PART F – PREREQUISITE: Volcanoes

Introduction

Scarcely a day goes by without a volcanic eruption occurring somewhere in the world. At the end of PART D – PREREQUISITE we noted the correlation of the majority of earthquakes with plate boundaries; the correlation between boundaries and volcanic eruptions is equally strong (Fig. F1). Every century in historic time has had its share of large volcanic eruptions. This century has been no exception: Soufriere in St. Vincent, Pelee in Martinique, Mount St. Helens, and Pinatubo - among others.

In Indonesia, during the summer of 1883, an apparently dormant volcano, Krakatau, began emitting steam and ash. At that time, Krakatau was a volcano which dominated a moderate size island Fig. F2). On Sunday, August 26 of that year, the activity of the volcano increased noticeably and on the next day it blew up and the island disappeared below sea level. On that site now is simply a collection of tiny islands. The explosion was heard 4600 km away (Fig. F3). As we've already seen, huge waves were responsible for the deaths of more than 36,000 people.

The effects of Krakatau were felt around the world. We will take another short look at Krakatau later when we put it in perspective to the active plate tectonics there. About 20 km³ of volcanic debris was ejected during the eruption, with some particles blasting as high as 50 km into the stratosphere. Within 13 days the stratosphere dust had encircled the world, and for 9 months afterward the dust could be seen at sunset. The suspended dust made the atmosphere so opaque to the Sun's rays that the average temperature around Earth dropped at least 0.5^oC during 1884. Within 5 years all the dust had fallen to the ground and the climate returned to normal.

Every now and then, a truly devastating eruption reminds us of the enormous magnitude of volcanic forces. The 1991 eruption of Mount Pinatubo in the Philippines stands out as one such event. We will recount the story of Pinatubo later.

Volcanism

The first solid rocks to form on Earth’s surface were volcanic rocks. Even today, the majority of Earth’s crust – both oceanic and continental – consists of volcanic rock. Volcanic rocks and other deposits are formed from magma [magma: molten rock material that forms igneous rocks upon cooling; magma that reaches surface is called lava]. Obviously, to produce magma, we need to melt something below Earth’s surface. In PART C – PREREQUISITE section we looked at a temperature profile through Earth (take another look at Fig. D9); while the lithosphere is relatively cold and brittle, we know that the asthenosphere, which extends from about 75 to 250 km depth, reaches 1100^oC to 1200^oC – certainly high enough for rocks there to begin melting. In fact, the asthenosphere is the main source of all magma.

The melt that forms in the asthenosphere rises buoyantly – that is, it floats upward because it is less dense than the surrounding rocks. The reason is simply that the melt will be slightly richer in relatively light elements because they tend to melt out of mantle materials at lower temperatures than will the heavier elements. In some places, the buoyant magma will find a path to surface through fractures in the lithosphere (such as commonly develop at plate boundaries); in other cases, the magma may actually melt a channel to surface. In either case, the result will be a volcanic eruption at surface.

Types of Lava

Basaltic Lava

Basaltic lava, dark in color, erupts at 1000^oC to 1200^oC – close to the temperature of the asthenosphere. Basaltic lava is extremely fluid and can flow downhill fast and far. Streams of hot lava have been measured as fast as 100 km/hour, and you can see these streams almost any day of the year in places like Hawaii. In Figure 4 we see a spectacular night photo of a stream of lava rolling down the side of Mount Mayon in the Philippines in 1993. Cooling basaltic lava flowing downhill falls into one of two categories, according to its surface form. Pahoehoe (Fig. F5 lower part) forms when a highly glassy ‘skin’ congeals on its surface. The skin is dragged and twisted (by underlying flow motion) into coiled flows that resemble rope. Aa (Fig. F5 top part) is the resultant rough, jagged, and blocky form of basalt that forms when the magma loses its gases and consequently becomes more viscous than pahoehoe. It tends to move so slowly that a human can readily keep out of its path. The most common form taken by basaltic lava, however, is pillow lavas – the form developed by underwater eruptions. Geologist divers have watch pillow basalts form in such places as the ocean floor off Hawaii. Tongues of magma erupt from fractures, and soon develop tough cool skins while the interior remains fluid, resulting in ‘sausage-like’ tubes piled one on another. In Figure F6 that sausage form is apparent; in most weathered cross sections of such deposits, the individual tubes look more like pillows – thus the name.

Rhyolitic Lava

Rhyolite is light in color, is much richer in elements like Si, K and Na than basalt, has a much lower melting temperature than basalt, and erupts at temperatures of 800^oC to 1000^oC. It is not nearly as fluid as basalt, thus tends to move at least 10 times more slowly. Because it is so viscous (the opposite of fluid), it tend to pile up in thick, bulbous deposits.

