Volcanism

Volcanism Volcanoes were my first love as a geology student. After a field trip to witness erupting volcanoes in Central America, I was hooked. I went on to return to Central America two more times to undertake field work for my Senior Thesis and then my Master's Thesis. After that, I found other things in geology that were just as intriguing to me but I still love to go and visit volcanoes wherever they are. It is a high 'sex appeal' topic. Students never seem to tire of hearing about volcanoes. On this page we will try to address many different aspects of volcanism. I'll provide examples and photographs from volcanoes I have studied or visited and try to provide you with a strong background about the nuts and bolts of volcanism.
Volcanoes as Geosystems As we discussed in the chapter about igneous rocks, Earth has a geothermal gradient so that the deeper we go, the hotter it gets. Eventually, a depth is reached where it is hot enough to melt rock. Because magma is less dense than the surrounding rock, it rises toward the surface, eventually creating and occupying a magma chamber. When a magma chamber empties, a volcanic eruption occurs on the surface. The volcanic geosystem is the entire series of events and processes including the rocks and magmas that are components of volcanism. Volcanoes comprise an important geosystem because 1) volcanism is fundamental to the plate tectonic process for constructing Earth's crust; 2) eruptions can be a major natural hazard; and 3) rocks erupted from volcanoes provide important clues about Earth's interior.
Volcanic Deposits Volcanic products fall into two broad eruptive categories: 1) effusive rocks and 2) explosive rocks. Effusive rocks are lavas that flow slowly along the surface. Although they can cause a lot of property damage, they seldom cause death to humans because people can usually outwalk or outrun them. Explosive rocks are erupted when subterranean pressures exceed the strength of the rock and blow it apart. Volcanic ash is hurled high into the atmosphere and may sweep swiftly down the slopes of the volcano, trapping helpless fleeing villagers at its base.
Types of Lava Several important properties of lava determine the types of eruptions and volcanic products that are likely to be produced. Perhaps the most important property of a lava or magma is its viscosity. Viscosity is defined as the ability of a fluid to flow. The easier it flows, the less viscous it is. Oil is more viscous than water. All lavas are much more viscous than oil. Some lavas are more viscous than other lavas. Their viscosity depends on things like their water content, temperature, and chemical composition. The higher the water content, the less viscous the magma becomes. The higher the temperature, the less viscous the magma becomes. The higher the silica content, the more viscous the magma becomes. In the last chapter we learned that mafic (basaltic) magmas have higher melting temperatures than more sialic (rhyolitic) magmas. Sialic magmas have more silica (quartz) than mafic magmas. Because most mafic magmas are generated at mid-ocean ridges or through subduction, processes in which water is introduced to hot asthenosphere, it should not surprise you that mafic magmas have a higher water content than sialic magmas. Gases dissolved in magmas expand as the pressure on the magma drops as it approaches the surface. Less viscous magmas allow the gases to form bubbles. Viscous magmas do not allow bubbles to form. Instead, the internal pressure builds until it overwhelms the strength of the magma and blows it apart. Given this information, it is easier to understand why basaltic magmas tend to produce mostly effusive eruptions while rhyolitic magmas usually produce explosive eruptions.
Basaltic Lavas Basaltic lavas erupt at high temperatures, generally between 1000º-1200º C and are usually very fluid. They can flow great distances (50 to 100 km) before they cool, although most cool within 10-20 km from their source. On very steep slopes these flows can move at 100 km/hr but typical flat land speeds are just a few km/hr. The largest eruption in recorded history occurred in Iceland in 1783, covering more than 400 square kilometers.
		Basaltic lava from Kilauea on the island of Hawai'i. (Photo by Jim Reynolds)
Flood Basalts Perhaps the most spectacular type of eruption is that of flood basalts. These very fluid lavas can travel hundreds of kilometers and cover thousands of square kilometers. Fortunately, the only historical flow that would come close to fitting into this category is the Lakagígar flow in Iceland in 1783, mentioned above. Flood basalt flows in the Columbia River Basin of Washington and Oregon during the Miocene and Pliocene covered tens of thousands of square kilometers with successive flows with a total volume of millions of cubic kilometers.

The Lakagígar eruption in Iceland issued from a long fissure running between Katla and Laki volcanoes on top of the mid-ocean ridge. (Photo by Jim Reynolds)	The Columbia River Plateau is a predominantly flat area of stacked lava flows that were erupted during the Miocene and Pliocene. Rivers have cut through the lavas. (Photo by Jim Reynolds)	Iguazú Falls , on the border between Argentina and Brazil, is formed where a river, that flowed on top of two Cretaceous flood basalts, cascades over the side at the flows' edge. Each flow is about 100 m thick. (Photo by Jim Reynolds)
Pahoehoe Flows Even very fluid lava flows, such as those found in Hawai'i and Iceland, are still quite viscous. When the front of one of these flows ceases to move, new lava, arriving from behind, causes the more viscous earlier lava to accordion forming a ropy-looking lava surface. Pahoehoe is the Hawai'ian word for ropy.

