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 A volcano is the
landform that develops
around an opening in
Earth’s crust where
magma, gas, and dust
comes out.
 Magma rises because it’s
 1. Hot
 2. Less Dense than the
surrounding materials.
3 Locations for Volcano Formation
 Divergent Boundary
 Most of the magma that reaches Earth’s surface
comes out at a divergent boundary.
 This creates new oceanic crust (sea-floor spreading)
 Example: Mid-Ocean Ridge
 Subduction Zone
 Volcanoes always form on the top plate.
 These volcanoes tend to be explosive due to friction.
 Examples: Andes Mountains, M t. Saint Helens
 Hot Spot
 Ex: Hawaii
 ***The hot spot does not move as the plate moves
across it***
Two Types of Material in Volcanic
Eruptions
 Lava-liquid rock on the
surface
 Magma that reaches Earth’s
surface
 Pyroclastic material
 “Pyro”=fire




clastic=rock
Blocks-HUGE rocks
Bombs-big rocks
Lapilli-little rocks
Ash
Two Types of Eruptions
 Non-Explosive
(quiet)
 Causes a greater
threat to property
than human
life…why?.....humans
can run from it!
 Lava Flow- rivers of
red hot lava
 Lava hardens upon
itself
 Creates shield
volcanoes
 Explosive Eruption
 Clouds of hot debris and gases shoot out of the
volcano at supersonic speeds.
 Dust size particles can remain in the air for years
in the atmosphere.
 Large blasts can demolish rock formations and
shrink the volcano instead of helping it to get
bigger.
Mt. Saint Helens
Before
After
 Size of explosion depends on how much
water there is! Why?
 Water turns into vaporSteam
 Steam builds pressure
 Pressure causes explosions
 **Magma with a lot of water will explode!
So let’s review Volcano Vocab…
 Magma-hot, molten rock deep within Earth
 Lava-magma that erupts and reaches Earth’s
surface.
 Pyroclastic material- Magma that erupts as
fragments of molten rock and solidifies in the
air.
 Vent-opening in the Earth’s crust where
magma reaches the surface.
 Caldera-large hole that forms when the roof a
volcano collapses.
Crater Lake, Oregon
Types of Lava
 Aa (pronounced AH-AH)

very stiff & jagged. Moves
slowly.
 Pahoehoe Ropy, flows like candle
wax, shiny surfaces with
wrinkles.
 Pillow Lava
 Forms when lava meets
the surface under water
and is in the size & shape
of a pillow.
3 types of Volcanoes
1. Shield Volcanco

Built of out layers of non-explosive eruptions
Lava is very runny (thin) so it spreads out over a
wide area




Creates gentle sloping sides.
Formed by hot-spots and divergent boundaries
Example: Mauna Loa (Hawaii-Largest Mountain
from the sea floor)
2. Cinder Cone Volcano

Small volcanic cones made entirely of
pyroclastic material from moderate eruptions
 Steeper slope, narrow base
 Usually only erupt for a short time
 Erodes quickly because the material is not
cemented together like lava.
 Example: Paricutin (Mexico)
3. Composite Cone Volcano

Aka: STRATOVOLCANO
 Most common type of volcano
 Explosions of pyroclastic material and lava flow.
 Alternating layers of pyroclastic material and
lava
 Broad bases and sides
 Form at subduction boundaries
 EXAMPLE: Mt. Fuji (Japan) & Mt. Saint Helens