Transcript - ILM.COM.PK

Chapter 10
Volcanoes and other Igneous Activity
Extra Credit – Inquiry Activity page 279….
Internet research and map plotting…..
The Nature of Volcanic Eruptions
Review the Mt. St. Helens eruption on May 18,
1980.
Mt. St. Helens
Nearly a cubic kilometer of ash and rock
debris ejected
Yakima, Washington, 130 kilometers to the
east, so dark at noon it appeared to be
midnight.
Factors Affecting Eruptions
The primary factors that determines whether
a volcano erupts violently or quietly
include:
1. magma composition
2. magma temperature
3. dissolved gases in the magma
Viscosity
is a substance’s resistance to flow.
Example: Maple syrup is more viscous than
water. When you heat the syrup is becomes
more fluid and less viscous.
The mobility of lava is strongly affected by
temperature. As the lava cools its viscosity
increases, its mobility decreases and the
lava comes to a halt.
The viscosity of magma is directly related to
its silica content.
Generally, the higher the silica content the
higher the viscosity.
Because of the silica content, rhyolitic lavas
are very viscous and don’t flow easily.
Basaltic lavas, which contain less silica, tend to
be more fluid.
Dissolved Gases
During explosive eruptions, the gases
trapped in magma provide the force to
eject molten rock from the vent, or opening
to the surface.
Volcanic Gases
1. Water
vapor
2. CO2
3. SO2
4. H2
5. CO
6. H2S
7. HCl
8. HF
Magma that with lots of dissolved
gases tends to be more violent or
explosive
Air Pollution through a SO2
Cloud
As magma rises its pressure is reduced
allowing the dissolved gases to be released
suddenly.
Very fluid basaltic magmas allow expanding
gases to bubble upward and escape
relatively easily.
Therefore, eruptions of fluid basaltic lavas,
as in Hawaii, are relatively quiet.
High viscous lava magmas slow the upward
movement of expanding gases.
The gases collect in bubbles and pockets
that increase in size until they explosively
eject the molten rock from the volcano.
Example: Mt. St. Helens
Volcanic Material
Lava Flows
Pahoehoe – (looks like rope) hot basaltic lavas are
usually very fluid because of their low silica
content. Flow rates of 10 – 300 meters per hour
are common. When basaltic lavas cool they form a
relatively smooth skin with wrinkles as the still
molten subsurface lava continues to flow.
A-A – (block-like) its surface is rough, jagged blocks
with dangerously sharp edges.
Gases
The actually percentage of gases in magma
is about 1-6 percent of total weight.
The actual quantity of gases emitted during
an eruption can exceed thousands of tons
each day.
Gases & amounts
Water vapor – 70%
CO2 – 15%
Nitrogen – 5%
Sulfur – 1%
Chlorine
Hydrogen
Argon
Pyroclastic Materials
Ejected materials propelled to great
heights by dissolved gases.
The fragments ejected during eruptions
range in size from very fine dust and
volcanic ash to pieces that weigh
several tons.
Size
Small bead to walnuts (cinders) are called
lapilli
Larger than 64 millimeter in diameter are
called blocks
Ejected glowing lava are called bombs
Fragments of tephra ejected
1. Dust
from volcanoes
2. Ash
3. Cinders
4. Bombs
Cinders
Bombs
Types of Volcanoes
Three main types of volcanoes
1. Shield volcanoes
2. Cinder cones
3. Composite Cones
Anatomy of a Volcano
Shield Volcanoes – formed by the
accumulation of fluid basaltic lavas.
These volcanoes are very broad and slightly
domed. Most shield volcanoes have grown
up from the ocean floor and formed islands.
Examples: Hawaii & Iceland
Mauna Loa
Belknap, Oregon
Kilauea
Cinder Cones – Ejected lava fragments the
size of cinders, which harden in the air, build
a cinder cone.
These volcanoes result from gas-rich
basaltic magma. They sometimes extrude
lava.
Cinder cones have a steep sided shape. They are
usually the product of a single eruption that
sometimes lasts only a few weeks and rarely more
than a few years.
Once the eruption ends, the magma in the pipe
connecting to the magma chamber solidifies, and
the volcano never erupts again.
Their cone is usually small from 30 – 300 meters
in height.
There are large numbers of cinder cones
around the Earth.
Many exist in volcano fields like Flagstaff,
Arizona, which consists of 600 cones.
Others form on sides of larger volcanoes,
for example, Mt. Etna has dozens of cinder
cones dotting its flanks.
Intermediate – Cinder Cones
Composite Cones – or stratovolcanoes –
most are located around the Pacific Ocean
in an area called the Ring of Fire.
Examples: Andes of South America, Cascade
Range in the western U.S. The Cascade
Range includes Mt. St. Helens, Mt. Rainier,
and Mt. Garibaldi.
The most active regions on the Ring of Fire
are located long curved belts of volcanic
islands next to deep ocean trenches.
Examples of these include the Aleutian
Islands, Japan, the Philippines, and New
Zealand.
A composite cone is a large, nearly symmetrical
structure composed of layers of both lava and
Pyroclastic deposits. They generally produce very
viscous lavas that are gas-rich.
Composite cones may generate the most
explosive eruptions that eject huge amounts of
pyroclastic material.
Examples of the classic composite cone include
Fujiyama in Japan, Mt. Shasta in California.
About 50 composite volcanoes have erupted in
the U.S. in the last 200 years.
Costa Rica
Mt. Pelee
Mt. Rainier
Dangers from Composite Cones
Pyroclastic flows – they consist of hot
gases, glowing ash, and larger rock
fragments. They can race down the sides of
the cone at nearly 200 mph.
Lahars – mudflows, many result when large
amount of snow and ice melts during an
eruption. In other cases, rain saturates the
ground resulting in a mudflow.
