Download Volcanoes - amsscs.sharpschool.com

Volcanoes
Volcanoes & Plate
Tectonics
6.E.2.2 Explain how heat flow and volcanoes
reflect forces within the earth.
What is a Volcano?
• A volcano is a weak spot in the crust
where molten material, or magma,
comes to the surface.
• Magma is a molten mixture of rockforming substances, gases, and water
from the mantle.
• When magma reaches the surface, it is
called lava.
Location of Volcanoes
• There are about 600 active volcanoes
on land.
• Many more are found beneath oceans
• Most occur in belts that extend across
continents and oceans
• A major belt is the “Ring of Fire” which
encircles the Pacific Ocean
Volcanic belts form along the
boundaries of Earth’s plates.
Most volcanoes occur along diverging
plate boundaries such as the mid-ocean
ridge, or in subduction zones, around
edges of oceans
Volcanoes often form where two or oceanic plates collide
or where an oceanic plate collides with a continental
plate. In both situations, an oceanic plate sinks beneath a
trench. Rock above the plate melts to form magma, which
then erupts to the surface as lava.
• “Hot Spot” volcanoes form where
magma from deep within the mantle
melts through the crust like a blow torch
• Examples:
– Hawaiian Islands
– Yellowstone National Park
Inside a Volcano
• All volcanoes have a pocket of magma
beneath the surface and one or more cracks
through which magma forces its way through
• Magma pocket is called a magma chamber
• A pipe is a long tube in the ground that
connects the magma chamber to the earth’s
surface
• Molten rock and gas leave the volcano
through an opening called a vent
• A lava flow is the area covered by lava
as it pours out of a vent
• A crater is a bowl-shaped area that
may form at the top of a volcano around
the central vent
A Volcanic Eruption
• When a volcano erupts, the force of the
expanding gases pushes magma from
the magma chamber through the pipe
until it flows or explodes out of the vent.
• Once magma escapes from the volcano
and becomes lava, the remaining gases
bubble out.
Types of Volcanic Eruptions
• Geologists classify volcanic eruptions as quiet
or explosive.
• The silica content of magma helps to
determine whether the volcanic eruption is
quiet or explosive
• Silica is the material formed from the
elements silicon and oxygen
• Silica is one of the most abundant
materials in Earth’s crust and mantle
• The more silica that magma contains,
the thicker it is
Quiet Eruptions:
• A volcano erupts quietly if its magma is low in
silica.
• Magma flows easily; the gas dissolved in the
magma bubbles out gently
• Examples:
– Hawaii
– Iceland
Quiet eruptions produce two types of lava:
1. Pahoehoe - fast moving, hot lava;
Surface looks like a solid mass of
wrinkles, billows, and rope-like coils.
2. Aa - cooler, slower-moving lava; when
hardens, forms a rough surface
consisting of jagged lava chunks.
Explosive Eruptions:
• A volcano erupts explosively if its
magma is high in silica.
• Magma is thick and sticky
• Magma slowly builds up in the
volcanoes’ pipe
• Dissolved gases cannot escape
• Trapped gasses build up pressure until
they explode
• A pyroclastic flow occurs when an
explosive eruption hurls out ash, cinder,
bombs, and gasses
• Pebble-sized particles are called
cinders.
• Larger pieces, called bombs, may range
from the size of a baseball to the size of
a car.
Stages of a Volcano
• Active - is erupting, or has shown signs
that it may erupt in the near future
• Dormant - does not show signs of
erupting in the neat future
• Extinct - unlikely to erupt
Related Volcanic Activities
• Hot Spring - groundwater heated by a
nearby body of magma rises to the
surface and collects in a natural pool
• Geyser - forms when rising hot water
and steam become trapped
underground in a narrow crack
• Pressure builds until the mixture
suddenly sprays upward, clearing the
crack
Volcanoes
Volcanic Landforms
Types of Volcanoes
• Shield Volcano: nonexplosive
eruptions, fluid basaltic lava, gentle
broad slopes
– Thin layers of lava pour out of a vent
and harden on top of previous layers
– Example: Hawaiian Islands
Shield Volcano
• Cinder Cone Volcano: explosive
eruptions, small but steep slopes,
pyroclastics
– Form when cinders from a vent, pile
up around the vent, forming a steepcone-shaped mountain
Cinder Cone Volcano
• Composite Volcano: alternating
between lava and pyroclastics,
explosive and nonexplosive eruptions,
steep and tall
– Layers of lava alternate with layers of
ash, cinders, and bombs
Composite Volcano
Related Volcanic Landforms
• Caldera:
– A large hole at the top of a volcano
formed when the roof of a volcano’s
magma chamber collapses
Formation of a Caldera
• Dike:
– A slab of volcanic rock formed when
magma forces itself across rock
layers
– (vertical rock formation)
Dike
• Sill:
• A slab of volcanic rock formed when
magma squeezes between layers of
rock
• (horizontal rock formation)
Sill
• Batholith:
• A mass of rock formed when a large
body of magma cooled inside the crust
Batholith
• Dome Mountains:
– Rising magma within the crust is
blocked by layers of rock
– Magma forces layers of rock to bend
upward into a dome shape
– Example: Black Hills
Dome Mountains
Questions
When magma reaches the surface, it is
called?
A. Larva
B. Lava
C. Lagoon
D. Lump
What is a magma pocket called?
A. Magma chamber
B. Hot pocket
C. Hot chamber
D. None of the above
What type of lava is fast moving?
A. Aa
B. Naa
C. Pahoehoe
D. HoHoHo
The End