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Mountains and Volcanoes Notes
Earth's surface is constantly evolving as high mountains
gradually wear down and new mountains are continuously
being formed, over and over again.
- 4 types of Mountains:
How do fault block
mountains form?
How are folded mountains
formed?
How are upwarped or dome
mountains formed?
What is a volcano and how
does it form?
(a.) fault block mountains - Mountains sometimes form when
many layers of the Earth's crust are moved vertically upward
at fault lines by pressures caused by plates colliding. Both
normal and reverse faults can cause the uplift of blocks of
crustal rocks. The Sierra Nevada mountains of California, and
the mountains in the Tetons, and Basin and Range province of
the western U.S. were formed by faulting processes and are
thus fault block mountains.
(b.) fold & thrust mountains - All rock that is put under
extreme pressure for long periods of time (thousands or
millions of years) will fold like clay. Folding is a process in
which the Earth's plates are pushed together in a roller coaster
like series of anticlines and synclines. Folding bends many
layers of rocks without breaking them. The Himalayan
Mountains (currently the highest on Earth) are mountains of
this type and were formed as a result of the Indian Plate
colliding with the Eurasian plate. The Appalachian Mountains
in the Eastern United States were formed about 400 million
years ago when North America and Africa collided. Similarly
the Alps of Europe were formed by such processes.
(c.) upwarped / dome mountains- These type of mountains,
are caused by a broad arching of the crust. The Black Hills of
South Dakota and Adirondacks of New York are examples of
these types of mountains. The Black Hills of South Dakota
and Wyoming, and the Adirondack Mountains of New York are
low mountains that were formed when the crust was heaved
upward without folding or faulting into a rounded dome by
broad arching of the crust. Upwarped or dome mountains are
much higher in elevation than the surrounding land and
because of this erosion occurs at a very fast rate.
(d) volcanic mountains - Volcanic mountains, are not formed
by deformational processes, but instead by the outpouring of
magma onto the surface of the Earth. The Cascade Mountains
of the western U.S., and of course the mountains of the
Hawaiian Islands and Iceland are volcanic mountains. Can be
dormant or active.
- volcano - a mountain that forms when layers of lava and
volcanic ash erupt and build up. Can be dormant or active.
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- how a volcano forms:
(1) heat and pressure eventually causes rocks to melt back
into magma.
(2) magma is less dense than rock so it rises to the surface
and finds a vent (opening at Earth's surface) to flow out of.
(3) As magma cools the layers build until a volcano is formed.
The opening is called a crater.
Where are volcanoes found?
- where volcanoes occur:
1) Divergent Plates
2) Converging Ocean-Ocean Plates, Converging Ocean –
Continent Plates
3) Hot Spots
What is the Pacific Ring of
Fire?
Pacific Ring of Fire - edge around the Pacific plate where
most volcanoes and earthquakes occur.
Name 3 factors that affect
how thick the lava is.
-3 Factors affecting Viscosity (- a measure of a fluid's
resistance to flow):
1. Chemical Composition - more silica in magma, increases
viscosity
Magma
Silica
Viscosity
Composition Content
Basaltic
50%
least
viscous
Andesitic
60%
intermediate
viscosity
70%
most
viscous
Rhyolitic
Lava
Flows
long,
thin
flows
short,
thick
flows
2. Temperature - as lava cools, viscosity increases, therefore
mobility decreases
3. Dissolved Gases - dissolved gases increase fluidity,
expansiveness of gas adds buoyancy, helps molten lava
propel from a vent, highly viscous magmas block the upward
migration of gases, pressure increases until violent eruption
occurs
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What are the 4 types of
volcanoes?
Types of Volcanoes:
1) Shield - broad, gently sloping landform, small percentage of
pyroclastic material, greater width than height, slope angles
less than 12 degrees
2) Cinder (Scoria/Tephra) Cone - conical hill composed of
pyroclastic debris, cinders, low height, ~ 400 meters, gascharged magma, after gas is expelled from magma, lava pours
from vent, generally flows under the cinder cone.
3) Composite/ Stratovolcano - steep-sides, symmetrical
landform, built of alternating layers of pyroclastic debris
capped by high viscosity lava flows, Nuee Ardente and
ignimbrites associated
4) Dome - the lava, even though emitted at high temperature,
is extremely viscous, Eruptions are separated by long
intervals, beginning with a preliminary phase characterized by
emissions of smoke and cinders, followed by an enormous
explosion.
Rock Types Associated with Volcanoes:


Shield: basalt (low silica, high iron and magnesium),
therefore fluid; lavas known as aa (slow-moving, blocky
and sharp) and pahoehoe (faster-moving, and ropy)
basalt (low silica, high iron and magnesium), therefore
fluid; lavas known as aa (slow-moving, blocky and
sharp) and pahoehoe (faster-moving, and ropy)
Composite: andesite (high silica, medium-low iron and
magnesium), therefore sticky; rhyolite (very high silica
and low iron and magnesium), therefore very sticky;
pumice and obsidian (volcanic froth and glass) andesite
(high silica, medium-low iron and magnesium),
therefore sticky; rhyolite (very high silica and low iron
and magnesium), therefore very sticky; pumice and
obsidian (volcanic froth and glass)
Other Igneous Features
Intrusive:
- batholith - the largest igneous rock bodies, formed when
magma cools underground.
- dike - formed when magma is squeezed into a vertical crack
that cuts across rock layers and hardens.
- sill - formed when magma is squeezed into a horizontal
crack that cuts across rock layers and hardens.
- laccolith - a dome of rock created from a sill that pushes
rock layers upward.
Extrusive:
- volcanic neck - the solid core that is left behind after the
volcano stops erupting and erosion wears away the cone of
the volcano.
- caldera - the very large opening left after a volcano erupts
and collapses into the partially emptied magma chamber.
- lava flows - can cover hundreds of square miles of
horizontal beds of lava; seen on the Columbian Plateau in
Washington state and the Deccan Plateau in India
- ash falls - can be hundreds of feet thick and form new rock
called welded tuff
- pillow lava - formed as lava is cooled quickly as it hits sea
water; found at deep oceanic rifts, off volcanic islands and at
ancient plate boundaries such as those found in central
Canada; lava with a rounded, "pillow" shape.
Summaries