Igneous Rocks

Extrusive rocks and Volcanism

Magma types - Basaltic, Andesitic, Rhyolitic

Composition of magma

Gas, viscosity, and eruption style

Tephra, nuee ardente

Volcano types - shield, composite, cinder cone

Fissure eruptions

The only places where one can see igneous rocks in the process of forming are where molten rock is reaching the surface of the earth ? volcanoes (from the Roman God of fire, Vulcan).

Volcanoes are places where molten rock (magma) is extruded as lava. In addition to molten rock, magma also contains some solid crystals, as well as dissolved gases (mostly water and CO2).

This heterogeneous mixture is often referred to as a melt. Magma that reaches the surface of the earth generally has a temperature between 1000° and 1400° C.

There are a range of different compositions that magma can have that produce 3 different types of volcanoes.

Volcanic eruptions can also range from explosive to non-explosive. To understand the different types of volcanoes and their eruptions, we need to know something about magmas.

Differences in magmas: The primary compositional difference in magma is in the amount of silica that makes-up the melt. Three distinct types of magma are most common. All are dominated by silica, but the silica content can vary from 50% up to about 70%.

Basaltic magma 50% silica.

Andesitic magma 60% silica.

Rhyolitic magma 70% silica.

The principle structural difference between these magmas is in their viscosity. The complex silica ion likes to bond to others of its kind to form complex three dimensional networks. The greater the amount of silica in the melt, the more bonding occurs and the stickier or more viscous the magma becomes. Also, as magma cools it becomes more viscous.

Another important difference between magmas is in the amount of dissolved gas that they contain. Gas content typically varies from .2% to 3%. Volcanic gases are mostly water vapor and carbon dioxide, with smaller amounts of more noxious gasses such as hydrogen sulfide.

Gas in magma behaves just as gas in carbonated beverages does. Deep in the crust the magma is under pressure and can thus hold more dissolved gas. As magma rises to the surface, the pressure on the magma decreases and the gas comes out of solution, sometimes explosively.

Viscosity and Gas: The more viscous the magma is, the more difficult it is for gas to escape quickly through the lava. Gas pressure that cannot escape from magma will build-up beneath and within the magma.

We are now ready to understand the three general types of volcanoes...

Basalt magma eruptions (Shield Volcanoes)

Because basaltic magma is relatively low in silica (50%) it tends to erupt non-explosively because the fluid nature of the lava permits the gas to escape easily. Even so, the initial release of pressure on a gas-rich basaltic magma can produce a burst of spectacular fountaining. However, when the fountaining dies down after most of the gas has escaped, the lava simply oozes out of the volcanic opening.

Initially, basaltic magmas flow very easily because of their high temperature. They solidify to form a relatively smooth, ropey lava called Pahoehoe.

However, after moving for some distance the lava mass begins to cool and clump, slowing down its rate of flow. The lumpy, jagged mass of rock produced now is called AA.

Because of their fluidity, basaltic lavas can flow for great distances, aided by their tendency to form lava tubes ? roofed channels through which the lava can flow and remain hot.

If basaltic lavas flow from a point source they will form volcanoes called shield volcanoes. Shield volcanoes are extremely wide (because the lava can flow and spread-out over a wide area). Shield volcanoes are by far the largest volcanoes in the solar system by virtue of the incredible volume of lava they can disgorge.

The largest shield volcano on the earth is Mona Loa, the large volcano on the Big Island of Hawaii. If measured from its base on the sea-floor to its summit, Mona Loa is 5.5 miles high, significantly higher than Mt. Everest. Other shield volcanoes include the Tahiti, Samoa, and the Galapagos Islands.

Shield volcanoes are associated with hot spots, localized regions of high heat flow in the mantle that are not particularly well understood. The Hawaiian islands have been produced by the pacific plate moving west over a stationary hot spot in the mantle.

Another type of basalt flow is associated with larger regions of mantle upwelling along mid-ocean ridges and continental rifting zones. These are linear regions of lava eruption called fissure eruptions. Because of their linear nature, fissure eruptions produce broad expanses of basalt.

In the case of mid-ocean ridges, this basalt forms new seafloor where lithosphere plates are diverging.

