash, pumice, rock
fragments, and volcanic gas
– Can travel up to speeds of 100 km/h
– Temperatures can exceed 500
degrees Celsius
– Ability to destroy everything in path
To explain damage, one of the most dangerous flows occurred
on the island of Martinique, where more than 29,000 people
suffocated or were burned to death in 1902 due to the
eruption of Mt. Pelee
Plinian Eruptions
Plinian Eruptions are large explosive
events forming dark columns of
tephra and gas high into the
stratosphere
Sometimes results in so much magma
withdrawal from below the
volcano that a caldera forms
Sometimes leads to detectable cooling
due to large quantities of aerosols
being injected into the stratosphere
Eruption Clouds
A cloud of tephra and gases forming
downwind of an erupting volcano
(vertical pillar directly above vent is
eruption column)
Often dark colored, but can be white like
weather clouds
May drift for thousands of kilometers
downwind and spread out the further
they move from the vent
Large eruption clouds can encircle the
Earth within days
Consists of rock, mineral, and volcanic
glass fragments that is hard and does
not dissolve in water (like ash from
wood burning fires)
Extremely abrasive, mildly corrosive,
electrically conductive (when wet)
Created during explosions by shattering
of rock and violent separation of
magma into tiny pieces
When it accumulates, it is very heavy
and leads to building collapses. It
also interferes with telephone and
radio communications and effects
ventilation as it clogs filters. Due to
it’s ability to conduct electricity, it
can also lead to power outages
Ash Fall
Lahars
Rapidly flowing mixture of rock debris
and water that develops on the slopes
of the volcano
Also known as volcanic mudflows or
debris flows
Form in a variety of ways:
– rapid melting of snow and ice by the
pyroclastic flow
– Intense rainfall on loose volcanic rock
deposits
– Breakout of a lake once dammed by
volcanic rock fragments
– Consequence of debris avalanches
Yellowstone’s Supervolcano
Beneath Yellowstone Park is a monstrous plume of hot rock causing the Earth to tremble and
heave. Past volcanoes have erupted with 1000 times the magnitude of Mount St. Helens.
Scientists claim the future is anybody’s guess, but its eruption will have a global impact.
Formation of Supervolcanoes
1. Supervolcanoes ultimately form in calderas. When a normal
volcano erupts lava gradually builds up in the mountain before
release. In a supervolcano when magma nears the surface it does
not fully reach it and instead begins to fill massive underground
reservoirs.
2. Magma then melts the nearby rock to form more extremely thick
magma, making it so viscuous that volcanic gases that normally
trigger an eruption cannot pass through. As result, a massive
amount of pressure begins to build up.
3. The build up of pressure continues for hundreds of thousands of
years until an eruption occurs, which blasts away a huge amount
of ground and forms a new caldera.
Dangers of Eruption
Immediately before the eruption, there would be extreme seismic activity in
the Yellowstone region, causing the ground to swell further – causing
most of Yellowstone to be uplifted – until the seismic activity eventually
breaks through the layer of rock holding in the magma.
Magma would be then be flung 50km into the atmosphere, virtually killing
all life by falling ash, lava flows, and the sheer explosive force of the
eruption.
Volcanic ash would then completely coat places as far away as Iowa and
the Gulf of Mexico. Enormous amounts of lava would pour out of the
volcano (enough to coat the whole of the USA with a layer 5in thick).
Within minutes of the eruption tens of thousands would be dead.
Long Term Effects of Eruption
The long-term effects to an eruption of this magnitude would be
absolutely devastating.
The ash shooting into the atmosphere could block out light from the
sun, making global temperatures plummet (nuclear winter).
–
–
–
–
Would kill a large percentage of the world's plant life
The grain harvest of the Great Plains would disappear in hours
Global effect of massive food shortages
Declining temperatures (by at least 21 degrees) could cause mass extinction
of wildlife
– Could possibly lead to extinction of human race as long term effects of the
eruption play out
Where do Volcanoes Occur?
Volcano distribution is not random. Most form at plate boundaries.
Convergent Plate Boundaries – make up about 80% of all
volcano locations
Divergent Plate Boundaries – make up about 15% of all volcano
locations
Away from any plate boundaries – only approximately 5% of all
activity
Convergent Volcanism
There are two major belts along
convergent plate boundaries:
1. Circum-Pacific Belt (Pacific
Ring of Fire)
This stretches along the
western coasts of North and
South America, across the
Aleutian Islands, and down
the eastern coast of Asia.
2. Mediterranean Belt (Italy)
Includes Mount Etna and
Mount Vesuvius
Divergent Volcanism
As plates move apart, fractures and
faults are created. This results in
major separations called rift zones.
Most of the world’s rift volcanism
occurs under water along deep
ocean ridges. This results in a
process referred to as seafloor
spreading.
Rift Volcanism can be observed above
water in Iceland, which is part of the
Mid-Atlantic Ridge. Several
volcanoes are present within this
area.
Hot Spots
Some volcanoes are far from plate boundaries
and form from hot spots (unusually hot
regions of Earth’s mantle).
Chains of volcanoes that form over hot spots
provide important information about plate
motions
Some of Earth’s best known volcanoes form
from hot spots under the Pacific Ocean
Mt. Kilauea
The worlds most active volcano
is currently located over a hot
spot on the big island of Hawaii.
Sometimes, hot spots can result in
flood basalts. Flood basalts erupt
from fissures rather than a central
vent, forming flat plains or
plateaus
The volume of basalt in these
eruptions can be tremendous.
Kilauea has what is called a “fire
hose” dumping lava straight off
the side of the volcanic cliff into
the Pacific Ocean.