VOLCANOES.ppt..................................

A volcano is a mountian that forms when molten rock
(magma) is forced to the earth’s surface

What causes volcanoes?
Tectonic Plate Boundaries!!!
~75% world’s active volcanoes in Ring of Fire

Where do volcanoes occur?
 Divergent boundaries
 Convergent boundaries
 Hot spots

Volcanoes form around vents
that release lava, which is the
magma that reaches the
earth’s surface.

Non-explosive Eruptions
• Quiet eruption in which magma flows easily
• Usually gas dissolved in magma escapes easily
• Thin, runny lava oozes quietly from vent
• Mafic lava has low silica (silicon dioxide) content, so it is not
viscous
• Example: Hawaiian Islands

Lava: Pohoehoe
 Flows slowly, like
wax dripping from a
candle.
 Forms glassy surface
with rounded
wrinkles ‘ropey’
appearance.

Lava: A’a
• A’a is slightly stiffer.
• Pours out quickly and forms a brittle crust
which is torn into jagged pieces as molten lava
underneath continues to move.

Lava: Block
 Cooler and stiff
 Oozes from volcano
 Forms jumbled heaps
of sharp-edged
chunks.
Mexico’s Colima Volcano

Lava: Pillow
 Forms when lava
erupts underwater at
sea floor spreading
zones.
 Forms rounded
lumps the size and
shape of pillows.

Explosive Eruptions
 Thick and sticky magma will not flow out of a volcano.
Instead, it plugs the vent like a cork in wine bottle.
 Felsic lava: high silica content results in high viscosity
(resistance to flow)
 Dissolved gasses and water cannot escape the thick magma,
build up pressure that contributes to explosive eruptions.
 Example: Mt. St. Helens

Explosive Eruptions
 Clouds of hot debris and gasses shoot out from the volcano,
often at supersonic speeds.
 Molten rock is blown into millions of pieces that harden in the
air.
 Dust-sized particles can circle the globe for years in the
upper atmosphere.
 Larger pieces fall closer to the volcano.
 Can blast millions of tons of solid rock and, in just a few
minutes, can demolish rock formations that took hundred or
thousands of years to accumulate.
 Volcanoes can actually shrink due to repeated eruptions.

What Erupts from an explosive Volcano?
Pyroclastic material - rock fragments created by
explosive eruptions
•magma explodes from volcano and solidifies in the air
•existing rock is shattered by powerful eruptions
Volcanic blocks
Volcanic bombs
Lapilli
Volcanic ash

Pyroclastic Material: Size
 Volcanic blocks: solid rock blasted out of volcano
 Volcanic bombs: large blobs of magma that harden
in air
 Lapilli: “little stones” (Italian) – pebble-like bits of
magma that become solid before hitting the ground.
 Ash: forms when gases in stiff magma expand rapidly
& walls of gas bubbles explode into tiny glasslike
slivers.

Types of Volcanoes
Shield volcano
Cinder cone volcano
Composite volcano

Shield Volcanoes are produced by the accumulation of
basalt lava flows.
• Broad, dome shape
• Frequent non-explosive
eruptions
• low silica = low viscosity

The Hawaiian Islands are hot spot volcanoes (shield)

Composite Volcanoes
 Switches between quiet eruptions of flowing lava and violent
eruptions of thick, gas-rich lava (pyroclastic material). This type
of volcano has the most powerful eruptions of all.
 Composite volcanoes, also called stratovolcanoes, form tall
conical mountains.
 Example: Cascade Volcanoes.

Cinder Cone Volcanoes
 Built up from ashes, cinders, and rocks that burst from Earth during a
violent eruption.
 The rocks fall back to Earth near the opening, to form a cone. Ash
covers a larger surrounding area.
 Erupt for a short period of time, so most are not taller than 300m
(about 1000 ft)
 Occur in clusters or on sides of shield and composite volcanoes.
 Erode quickly due to pyroclastic material not being cemented together
by lava.
Mt. Etna, Italy

Craters, Calderas, and Lava Plateau
Crater
• From explosions of material out of the
vent and the collapse of material back
into vent
Caldera
• Much larger depression that forms
when magma chamber empties and
its roof collapses
Lava Plateau
• Forms when lava erupts from long cracks, or fissures, and
spreads out evenly (thousands of km)

Crater Lake is an example of a caldera, created when Mt. Mazama erupted
an collapsed around 5600 BC

VOLCANOES.ppt..................................

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