Volcanoes Chapter 13 Volcanic eruptions cause some of

Volcanoes Chapter 13

Volcanic eruptions cause some of the most dramatic changes on the earth’s surface.

Some of them are more powerful than an atomic bomb.

Scientists can learn more about volcanoes by studying the relationship of temperature and pressure deep within the earth.

In most of the earth, the extremely high pressures at extreme depths keeps the rock solid. However, the smallest of fluctuations in pressure can cause the rock to melt and form magma (liquid rock under the earth’s surface).

When magma reaches the surface, it changes. It loses gases (sulfur dioxide and carbon dioxide) and water vapor and certain chemical reactions may occur with oxygen. Liquid rock above the earth’s surface is called lava.

Magma can form under three conditions: § An increase in temperature § A decrease in pressure § The addition of fluids which may affect melting point.

As magma forms, it melts surrounding rock and, since it is less dense than the surrounding rock, it travels upward along fissures. Any activity that includes the movement of magma on the surface of the earth is called volcanism.

Volcanism happens along major plate boundaries (both convergent and divergent). One of the most active volcanic zones is the Pacific Ring of Fire.

Major volcanic zones may occur along subduction zones.

When magma moves upward but does not reach the surface, the action is termed intrusive. The results of this activity are called plutons. They may be as large as batholiths and form the root for entire mountain ranges or as small as paper thin dikes.

Volcanic Eruptions Section Two

Volcanoes are formed by two general types of magma: • Free flowing mafic magma that is rich in iron and magnesium. It is dark in color and flows freely leading to quiet eruptions. Mafic magma is formed by the melting of the ocean plates. • Firm, light-colored felsic magma that is rich in silicates. It is light in color and does not flow but creates explosive plugs. It is formed by the melting of continental plates.

Eruptions may be: l l l Quiet – Eruptions occur Hawaiian style with lakes and rivers of free flowing lava. Explosive – Pyroclastics are blown over much of the countryside. Varied – The volcano goes through both explosive and quiet stages.

The largest amount of magma comes to the surface at the midocean ridges quietly as pillow lava. An exception is Iceland (the only midocean ridge which is above the surface of the ocean) where a rift zone tears the country in two.

Quiet eruptions of the ocean floor variety (mafic lava) also occur in Hawaii, a typical hot spot.

As felsic lavas erupt various pyroclastics are ejected: Ø Volcanic ash (less than 2 mm) is launched high into the atmosphere. Ø Particles less that. 25 mm are considered dust. Ø Pebble sized particles are called cinders or lapilli. Ø Bombs are ejected as liquid lava and solidify as they travel through the air. Ø Blocks are segments of rocks the size of a house that are launched into the air.

Sometimes pyroclastics are exploded, tumbled, and ground together, mixed with heavy gases such as sulfur and carbon dioxide and sent down the mountain side as a pyroclastic cloud.

Eruptions may produce deadly mudflows and flooding as the billions of tons of glaciers melt on top of the mountain.

Quiet mafic flows may produce: Pahoehoe or rope lava Aa or jagged chunks Block lava

The type of eruption affects the formation of the volcanic cone.

Quiet eruptions form gently sloping shield cones.

Varied eruptions form composite cones or stratovolcanoes.

Explosive eruptions form cinder cones.

The magma chamber may empty so fast that the cone collapses into the ground forming a cauldera.

Scientists constantly look for a pattern of events that may indicate a volcanic eruption. • Changes of temperature and pressure in rocks. • Small earthquakes • Surface bulges on the volcano • Emission of gases • Knowledge of prior eruptions • Animal behavior