3. Composite Cones

Earth’s most picturesque yet potentially dangerous volcanoes are composite cones or stratovolcanoes. Most are located in a narrow zone that rims the Pacific Ocean, called the Ring of Fire. This active zone consists of a chain of continental volcanoes that are distributed along the west coast of the Americas, including the large cones of the Andes in South America and the Cascade Range of the western United States and Canada.

The classic composite cone is a large, nearly symmetrical structure consisting of alternating layers of explosively erupted cinders and ash interbedded with lava flows. A few composite cones, e.g. Italy’s Etna, display very persistent eruption activity, and molten lava has been observed in their summit craters for decades. Mount Etna has erupted, on average, once every 2 years since 1979. Just as shield volcanoes owe their shape to fluid basaltic lavas, composite cones reflect the viscous nature of the material from which they are made. In general, composite cones are the product of gas-rich magma having an andesitic composition. However, many composite cones also emit various amounts of fluid basaltic lava and occasionally pyroclastic material having rhyolitic composition. Relative to shields, the silica-rich magmas typical of composite cones generate thick viscous lavas that travel less than a few kilometers. In addition, composite cones are noted for generating explosive eruptions that eject huge quantities of pyroclastic material. A conical shape, with a steep summit area and more gradually sloping flanks, is typical of many large composite cones.

During the early stages of growth, lavas tend to be more abundant and flow greater distances from the vent than lavas do later in the volcano’s history. This contributes to the cone’s broad base. As the volcano matures, the shorter flows that come from the central vent serve to armor and strengthen the summit area. Consequently, steep slopes exceeding 40 degrees are sometimes possible. Two of the most perfect cones—Mount Mayon in the Philippines and Fujiyama in Japan—exhibit the classic form of a composite cone, with its steep summit and gently sloping flanks.

Task 3. Find all the proper nouns (geographic names) in the text, copy them out into the notebook, transcribe them. Be sure to know the location of the objects on the map.

Task 4. Compare and contrast the three main types of volcanoes (consider size, composition, shape, and eruptive style).

Individual work

Task 1. Read the texts and translate them into Ukrainian (in written form). Build up a list of key terms to the text.

Materials Extruded during an Eruption: gases and pyroclastic materials

Gases. Magmas contain varying amounts of dissolved gases (volatiles) held in the molten rock by confining pressure, just as carbon dioxide is held in cans and bottles of soft drinks. As with soft drinks, as soon as the pressure is reduced, the gases begin to escape. Obtaining gas samples from an erupting volcano is difficult and dangerous, so geologists usually must estimate the amount of gas originally contained within the magma. The gaseous portion of most magmas makes up from 1 to 6 % of the total weight, with most of this in the form of water vapor. Although the percentage may be small, the actual quantity of emitted gas can exceed thousands of tons per day.

Occasionally, eruptions emit colossal amounts of volcanic gases that rise high into the atmosphere, where they may reside for several years. Some of these eruptions may have an impact on Earth’s climate.

The composition of volcanic gases is important because they contribute significantly to our planet’s atmosphere. Analyses of samples taken during Hawaiian eruptions indicate that the gas component is about 70 % water vapor, 15 % carbon dioxide, 5 % nitrogen, and 5 % sulfur dioxide, with lesser amounts of chlorine, hydrogen, and argon. Volcanoes are also natural sources of air pollution—some emit large quantities of sulfur dioxide, which readily combines with atmospheric gases to form sulfuric acid and other sulfate compounds.

Pyroclastic materials. When volcanoes erupt energetically they eject pulverized rock, lava, and glass fragments from the vent. The particles produced are referred to as pyroclastic materials (pyro = fire, clast = fragment). These fragments range in size from very fine dust and sand-sized volcanic ash (less than 2 mm) to pieces that weigh several tons. Ash and dust particles are produced when gas-rich viscous magma erupts explosively. As magma moves up in the vent, the gases rapidly expand, generating a melt that resembles the froth that flows from a bottle of champagne. As the hot gases expand explosively, the froth is blown into very fine glassy fragments. When the hot ash falls, the glassy shards often fuse to form a rock called welded tuff.

Somewhat larger pyroclasts that range in size from small beads to walnuts are known as lapilli (“little stones”). These ejecta are commonly called cinders (2–64 mm). Particles larger than 64 mm in diameter are called blocks when they are made of hardened lava and bombs when they are ejected as incandescent lava. Because bombs are semimolten upon ejection, they often take on a streamlined shape as they hurtle through the air. Because of their size, bombs and blocks usually fall near the vent; however, they are occasionally propelled great distances.

Some materials are also identified by their texture and composition. In particular, scoria is the name applied to vesicular ejecta that is a product of basaltic magma. These black to reddish-brown fragments are generally found in the size range of lapilli and resemble cinders and clinkers produced by furnaces used to smelt iron. When magmas with intermediate (andesitic) or felsic (rhyolitic) compositions erupt explosively, they emit ash and the vesicular rock pumice. Pumice is usually lighter in color and less dense than scoria.

Task 2. Look at Figures 7.2. Try to define the volcanic rocks A and B. What do you know about them?

Task 3. Look at Figure 7.3. It shows pyroclastic materials. A. Volcanic ash and small pumice fragments (lapilli) that erupted from Mount St. Helens in 1980. Inset photo is an image obtained using a scanning electron microscope (SEM). This vesicular ash particle exhibits a glassy texture and is roughly the diameter of a human hair. B. Volcanic block. Volcanic blocks are solid fragments that were ejected from a volcano during an explosive eruption. C. These basaltic bombs were erupted by Hawaii’s Mauna Kea volcano. Volcanic bombs are blobs of lava that are ejected while still molten and often acquire rounded, aerodynamic shapes as they travel through the air. Describe pyroclastic materials shown in Figure 7.3.