Transcript volcanoes - Learn District 196

VOLCANOES
One Hot Topic!!!
You are going to LAVA it!!!
How and Where Volcanoes
Form
 Volcano-opening in the earth’s crust
through which molten rock, gases, and
ash erupt and the landform that
develops around the opening
 Volcanic eruptions occur when magma
from inside the Earth reaches the
surface
3 Conditions that allow
magma to form
 A decrease in pressure can lower the melting
point of rock (along a rift valley)
 An increase in temperature (at a hot spot)
 An increase in the amount of water can lower
the melting point (at subduction boundaries)
 Most volcanoes are found along divergent or
subduction boundaries
Magma
 Magma rises to the surface because it is
less dense than the surrounding solid
rock, this is called convection
 How fast the magma rises (the rate)
depends upon how much silica the
magma contains
Subduction Boundaries
(Pg 195)
 In a continent-ocean plate situation, the
volcanoes always form on the overriding
continental plate (Cascade Mountains)
 In an ocean-ocean plate situation,
volcanoes form in a chain on the
overriding plate (Marianas Islands)
Divergent Boundaries (Pg
196)
 Most of the magma that reaches the
surface does so at divergent boundaries
 Most divergent boundaries are below the
ocean, but not in Iceland
 Less dense magma rises to fill the space
left open by the separating plates
 Mid-Atlantic Ridge is a great example
Hot Spots (Pg 197)
 Volcanic eruptions that occur far from a
plate boundary is a hot spot
 Hawaii and Yellowstone are good hot
spot examples
 Often the volcanoes are in a chain
Magma and Erupted
Materials
 The viscosity of magma refers to its
resistance to flow (How thick it is.)
 The viscosity of magma is determined by
how much silica it contains
 High gas content can make magma very
explosive
Lava Flows (Pg 200)
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Lava is magma that reaches the surface
Basaltic lava flows are less explosive
Pahoehoe-ropey lava
Aa-sharp, jagged lava pieces
Pillow lava-forms under water
Ash and Rock Fragments
(Pg 200)
 Pyroclastic material-material blasted out
of volcanoes in an eruption
 Ash is the smallest type of pyroclastic
material, lapilli is intermediate size and
blocks and bombs are the largest
 Pyroclastic flow is a dense, superheated
cloud that combines pyroclastic material
and gas that travels downhill
Volcanic Landforms (Pg
202)
 Shield Volcanoes-broad based, gentile sloping
sides, made up of layers of repeated lava
flows
 Shield Volcanoes are generally less explosive,
(Mauna Loa is an example)
 Cinder Cone-fragments of material that form
around a blast hole (cinder cones tend to be
small) Capulin in New Mexico is an example
3 types of volcanoes
Volcanic Landforms
 Composite Volcanoes-develop when layers
from successive explosive eruptions form
around a vent
 Mt. St. Helens and Mt. Shasta are examples
 Composite volcanoes tend to be very
explosive
 Sometimes melted snow and ice mix with ash
to form a lahar, which is like one nasty
mudslide
Extraterrestrial Volcanoes
 The Moon has evidence of lava flows Galileo
thought these were water, that’s why he called
them SEAS (Maria)
 Mars has the largest known volcano in the
Solar System, Olympus Mons
 Venus-has evidence of volcanoes, some may
be active today
 Io- Jupiters 3rd largest moon is very
volcanically active
More Landforms
 Calderas-large crater shaped basin
formed after a volcano collapses
 Crater Lake in Oregon is an example
 Lava plateau-when lava pours out of a
long narrow crack in the Earth and
spread out over a large area (Columbia
Basin in NW USA)