Pahoehoe texture forms readily at all scales on the fluid Hawai'ian lava. The glowing portion of the central blob is about 50 cm across. (Photo by Jim Reynolds)	Pahoehoe splays with the classic ropy texture are seen all over Hawai'i. (Photo by Jim Reynolds)	When pahoehoe flows reach a complex obstacle, all sorts of fanciful shapes can form. (Photo by Jim Reynolds)
Aa Flows Aa is another Hawai'ian word and is reputedly what people saw when they walk on this sharp, clinkery lava with bare feet. Since the natives did not wear shoes, there may be some truth in it. Aa is more viscous than pahoehoe because it has lost most of its gases, which are mostly water vapor. A lava flow often shows pahoehoe texture near its source and aa textures farther away. Thee aa texture forms because the cooled rock breaks instead of coiling when pressure is exerted on it from behind by advancing lava.

In these three images, aa is the dark lava and pahoehoe is the light. Even though they are from the same lava flows, this difference occurs because the smoother pahoehoe reflects light better than the rough aa.(Photo by Jim Reynolds)	The image at the left and this image were taken from the top of the Holei Pali, a steep fault escarpment on the southeastern side of Hawai'i. Lava flows from Kilauea's East Rift Zone flow over the edge of this escarpment, down to the plain, on onward to the ocean. (Photo by Jim Reynolds)	This image of the Holei Pali is taken from the ocean looking at the scarp. Again, the lighter colored lavas are pahoehoe and the dark are aa. The yellowish areas are vegetation that had not yet been covered by lava. (Photo by Jim Reynolds)
Blocky Flows Another type of lava flow that most texts don't describe is the blocky flow. These are usually associated with more viscous lava types but I choose to include it here with the other flow types. A blocky flow is just what it says it is: big blocks of lava. Like aa, they are broken by pressures from upflow but instead of little bits, they make big blocks.

This blocky basalt flow east of in the MacKenzie Pass, Oregon flowed across the surface around 1500 years ago, issuing from Belknap Peak. (Photo by Jim Reynolds)	The large gray flow at the center of the image is an 800-year old blocky obsidian lava flow at Landmannalaugar, Iceland. Obsidian is nearly 100% volcanic glass, usually with some pumice mixed in. Although dark in color, almost all obsidian is rhyolitic in composition. (Photo by Jim Reynolds)	This blocky dacitic lava flow is descending off of Santiaguito volcano in Guatemala. Notice the levee built up on its far side. Because it is so viscous, it only moves a few meters/day. The surface is cool enough to walk on but it is still molten deeper down. (Photo by Jim Reynolds)
Pillow Lavas Lavas that erupt underwater tend to form bulbous pillow-like masses. This is because when fluid lava flows underwater, the outer surface cools quickly. The process is analogous to squeezing a tube of toothpaste. New lava pushing from behind blows the pillow mass up like a balloon until it cracks open extrudes a new pillow. A chain of pillows gradually snakes across the ocean or lake floor.
	Steam rises as basaltic lava from Hawai'i's Kilauea volcano enters the sea along the southeastern coast of the island. Divers have shot video of these lavas forming pillows just a few meters out from the shoreline. (Photo by Jim Reynolds)	Ordovician-aged pillow lavas near Thetford Mines, Quebec indicate that this area was once on the ocean floor. In fact, the upper part of oceanic crust is composed almost entirely of pillow lavas and covered by a thin veneer of sediment. (Photo by Jim Reynolds)

Rhyolitic Lavas Rhyolitic lavas are usually a light color. An exception to this is obsidian. Obsidian is just about 100% volcanic glass. Obsidian lavas are probably hotter than most rhyolitic lavas because they tend to have few, if any, included crystals. As they rise to the surface the lower overlying pressure allows the gases inside the lava to expand and break the glass into fine bits called pumice. It is not uncommon to see pumice and obsidian intermixed.All rhyolites are very viscous and their flows move just a few to a few 10's of meters/day.
	The gray mass at the center of the photo is the Big Obsidian Flow that erupted in Oregon's Newberry Caldera about 1300 years ago. (Photo by Jim Reynolds)	The obsidian in the Newberry Caldera is dark black but is laced with large amounts of pumice. The weight of the lava caused enough subsidence to create a sag pond at the front of the flow. (Photo by Jim Reynolds)
Andesitic Lavas Andesitic lavas are intermediate between basaltic and rhyolitic flows. They tend to produce more blocky flow than any other type. Most of the world's classic volcanic cones are composed primarily of andesitic lavas.		The three andesitic cones south of Guatemala City are, from left to right, Fuego, Acatenango (in clouds) and Agua. Fuego is a moderately active volcano, erupting several times each century. Acatenango has had a couple of historic eruptions. Agua, however, remains silent. Because the suburbs of the capital now come right to its base, it is consider to be one of Central America's most dangerous volcanoes. (Photo by Jim Reynolds)
Textures of Lavas Lavas can have a few other important textures. The most common occurs when gas bubbles become frozen in the lava before they can bubble out of the top of the flow. These bubbles are called vesicles. A lava with many vesicles is called a vesicular lava. Not surprisingly, lavas tend to be most vesicular at the tops of lava flows so this texture can be used to distinguish between flows. When lavas are buried, low-grade metamorphic minerals called zeolites grow in the vesicles. A basalt exhibiting this is called an amygduloidal basalt. Extremely vesicular lava, such as that found in the cinders of cinder cones is given the name scoria. Scoria is most often associated with basaltic and andesitic eruptions. The extremely vesicular type of rhyolite is called pumice. Pumice is so light that it can float on water. After the great eruption of Krakatoa and its subsequent tsunami in Indonesia, in 1883, rafts of pumice carrying the skeletons of people who had survived the tsunami washed up on the east coast of Africa.