Chapter 6

Volcanoes

SECTION 1: VOLCANIC ERUPTIONS

• Volcanoes are areas of Earth’s surface through which magma and volcanic gasses pass

Nonexplosive Eruptions

• Are occurring at this very moment on earth and on the ocean floor

• These are the most common eruptions.

• They produce relatively calm flows of lava

• Can release huge amounts of lava.

• Vast areas of the Earth’s surface, including much of the sea floor and the Northwest region of the United States, are covered with lava from nonexplosive eruptions.

Explosive Eruptions

• Are much more rare, but the effects can be much more destructive.

• During an explosive eruption, clouds of hot debris, ash and gas rapidly shoot out of a volcano.

• Instead of lava flows, explosive eruptions cause molten rock to be blown into tiny particles that harden in the air.

• The dust sized particles, called ash, can reach the upper atmosphere and can circle the Earth for years.

• Larger pieces of debris fall closer to the volcano

• Can also blast millions of tons of lava and rock from a volcano.

• In a matter of seconds, an explosive eruption can demolish an entire mountainside

What is inside a Volcano?

• A magma chamber is a body of molten rock deep underground that feeds a volcano

• Magma rises from the magma chamber through cracks in the Earth’s crust to openings called vents.

• Magma is released through these vents during an eruption.

What makes up magma?

• The composition of the magma affects how explosive an eruption is. The explosiveness lies in the silica, water and gas content of the magma.

• If the water content of the magma is high, an explosive eruption is more likely.

– Just like shaking a can of soda….The water in the magma underground remains dissolved because of high heat and pressure. When the magma moves to the surface the water and other compounds become gases. As the gases expand rapidly, an explosion can occur.

– Because some lava is so frothy from the gases when it hardens it becomes pumice!

Magma with high silica content is also explosive.

• The silica-rich magma is stiff and flows slowly, occasionally blocking the vents. As more magma pushes up below, the pressure increases and if enough pressure exists an explosion can occur.

• Stiff magma can also prevent water vapor and other gases from escaping; these gases can expand until the explode shattering the magma and ash and pumice are blasted from the vent.

What Erupts from a Volcano?

• Lava is liquid magma that flows from a volcanic vent

– Lava is released mostly from nonexplosive volcanoes

– The viscosity of lava varies greatly. High viscosity is stiff and low viscosity is more fluid

– Blocky lava and pahoehoe have a high viscosity and flow slowly

– Other types such as aa and pillow lava, have lower viscosities and flow more quickly.

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

• Produced from mostly explosive volcanoes

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

• Also from eruptions that shatter existing rock

• Range in size from tiny pebbles to huge boulders the size of houses

• Four types are vocanic bombs, Lapilli, volcanic ash, and volcanic blocks…see page 160.

• Pyroclastic Flows are produced when enormous amounts of hot ash, dust, and gases are ejected from a volcano

• This glowing cloud of pyroclastic material can race downhill at speeds of more than 200 km/hr – faster than most hurricane winds!!

• Temps at the center of this flow can exceed 700⁰C

SECTION 2: EFFECTS OF VOLCANIC ERUPTIONS

Volcanic Eruptions and Climate Change

• As volcanic ash and gases spread throughout the atmosphere they can block enough sunlight to cause global temperatures to drop.

• A small shift of temperature, such as 0.5⁰C can disrupt climates all over the world.

Ash from the eruption of Mount Pinatubo blocked out the sun in the Philippines for several days.

The eruption also affected global climate.

DIFFERENT TYPES OF VOLCANOES

Shield Volcanoes

• Are built of layers of lava released from repeated nonexplosive eruptions

• Lava is runny and spreads over a large area, creating layers, and eventually a volcano that has gently sloping sides

• Can be enormous

• Hawaii’s Mauna Kea is the tallest mountain on Earth, taller than Mount Everest when you measure from its base at the ocean floor.

http://www.youtube.com/watch?v=byJp5o49IF4

Cinder Cone Volcanoes

• Made of pyroclastic material usually produced from moderately explosive eruptions

• Forms steep slopes

• Cinder cones are small and usually erupt for only a short time

• Cinder cones often occur in clusters, commonly on the sides of other volcanoes.