On continents, fissure eruptions associated with hot spots or rifting can also produce large regions of basaltic lava called flood basalts. For example, the Columbia River Plateau in Washington, Oregon, and Idaho formed from lava flows that erupted from a series of fissures between 16 and 13 million years ago to cover an area of about 200,000 km2 with an average of 50 meters of basalt. The Watchung Hills of northern New Jersey are the eroding remains of basalt lava flows that erupted when North America and Africa rifted apart some 200 million years ago.

Shield volcanoes can also be observed on Venus and Mars, and every time you look at the moon you see in the dark areas the massive flood basalts that filled the mare basins some 3 to 4 billion years ago.

Andesitic magma eruptions (Composite/strato Volcanoes)

Because of the higher silica content of andesitic magma it is more viscous than basaltic magma. Andesitic volcanoes tend to be associated with subduction zones where seafloor is sliding back into the mantle. The ‘ring of fire’ around the margin of the pacific ocean is an almost continuous circle of andesitic volcanoes sitting above subduction zones.

The andesitic magma forms from a combination of water saturated seafloor basalt and sediments from the continent (continental crust is high in silicates) that is melted as the plate descends at a subduction zone.

Andesitic magmas do not flow as readily as basalts, so they tend to pile-up to form large volcanoes. Furthermore, their higher viscosity makes them prone to highly explosive eruptions. Andesitic magmas form what are called stratovolcanoes. Stratovolcanoes build to great heights through a combination of explosive eruption followed by (once the gas pressure is released) viscous flows that cover and cement the tephra. This is why these volcanoes are also called composite volcanoes.

Wait a minute you ask, tephra, what is tephra?

Explosive eruptions blast large quantities of volcanic rock out of the volcano. Some of this rock is excavated from the volcano itself, the rest consists of fragments of lava that has cooled in the air. This material accumulates around the volcano as pyroclastic deposits of tephra. Large fragments are called bombs, gravel size tephra is called lapilli, fine-grained tephra is called ash.

When an andesitic volcano erupts, it can blow a column of tephra straight up into the air, with the ash particles reaching the stratosphere where the quickly spread around the globe.

Erupting tephra can also rush down the slopes of the volcano carried and buoyed by hot gases. This flowing mass of red-hot material is called a nuee ardente (Fr. ‘glowing cloud’) has been known to travel up to 60 miles away from the volcano, reaching speeds of hundreds of miles per hour.

A.D. 79, Pliny the Younger is visiting his uncle, the Roman naturalist Pliny the Elder in the Roman resort city of Pompeii on the coast of Italy at the bay of Naples. A stratovolcano, Mt. Vesuvius, erupted sending hot clouds of ash rushing down on the city, burying it and its inhabitants. Elder Pliny died trying to rescue a friend. His family and nephew survived to write an account of the eruption. Pompeii was completely buried and almost forgotten until modern archeological excavations revealed the entombed town.

1883, May 8 Mt. Pelee on the island of Martinique erupts explosively, sending a nuee ardente rushing down on the city of St. Pierre (considered by many to be the Paris of the Caribbean) at app. 95 mph. The heat within the cloud was enough to melt glass and twist metal. All 30,000 inhabitants of the town were killed instantly. Only two survived, a prisoner in solitary confinement, and a one lucky shoemaker. Both were badly burned.

1980, May 18, 8:39 am. Mt St. Helens in the Cascade range in Washington State erupts with both a sideways and upward blast that removes more than a cubic mile of rock from the mountain in an instant. Loss of human life was about 100 people (including 5 geologists who were monitoring the eruption) and property damage was in the billions. As fell over cities (Yakamha, Vancouver, Missoula) up to 400 miles away like snow. Spirit lake at the base of the volcano was almost totally destroyed, trees around the mountain were flattened like matchsticks for miles away from the blast. The Toutle River went up in temperature from its normal 50° F to nearly 90° F, causing trout to be seen leaping onto the shore. 70 million fish are estimated to have died.

Rhyolitic volcanoes (Cinder Cone Volcanoes)

Because the rhyolitic magma is so viscous, rhyolitic volcanoes tend to erupt mostly tephra that accumulates into a distinctive cinder cone. The size and shape of the cone depend on the stability of the tephra as it piles up. Rhyolitic volcanoes are primarily found on continents where rifting and hot spots can melt the silica-rich continental crust.

Rhyolitic volcanoes often produce layered deposits of volcanic ash called tuff (as do stratovolcanoes as well). Additionally, the silica-rich rhyolitic magmas commonly produce solid volcanic glass (obsidian), and frothy volcanic glass (pumice).