Transcript Shield volcanoes

Volcanoes and
Earth’s Moving
Parts
What is a Volcano?
• A volcano is an opening in the
Earth that erupts gases, ash,
and lava.
• Volcanic mountains form when
layers of lava, ash, and other
material build up around these
openings.
Kilauea
• Kilauea is the world’s most
active volcano.
• It has been erupting for
centuries although not
explosively.
• Its lava moves very slowly.
• Iceland is also famous for its
active volcanoes. It sits on
an area where the Earth’s
plates move apart. ( a -----boundary).
How do volcanoes form?
• Deep inside the earth, heat, and pressure
cause rock to melt, forming magma (liquid
rock).
• Magma is forced upward because it is less
------- than the rock above it, so it is forced
toward the Earth’s surface.
• After thousands or millions of years, the
magma reaches the Earth’s surface and
flows out of an opening called a vent.
volcano
Where do volcanoes
occur?
• Divergent plate
boundaries
• Convergent plate
boundaries
• Hot spots -Hawaii
What controls eruptions?
Some eruptions are quiet
while others are explosive.
Two factors control if an
eruption is quiet or
explosive.
One factor is the amount of
water vapor and trapped
gases in the magma.
The second factor is the
viscosity of the magma – how
quickly or slowly the magma
flows (determined by how
much silica is present).
• Basaltic Magma is relatively
low in SILICA has a low
viscosity.
• It is fluid and produces
quiet, non-explosive
eruptions (like those at
Kilauea.)
Kilauea
Explosive Eruptions
• Silica-rich or GRANITIC magma
produces explosive eruptions. It
has a high viscosity. This magma
sometimes forms where the
Earth’s plates are moving
together where one plate slides
under another – (AKA ------)
Forms of Volcanoes
•Shield Volcanoes
•Cinder Cone
Volcanoes
•Composite Volcanoes
• Shield volcanoes are volcanic
mountains built up by the eruption
of fluid, basaltic lava flowing out
of a central vent. They have
broad bases with very gentle
slopes, and are the largest
volcanoes on Earth. The volcano
gets its name from its
resemblance to the slightly
domed shape of a warrior's
shield.
• Shield volcanoes do not erupt
violently. Rather, fluid, basaltic lava
erupts onto the surface and flows
freely across the ground for great
distances forming a broad cone. Lava
tubes can form beneath the surface of
the lava flow which allows the lava to
flow for tens of kilometers (miles)
without cooling. Each successive flow
increases the size of the shield
volcano. Sheild volcanoes have
magma with a low viscosity.
Hot Spots
• Weak areas in the earth’s crust where
magma plumes punch through.
• Volcanoes eventually form islands chains
like the Hawaiian Islands.
•Cinder cone
volcanoes consist of
steep, loosely packed
material.
Cinder Cone
Cinder Cone
• Cinder cones are the simplest type of
volcano. They are built from particles and
blobs of congealed lava (tephra) ejected
from a single vent. As the gas-charged
lava is blown violently into the air, it breaks
into small fragments that solidify and fall
as cinders around the vent to form a
circular or oval cone.
• Most cinder cones have a bowl-shaped
crater at the summit and rarely rise more
than a thousand feet or so above their
surroundings. Cinder cones are numerous
in western North America as well as
throughout other volcanic terrains of the
world.
• Cinder cone volcanoes have lava with high
viscosity.
Composite Volcano
• Structure of a Composite Volcano
• Composite volcanoes are also known as
strata volcanoes.
• They have alternating layers of tephra and
lava. They have a mountainous form, and
usually form at subduction zones.
• Mount St. Helen’s is a composite volcano.
•The End