Chapter 3 Volcanoes – Flashcards

Most volcanoes occur along diverging plate boundaries, such as the mid-ocean ridge, or in subduction zones around the edges of oceans.

Island arcs are formed from subduction. Magma forming from the subducting plate seeps upward through cracks in the crust and breaks through the ocean floor, creating volcanoes.

Magma from deep within the mantle melts through the crust above it.

Eventually, the plate's movement will carry the island of Hawaii away from the hot spot.

Because liquid magma is less dense than the surrounding solid material, magma flows upward into any cracks in the rock above. As magma rises toward the surface, the pressure decreases. The dissolved gases begin to separate out, forming bubbles. A volcano erupts when an opening develops in weak rock on the surface. During a volcanic eruption, the gases dissolved in magma rushes out, carrying the magma with them.

A volcano erupts quietly if its magma flows easily. Quiet eruptions produce two different types of lava: pahoehoe and aa. Lava flows pour from vents, setting fire to and then burying everything in their path. If its magma is thick and sticky, a volcano erupts explosively. During an explosive eruption, a volcano can belch out hot, burning clouds of volcanic gases as well as cinders and bombs. Although quiet eruptions and explosive eruptions involve different volcano hazards, both types of eruption can cause damage far from the crater's rim.

Volcanic ash can bury entire towns, damage crops, and clog car engines. If it becomes wet, the heavy ash can cause roofs to collapse. If a jet sucks ash into its engine, the engine may stop. Eruptions can also cause landslides and avalanches of mud, melted snow, and rock.

This type of lava is pahoehoe. It is fast-moving, hot lava that hardens into a rippled surface. The type of magma that produce it must be basalt. Basalt, a dark-colored volcanic rock, forms from low-silica lava that flows readily.

Rock and other materials formed from lava and ash create a variety of landforms including shield volcanoes, composite volcanoes, cinder cone volcanoes, calderas, and lava plateaus. Repeated lava flows during quiet eruptions gradually build up a broad, gently sloping volcanic mountain known as a shield volcano. Layers of lava alternate with layers of ash, cinders, and bombs in a composite volcano, which has both quiet and explosive eruptions. When cinders erupt explosively from a volcanic vent, they pile up around the vent, forming a cone-shaped hill called a cinder cone. A caldera is the large hole at the top of a volcano formed when the roof of a volcano's magma chamber collapses. The hole is filled with pieces of the volcano that have fallen inward, as well as lava and ash. Some eruptions of lava form high, level areas called lava plateaus.

First, lava flows out of several long cracks in an area. The thin, runny lava travels far before cooling and hardening. Again and again, floods of lava flows on top of earlier floods. After millions of years, these layers of lava can form high plateaus.

Features formed by magma include volcanic necks, dikes, and sills, as well as batholiths and dome mountains.

A dome mountain forms when rising magma is blocked by horizontal layers of rock. The magma forces the layers of rock to bend upward into a dome shape. Eventually, the rock above the dome mountain wears away.

Enormous eruptions may empty the main vent and the magma chamber beneath a volcano. The mountain becomes a hollow shell. With nothing to support it, the top of the mountain collapses inward. The huge hole left by the collapse of a volcanic mountain is called a caldera.

Venus has about 150 large volcanoes measuring between 100 and 600 kilometers across and about half a kilometer high. Like Earth, Venus has volcanic mountains and other features that are probably made of thin, runny lava. On Mars, there are large shield volcanoes similar to those on Venus and Earth, as well as cone-shaped volcanoes and lava flows. Mars also has lava plains that resemble the lava flows on the moon.

Io and Triton have volcanic features very different from those on Earth, Mars, and Venus. On Io, sulfur volcanoes erupt like fountains or spread out like umbrellas above the colorful surface. The eruptions on Triton involve nitrogen. It is possible that heat from the sun melts some of the frozen nitrogen underneath Triton's surface. The liquid nitrogen then expands and erupts through the planet's icy crust.

The biggest volcano on Mars is the largest mountain in the solar system. This volcano, Olympus Mons, is a shield volcano similar to Mauna Loa on the island of Hawaii, but much, much bigger.

There are far fewer volcanoes on Mars than on Venus. Olympus Mons on Mars is over eight times taller than Theia Mons on Venus. The volcanoes on Mars and Venus don't seem to be active, but have a long history of volcanic activity.