Volcanic Eruptions

Chapter 9 Section 1

Volcanic Eruptions

Volcanoes – areas of Earth’s surface through which magma and volcanic gases pass– In seconds a volcanic eruption can turn

an entire mountain into a cloud of ash and rock

– Helps form fertile farmland– Create some of the largest mountains on

earth

Volcanic Eruptions

During an eruption magma is forced to the Earth’s surface– Magma – molten rock below the Earth’s

surface– Lava – magma that flows on the Earth’s

surface

Nonexplosive Eruptions

Most common type of eruptions Produce relatively calm flows of lava Can release huge amounts of lava Vast amounts of the Earth are

covered with lava from non-explosive eruptions– Ex. The sea floor and the Northwest

region of the US

Nonexplosive Eruptions

Explosive Eruptions

Much rarer than non explosive eruptions

Effects can be incredibly destructive Clouds of hot debris, ash and gas

rapidly shoot out from a volcano No lava flows Molten rock is blown into tiny

particles that harden in the air

Explosive Eruptions

Ash – dust sized particles of molten rock that have hardened in the air

Ash can reach the upper atmosphere and circle the Earth for years

Larger pieces fall closer to the volcano

Explosive eruption can blas millions of tons of lava and rock from a volcano

In seconds an explosive eruption can demolish a mountainside

Explosive Eruptions

In what resembles a nuclear explosion, volcanic ash rockets skyward during the 1990 eruption of Mount Redoubt in Alaska

What is Inside a Volcano?

Magma chamber – body of molten rock deep underground that feeds a volcano

Vents – an opening at the surface of the Earth through which volcanic material passes

Magma is released from vents during an eruption

What Makes Up Magma?

The composition of magma affects how explosive a volcanic eruption is.

The key to whether an eruption will be explosive lies in the silica, water, and gas content of the magma

What Makes Up Magma?

Water and Magma are an Explosive Combination– If the water content is high an explosive

eruption is more likely– Underground there is a lot of pressure

that keeps water and other compounds dissolved in the magma

What Makes Up Magma

– When magma quickly rises the water and CO2 turn into gas which expand quickly

– When the gasses expand an explosion takes place

– Pumice – some lava is so frothy with gas when it reaches the surfaces that its solid form, pumice, can float in water

Silica-Rich Magma Traps Explosive Gases

Magma that has high silica content tends to cause explosive eruptions

Silica-rich magma has a stiff consistency

Flows slowly and tends to harden in volcano’s vents

Plugs the vent As more magma pushes up from

below pressure increases

Silica-Rich Magma Traps Explosive Gases

If enough pressure builds up an explosive eruption takes place

Stiff magma prevents water vapor and other gasses from easily escaping

Gas bubbles in the magma can expand until they explode

When they explode ash and pumice are blasted from the vent

Silica-Rich Magma Traps Explosive Gases

Magma with less silica has a more fluid runnier consistency

Because gas particles can escape more easily explosive eruptions are less likely to occur

What Erupts from a Volcano

Nonexplosive Eruptions– Lava – liquid

magma that flows from a volcanic vent

Explosive Eruptions– Pyroclastic

material – forms when magma is blasted into the air and hardens

Over years or during the same eruption a volcano’s eruption may alternate between lava and pyroclastic eruptions

Types of Lava

High Viscosity – Lava with high viscosity is stiff– Flows slowly– Ex. Blocky lava and Pahoehoe

Low Viscosity– Lava with low viscosity is more fluid– Flow more quickly– Ex. Aa and Pillow Lava

Types of Lava

Aa – lava pours out quickly and forms a brittle crust. The crust is torn into jagged pieces as molten lava continues to flow underneath

Pahoehoe – lava flows slowly, like wax dripping from a candle. Its glassy surfaces has rounded wrinkles

Types of Lava

Pillow Lava – forms when lava erupts underwater. Forms rounded lumps that are the shape of pillows

Blocky Lava – cool, stiff lava that does not travel far from the erupting vent. Blockly lava usually oozes from a volcano and forms jumbled heaps of sharp-edged chunks

Types of Lava

Types of Pyroclastic Material

Forms when magma explodes from a volcano and solidifies in the air

Also forms when powerful eruptions shatter existing rock

Size ranges from boulders the size of houses to tiny particles

Types of Pyroclastic Material

Volcanic Blocks – the largest pieces of pyroclastic matieral, pieces of solid rock erupted from a volcano

Volcanic Bombs – large blobs of magma that harden in the air

Types of Pyroclastic Material

Lapilli – “little stones” pebblelike bits of magma that hardened before they hit the ground

Volcanic Ash – forms when the gases in stiff magma expand rapidly and the walls of the gas bubbles explode into tiny, glasslike slivers. Makes up most of the pyroclastic material in an eruption

Types of Pyroclastic Material

Pyroclastic Flows

Pyroclastic Flow – Dangerous– Produced when enormous amounts of

hot ash, dust and gases are ejected from a volcano

– Can race downhill at more than 200 km/h

– Temperature at center can exceed 700°C

Pyroclastic Flows

1991 eruption of Mount Pinatubo in the Phillippines

Scientists predicted the eruption and saved 250,000 people