Download Chapter 9 - Volcanoes

Chapter 9 - Volcanoes
Volcano – An opening in the Earth’s surface through
which magma and volcanic gases pass.
Magma – Molten rock, gases and water vapor inside
the Earth.
Lava – Molten rock that flows out onto the surface of
the Earth.
Eruption Types:
Nonexplosive – The most common eruption
where huge amounts of lava are released.
Explosive – A much rarer but destructive type
of eruption with hot debris, ash and gases rapidly
shooting out from the volcano.
Ash – Dust-sized particles of rock that get thrown into
the atmosphere.
Magma Chamber – The underground body of magma that feeds
the volcano.
Vent – The opening in the Earth’s surface through which magma
is released.
The explosive potential of the volcano depends on the silica,
water and gas content of the magma. The more water, the more
explosive due to the underground pressure build up of the
steam. Silica rich magma is thicker and can build up more
pressure.
What erupts from a volcano:
Lava – Liquid magma that flows out through the vent.
Blocky – Flows slowly; oozes and doesn’t travel
very far.
Pahoehoe – Flows slowly; has a glassy surface and
resembles dripping wax.
aa –Flows quickly; forms a brittle crust.
Pillow lava – Flows quickly; forms underwater.
Pyroclastic Material – Magma that has been blasted into
the air and hardens before landing.
i
Types of Pyroclastic Material
• Bombs – Blobs of magma that cool as they are
thrown high in the air.
• Lapilli – Pebble-sized bits of magma that
harden before hitting the ground.
• Ash – Most of the material ejected during a
volcanic eruption.
• Blocks – House-sized pieces of rock that are
sheared from the vent throat and thrown into the
air.
• Pyroclastic Flows – Dangerous eruptions in
which huge amounts of hot ash, dust and gases
blast from the volcano with temperatures of
1300°F and speeds of 130 m.p.h.
Types of Volcanoes
• The effects of a volcanic eruption are not only
dangerous immediately but the enormous
amounts of ash and gases ejected into the upper
atmosphere can change the climate worldwide
resulting in famine and disease.
• Shield Cones - can be enormous but are usually
built on layers of lava in non-explosive eruptions.
• Cinder Cones – smaller and formed from
moderately explosive eruptions that last a short
time and are composed of pyroclastic materials
such as ash.
• Composite Cones – one of the most common
types formed from alternating explosive/nonexplosive eruptions and lava flows.
Volcanic Landforms
• Crater – A funnel shaped pit at the top of many volcanoes.
• Caldera – A large depression formed after the eruption and
much larger than the crater. A crater with collapsed walls.
• Lava Plateaus – Formed by repeated eruptions with massive
outpourings of lava spreading out over a large area. These are
usually formed by rift zones (huge cracks in the surface of the
Earth).
• Causes of Volcanic Eruptions: The rock of the mantle is hot
enough to melt but the pressure prevents the molecules from
liquefying. However, when the pressure is decreased, magma
forms and begins to rise because it is less dense than the
surrounding rock.
• Where Volcanoes form: 80% of the worlds volcanoes form at
convergent plate boundaries and 15% form at divergent
boundaries.
• The Ring of Fire – The area of the Pacific plate perimeter
where many volcanoes are found.
Tectonic Plate Boundaries
• Divergent Boundary – Areas where tectonic plates separate
forming rift zones (deep cracks). The pressure rapidly
decreases allowing the rock to melt and form magma which
rapidly rises to fill in the rift zones.
• Mid-Oceanic Ridges – Undersea rift zones that produce
volcanoes and mountains by producing new crust from erupted
lava.
• Convergent Boundary – An area where tectonic plates collide.
• Subduction Zones – Areas where one tectonic plate is pulled
underneath (subducted) under another. Oceanic crust will
subduct under continental crust because it is denser than
continental crust. This process generates tremendous heat and
pressure that helps in the formation of magma and new
volcanoes.
• Hot Spots – Volcanically active places that are far from plate
boundaries. These usually will form volcanic island chains.
A. Mantle Plumes – Areas with rising magma that form hot
spots.
B. Cracks in the Earth’s crust that permit magma to rise
upward.
Volcanic Activity:
A. Extinct – Volcano has not erupted within recorded history
and most likely never will.
B. Dormant – Volcano is currently not erupting but may
again.
C. Active – Volcano is currently erupting or showing signs of
erupting in the near future.
Eruption Prediction
1. Watch for an increase in earthquakes.
2. Observe the ratio of certain gases.
3. Tiltmeters and GPS detect changes in the volcano's
slope.
4. Infrared satellite images watch for an increase in
temperature.