Pyroclastic flows are
super heated volcanic materials
forming a cloud.
This cloud can include volcanic
glass
Landslides
Landslides are produced:
During the eruption by the explosion
By producing enough heat to melt the snow on
the volcanic peak
Lahars
Occur when snow melts quickly creating
mud flows and landslides
Move a tremendous amount of debris
Can clog streams and rivers
Like other volcanic events, they are very
dangerous
Other Volcanic Features
Caldera – is a large depression in a volcano.
They may form one of two ways:
1. collapse of the top of the composite volcano
after an explosive eruption.
2. the collapse of the top of the composite
volcano after the magma chamber is drained.
Example: Crater Lake, Oregon, which formed
about 7,000 years ago when Mt. Mazama violently
erupted and collapsed.
Necks and Pipes – most volcanoes are fed magma through
conduits, called pipes, connecting a magma chamber to the
surface.
When cinder cones erode leaving the pipe exposed, the
resistant rock about the surface is called a volcanic neck.
Example: The best-known volcanic pipes are the diamondbearing pipes of South Africa. The rocks filling these pipes
formed at depths of at least 150 km. Where the pressure
is great enough to form diamonds.
Ship Rock in New Mexico and Devil’s Tower
Extinct Volcano
Conduit or
volcano throat
Lava Plateaus – Material extruded from
fissures in great volume over a large area.
Example: Columbia Plateau in northwestern
U.S. Here large numbers of fissures
extruded lava in flows some of which were
50 meters thick and buried the landscape
under a plateau nearly 1.6 km. thick.
Fissures or Fissure Eruptions
Intrusive Igneous Activity
Plutons
Structures that result from the cooling and
hardening of magma at depth are called plutons.
What can these plutons that are formed at depth be
exposed at the surface?
Erosion
Intrusive igneous bodies, or pluton, are classified
according to their shape, size, and relationship to
the surrounding rock layers.
Sills and Laccoliths
Sills and laccoliths are plutons that form when
magma is intruded close to the surface.
Sills – form when magma is injected along
sedimentary bedding surfaces, parallel to
the bedding planes. Overlying sedimentary
rock must be lifted to a height equal to the
thickness of the sill.
Laccoliths – are similar to sills when magma is
intruded between sedimentary layers close
to the surface. The magma that forms
laccoliths is more viscous and collects in a
lens shape pushing the overlying strata
upward.
Laccolith
Dikes – some plutons form when magma is
injected into fractures, cutting across
preexisting rock layers. Dikes are vertical
features that can form when magma from a
large magma chamber invades fractures in
the surrounding rocks.
Batholiths – are the largest of the intrusive
features. Batholiths are very thick. The
Idaho batholith covers 40,000 sq. km. A
batholith must have a surface exposure
greater than 100 sq. km. to be considered.
Batholiths
A batholith is a mass of rock
formed when a large body of
magma cools inside the crust.
Several large batholiths form the
core of mountain ranges in
western North America. Half
Dome in Yosemite National
Park, California, is part of the
Sierra Nevada batholith.
Origin of Magma
Geologists conclude the magma originates
when essentially solid rock, located in the
crust and upper mantle, partially melts.
The most obvious way to generate magma
from solid rock is to raise the temperature
above the level at which the rock begins to
melt.
Role of Heat
The change of temperature with depth is
known as the geothermal gradient.
On average, temperature raises between
20-300C per kilometer in the crust.
100 kilometers of depth temperatures
range between 1400 - 16000C. This is near
but not quite at the melting point of rocks.
To reach melting point additional heat must be
generated. There are several ways:
1. subduction zones – heat is generated by friction
2. Hotter mantle rocks can rise intruding crustal
rocks
These two processes can create small amount of
magma.
Large amount of magma can be created without
additional heat.
Role of Pressure
Pressure also increases with depth.
Melting causes an increase in volume, occurs at higher
temperatures at depth because of greater confining
pressure.
An increase in confining pressure causes an increase in
confining pressure causes in increase in rock’s melting
pressure.
Reducing the confining pressure lowers a rock’s melting
pressure.
This is triggers decompression melting.
This process generates magma beneath Hawaii where
plumes of hot rock melt as they rise toward the surface.
Role of Water
Another important factor is water content.
Water causes rock to melt at lower
temperatures.
Plate Tectonics and Igneous Activity
Convergent Plate Boundaries
The connection between plate tectonics
and volcanism is that plate tectonics
provide the mechanisms by which mantle
rocks melt to generate magma.
Ocean-Ocean
Results in a chain of volcanoes on the
ocean floor.
Eventually, as these structures grow, island
arcs are formed.
Ocean – Continent
Results in magma rising beyond the
subduction zone creating a volcanic
continental arc.
The only difference is the magma is silica
rich as it pushes its way through the
continental crust.
Divergent Plate Boundaries
As material rises from the mantle its confining
pressure decreases.
The rocks undergo decompression melting;
producing lots of magma that is less dense than
the mantle material it originally came from.
This new basalt is now floating on top of the
mantle and is an actual part of the ocean crust.
Intraplate Igneous Activity
Intraplate volcanism occurs within the plate,
not at its margin.
Examples: Hawaiian Islands and Yellowstone
National Park
Most intraplate volcanism occurs where a mass of
hotter than normal magma material called a
mantle plume rises toward the surface.
Mantle plumes form at the core-mantle boundary.
As these plumes rise they undergo
decompression melting forming basaltic magma.
The result is a small volcanic region a few
hundred kilometers across called a hot spot.
40 hot spots have been identified.Volcanic
activity on Hawaii is the result of a hot
spot.
Another example is the Columbia Plateau
in the northwestern U.S.
Landforms From Lava and Ash