Andesite Lava

Andesite is intermediate in composition to basalt and rhyolite, and also has intermediate properties.

Magma Viscosity Control Factors

Viscosity is a complicated subject, where simplification almost always leads to half-truths. It deserves consideration, however, because viscous magmas generally erupt violently. Viscosity results from the interactions of many things, but the result is that the molecules in the magma are polymerized, or grouped together in clusters. Rhyolitic magma is highly viscous, andesitic magma moderately viscous and basaltic magma highly fluid. The reasons for the viscosity differences are, primarily, twofold: first, the hotter the magma, the more fluid its motion (basalt erupts at such a high temperature, it tends to be very fluid); secondly, magmas with higher contents of silica (silicon dioxide: SiO₂) are more polymerized, and thus more viscous, than those with lower contents. The reason for the importance of silica is that the silicon-oxygen bond is very strong; the more silica in a magma, the more bonds develop and the greater the complexity of structures built by those bonds (simply silica tetrahedra [one Silicon atom bonded to four Oxygen atoms], chains of tetrahedra, sheets of tetrahedra, etc.) – thus the greater the viscosity (Fig. F7).

Eruptive Styles and Landforms

The most common eruptive styles we see are those that develop around central volcanic vents. In Hawaii, we mostly see shield volcanoes under construction (Fig. F8). Mauna Loa, for example, at about 10 km high (all but 4 km below water), is the tallest structure on Earth, with a base that has a diameter of 120 km. It is a typical shield volcano in that it has been constructed by the accumulation of thousands of thin, fluid basaltic flows, producing gently sloping sheets.

The second most-common volcanic form is the stratovolcano or composite volcano (Fig. F9a; 9b). This is the form developed by magma that is more viscous than basalt. The sides are steep and the individual layers are a mixture of andesitic or even rhyolitic lava flows with alternating beds of explosive debris commonly called pyroclastic flows (pyroclasts: fragmentary volcanic material that has been ejected into the air by violent eruption). Figure F10 shows a very large pyroclastic flow rolling down the slopes of Mount Unzen, Japan in 1991. The scientists studying this volcano were overtaken by this flow and all three died. Because the magma emitted at stratovolcano sites is extremely viscous, the central vents commonly become temporarily plugged; it normally takes considerable gas pressure from below to blow the ‘plug’ out. Obviously, these are typically highly explosive volcanoes. Mount St. Helens and the other volcanoes that line the west coast of North America are stratovolcanoes (Fig. F11). In Figure F1 you noted the correlation of volcanoes with the edges of plates. In fact, the correlation of stratovolcanoes with the subduction zones of compression plate boundaries is virtually 100%; around the Pacific Ocean, that ring of stratovolcanoes forms what is commonly called “the Ring of Fire” (Fig.12) – it seems there’s always at least one somewhere in the ring that’s erupting explosively.

Without any doubt, the most dangerous and explosive volcanic eruptions are associated with single eruptive sites called calderas. A caldera, which may exceed 50 km in diameter, marks the surface expression of a volcano immediately over a huge magma chamber. In a single eruption, the volume of pyroclastic and lava thrown out may easily exceed 1000 km³ – 1000 times the volume of most eruptions like Mount St. Helens, for example. The hot springs and geysers at Yellowstone National Park (Wyoming) occur within a very large caldera structure (Fig. F13).Some 600,000 years ago that caldera erupted 1000³ km of pyroclastic debris over nearly the whole USA. Today, the (growing) chamber beneath the Yellowstone Caldera has easily enough magma in it to do it all again.

Often lava erupts along fissures (Fig. F14) rather than from specific centers. Most commonly these occur along the ridges that mark the boundary of two plates moving apart, thus these are the under-sea eruptions that take place in such locations as the Mid-Atlantic. Imagine non-stop highly fluid basaltic lava flowing out of a fracture some tens of kilometers long! It’s not long before huge areas can be covered – thus the name flood basalts. The only flood basalt to have been witnessed by humans took place in Iceland in 1783. With virtually no warning, a 32 km long fissure (Fig.F15) opened and began spewing basalt out, forming the single largest flow on land that has ever been witnessed. The volume of 12 km³ was all expelled in a few days. About 20% of the human population and nearly all domestic animals of Iceland were killed. In the 240A course, we’ll look at the Iceland eruption and also look at three other flood basalts of ancient time: The Columbia River Plateau (USA) basalts, the Deccan Traps of India, and the Siberian Traps of Russia.

Please return to the regular lecture series.