Volcanoes

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