Transcript 10.2: Volcanoes form as molten rock erupts

Chapter 10: Mountains and
Volcanoes
10.1: Movement of rock builds mountains
10.2: Volcanoes form as molten rock erupts
10.3: Volcanoes affect Earth’s land, air, and water
Review
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Motion that transfers heat within a material
Convection current
The layer formed by Earth’s crust and the
very top of the mantle
Lithosphere
Underwater mountain ranges found where
the ocean floor is spreading
Mid-ocean ridges
ç
Alaska's Mount Redoubt volcano
spews ash and steam during an
eruption in 1989.
Steam rises through the snow on
Mount Redoubt in Alaska last year
Iceland volcano, near the
Eyjafjallajokull glacier, early
Sunday, March 21, 2010.
first major eruption there in
nearly 200 years
Volcanoes erupt many types of material
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Volcano: opening in Earth’s crust through which
molten rock, rock fragments, and gases erupt
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Recall: the surface is cool rock, but most of the
Earth is extremely hot rock and molten metal
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A mountain built up from erupted material
Some heat escapes to the surface through volcanoes
Erupt violent (throw out rocks) or gently (rivers of
molten lava
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Depends on type of magma feeding the volcano
Magma
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Made of a large portion of silica
(silicon and oxygen), gases
High in silica: resists flowing, so
expanding gases are trapped in
it
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Pressure builds up – the gases
blast out in a violent dangerous
explosion
Poor in silica: flows easily, gas
bubbles moves up through it
and escape fairly gently
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Can still throw lava high into the air
– lava fountains – but it’s “safe” to
be nearby http://www.classzone.com/books/ml_sci_ca6/page
_build.cfm?id=resour_ch3&u=3#%23
Grinded silica
Magma
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Rises as long as it is less dense than the
surrounding rock
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When it stops rising, it collects in a “magma
chamber”
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From here can cool (igneous rock) or erupt (lava)
Rock Fragments
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Form as:
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Ash: size of dust to rice grains
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Can travel long distances, carried by
winds
Cinders: somewhat larger
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Escaping gas bubbles pop, tearing
magma apart
Larger pieces of lava are thrown into the
air and cool
Rocks of all sizes are ripped from the
volcano wall
Contains holes from escaping gases
Bombs and Blocks: up to the size of
houses!
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Fall quickly
Volcanic Gases
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What looks like smoke is actually a mix of ash and
gases
Main gases: water vapor and carbon dioxide
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Can combine with water in air to form acids
Pyroclastic flow: a dense cloud of superhot gases
and rock fragments that races downhill
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As hot as 800C (1500F)!,
travel faster than 160 km/hr
(100 mi/hr)
Most dangerous type of
volcanic eruption
classzone volcanoes
Most volcanoes form along plate
boundaries
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Common along tectonic plate boundaries
where oceanic plates sink beneath other
plates
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Plates sinking deep into a subduction zone – heat
and melt magma can build tall volcanic
mountains
Common where plates pull apart: allows
magma to rise from the mantle
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In the water, magma erupts at spreading centers
and cools to form new lithosphere
Most volcanoes form along plate
boundaries
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Less common: forming
over a hotspot (far from a
plate boundary)
More than 400 volcanoes
– 80% of all active
volcanoes above sea
level – are along
subduction zones in the
Pacific Ocean
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Active = erupting or has
erupted in recorded history
“Ring of Fire”
Volcanoes can have many shapes and sizes
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Shield Volcano – shaped like a broad, flat dome
Built up by many eruptions of lava that is relatively low in silica – flows
easily and spread out in thin layers
Cinder Cone: steep, cone-shaped hill formed by the eruption of cinders
and other rock fragments that pile up around a single crater
Form as gas-rich magma erupts
Escaping gases throw small chunks of lava into the air, where they
harden before landing
Composite Volcano: a cone-shaped volcano built up of layers of lava
and layers of rock fragments
Magma high in silica, pasty
Steep near the top and flattens out toward the bottom
Have violent eruptions:
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Expanding gases trapped in rising magma tend to cause explosions
Hardened lava from earlier eruptions often plugs openings in these volcanoes
and is blow out of the way before any more magma can escape
aka composite
Crater Lakes
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Both shield volcanoes and composite volcanoes can form calderas –
“huge crater formed by the collapse of a volcano when magma
rapidly erupts underneath it”
 Crater lakes: Oregon: formed 7700 years ago!
Mount Mazama Volcano and Crater Lake Caldera, Oregon - Wizard Island
Scientists Monitor Volcanoes
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Before Mount Pinatubo (Philippines) erupted in 1991, it
had not erupted in 500 years!
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Erosion had changed its shape and so did not appear to be a
composite volcano
Early indication that is was going to erupt - ash
Scientists Monitor Volcanoes
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Mount Pinatubo: scientists were able to warn locals and save
many lives
 Warning signs: many small earthquakes followed by explosions
of steam near the top
Indications that magma is moving below a volcano:
 earthquake activity
 changes in tilt of the ground
 Temperature at openings, springs, and lakes
 Amount and types of gases given off by the volcanoes
 Increasing temperatures and gas changes indicate that fresh
magma has moved in a shallow magma chamber
Study age and types of volcanic rocks
 Time passed between eruptions
 How violent the eruptions were
Discovery.com Volcano Explorer
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http://dsc.discovery.com/convergence/pompe
ii/interactive/interactive.html