• Usually erode quickly because the pyroclastic material is not cemented together.

http://www.youtube.com/watch?v=aS_xl3nu_mY

Composite Volcanoes

• Sometimes called stratovolcanoes

• Are the most common types of volcanoes

• Form from explosive eruptions of pyroclastic material followed by quieter flows of lava, forming alternating layers

• Have broad bases and sides that get steeper towards the top

• Include Mount Hood, Mount Rainier, Mount Shasta, and Mount St. Helens http://www.youtube.com/watch?v=1u1Ys4m5zY4

Other Types of Volcanic landforms

• Craters are funnel shaped pits found around the central vent at the top of many volcanoes

• Calderas are large, semicircular depressions that form when the chamber that supplies magma to a volcano partially empties and the chamber’s roof collapses

• Lava Plateaus are landforms that result from repeated eruptions of lava spread over a large area (usually from fissures or rifts)

The Columbia River Plateau formed from a massive outpouring of lava that began 17 million years ago.

SECTION 3: CAUSES OF VOLCANIC ERUPTIONS

The Formation of magma

• Magma forms in the deeper regions of the Earth’s crust and in the uppermost layers of the mantle where the temperature and pressure are very high

• Changes in pressure and temperature cause magma to form

• Rock melts when its temperature increases or when the pressure on the rock decreases

• A decrease in pressure is the most common cause of magma formation, often forming at the boundary between separating tectonic plates where pressure is decreased

The curved line indicates the melting point of a rock. As pressure decreases and temperature increases, the rock begins to melt.

Where Volcanoes Form

• A large number of volcanoes lie directly on tectonic plate boundaries

Tectonic plate boundaries are likely places for volcanoes to form. The Ring of Fire contains nearly 75% of the world’s active volcanoes on land.

When Tectonic Plates Separate

• At a divergent boundary, tectonic plates move away from each other.

• As tectonic plates separate, a set of deep cracks called a rift zone forms between the plates; mantle rock then rises to fill the gap. When mantle rock gets closer to the surface, pressure decreases causing the mantle rock to melt and form magma.

• Because magma is less dense than the surrounding rock, it rises through the rifts. When the magma reaches the surface it spills out and hardens, creating new crust.

• Lava that flows from undersea rift zones produces volcanoes and mountain chains called mid-ocean ridges.

• Most volcanic activity on Earth occurs at mid-ocean ridges.

• Iceland was created from by lava from the Mid-Atlantic Ridge and in 1963 enough lava poured out of the Mid-Atlantic Ridge near Iceland to form a new island called Surtsey.

When Tectonic Plates Collide• A convergent boundary is a place where tectonic plates collide

• When an ocean plate collides with a continental plate, the ocean plate usually slides underneath because it is more dense and thinner than continental crust

• The process of subduction is the movement of one tectonic plate underneath another

• Subduction causes an increase in temperature and pressure causing the water in the oceanic crust to be released; it mixes with the mantle rock, which lowers the rock’s melting point, causing it to melt. This body of magma can rise to form a volcano.

Hot Spots

• Hot spots are volcanically active places on the Earth’s surface that are far from plate boundaries (example are the Hawaiian islands)

• Two theories are that hot spots are directly above columns of rising magma called mantle plumes, or that they are the results of cracks in the Earth’s crust.

• Hot spots usually produce a long chain of volcanoes.

Predicting Volcanic Eruptions

Volcanoes are classified in three categories:

1. Extinct Volcanoes – have not erupted in recorded history and probably will never erupt again

2. Dormant Volcanoes – are currently not erupting, but the record of past eruptions suggests that they may erupt again

3. Active Volcanoes – are currently erupting or show signs of erupting in the near future.http://www.youtube.com/watch?v=D4QT215zYgY

• Most active volcanoes produce small earthquakes as the magma within them moves upward and causes the surrounding rock to shift.

• Before an eruption, the number and intensity of the earthquakes increase and the occurrence of quakes may be continuous. Monitoring these quakes may be the best way to predict a volcanic eruption.

• Can also study the volume and composition of volcanic gases. Changes in the sulfur dioxide to carbon dioxide ratios may indicate changes in the magma chamber below.

• As magma moves upward prior to an eruption, it can cause the Earth’s surface to swell, sometimes causing the sides of a volcano to bulge as the magma moves upward.

As if being this close to an active volcano is not dangerous enough, the gases being collected are extremely

poisonous

• An instrument called a tiltmeter helps scientists detect small changes in the angle of a volcano’s shape

• Scientists use GPS to detect the changes in a volcano’s slope that may signal an eruption

• Also using satellite images can predict eruptions by recording changes in surface temperatures and gas emissions of a volcano over time. If the site is getting hotter, the magma below is probably rising.

http://www.youtube.com/watch?v=FnDT_6V4qVw&feature=related

The Mount St Helens Story, 22:58