Lavas and life.
Perhaps you've watched it on TV or in a movie. If you were close enough to witness it in real life, odds are you were either scrambling off with family and friends in the fastest transports available. Depending on your vantage point, the eruption of an active volcano can be a thrilling drama or a terrifying experience.
Not all volcanoes erupt in the same way. Some blow their tops with fierce explosions, spewing clouds of hot gases and dust that spell almost instantaneous destruction for miles around. Others may belch out streams of hot lava with little or no explosive burst. The lava flows are equally destructive of life and property, but they move more slowly, generally giving enough warning and time for people to escape.
Lava flows consist of molten rock with dissolved and boiling gases. The molten state is caused by heat released during the decay of radioactive elements such as uranium and thorium in Earth's mantle and crust. The hot mixture, also called magma, may include minerals that crystallize either underground or during surface cooling. As the lava cools and solidifies, vegetation grows again and insects and birds begin to colonize the new land. The term lava flow is also applied to the solidified magma at the surface.
Most of us have heard of areas of active lave flows, such as in Hawaii and Iceland, but streams of lava have erupted in many more areas in the past. Wherever you live, it's likely you don't need to drive to far to see a flow preserved in the rock record. Yet most lava flows are submarine, occurring in all of the upper part of the oceanic crust, beneath a thin layer of sediment.
Lava flows are not confined to Earth. They are also common on the surfaces of rocky planets and moons of the Solar System. Much of these of our Moon is covered with lava flows.
Variety of flows
One system of classifying lava flows is based on their content of silica (silicon dioxide), which is usually a major constituent. In the Solar System, the commonest type are basalt's, with 47-52 percent silica. On Earth, submarine lavas are composed almost exclusively of basalt, but areas on land have a much greater diversity of compositional types, with silica varying from below 1 percent to over 75 percent.
The silica content affects the viscosity of the flow during the eruption. The viscosity, in turn, affects the appearance of the flow. A high silica content makes the lava very viscous, so it moves slowly to form a "dome," as at Mount St. Helens. Other factors that influence viscosity are the gas content, crystal content, and temperature.
The lowest viscosity lava flows on earth are those of an unusual magma type, called carbonatite, which erupts like muddy water from Ol Doinyo Lengai volcano in Tanzania. This lava's chemical composition is almost identical to washing soda. Elsewhere in the Solar System, lava flows can have even more exotic composition, such as the sulfur flows on Jupiter's moon Io.
Lava flows can also be classified according to their surface texture and internal structure. Take, for example, the two most common types of basaltic lava flows: pahoehoe and a'a, which are Hawaiian words. Pahoehoe, meaning satinlike, is characterized by a continuos, wrinkled or ropy surface that, when fresh, has a satiny sheen. A'a has a surface that's broken into blocks with sharp, spiny projections. The name may be an onomatopeia, reflecting the pain experienced by a person walking barefoot across this type of lava! A'a flows are formed when discharge rates are higher than pahoehoe flows. Another type of flow, with an appearance intermediate be-tween pahoehoe and a'a, has been given the more prosaic name "toothpaste lava."
Lava flows with higher silica content than basalts give rise to blocky flows that are similar to a'a but lack spiny projections. Block lava flows are rare because such magma usually erupts explosively. In a highly viscous magma, gas bubbles are unable to escape easily and usually build up to great pressures before being released with explosive force. The fragmented magma gives rise to pyroclastic rocks, not lava flows.
A pillow lava is a type of flow that forms only when magma erupts into water or wet sediment. It is the dominant type of lava flow in the submarine environment, but it also occurs in lakes and rivers and beneath glaciers by long, tubelike structures that in cross section resemble elliptical pillows.
Some extremely large sheets of lava, known as flood lavas, can cover thousands of square miles in a relatively short time. For instance, the Pomona lava flow in Idaho, Washington, and Oregon traveled at least 340 miles before coming to rest. There are no areas of active flood lavas at present, but they have been widespread in the past, in areas such as the Karoo in South Africa, and the Deccan plateau in India, and the Columbia River area in the United States. They are thought to be precursors to the breakup of continental plates and the formation of new oceans, and they have also been linked to some mass extinction events.
Life on lava flows
For the first few months after an eruption, living creatures find it hard to survive on lava flows. Initially, there is no soil to provide nutrients and no vegetation to provide cover. The flow surface is often exposed to intense sunlight and becomes dry. In addition, flows may leak toxic gases, such as hydrogen sulfide. After a period of cooling, however, lava flows begin to provide a new, open area for life to colonize again.
Solidified lava is usually composed of volcanic "glass," formed when the magma chills against air or water. (When many molten substances are cooled relatively, with insufficient time to form crystals, they become glasslike). Volcanic glass is unstable, and within months it begins to break down to form a protosoil, consisting of clays and other minerals. Basaltic lava flows produce extremely fertile red soils, especially in areas of high rainfall.
The protosoils provide nutrients for the early plant colonizers, such as algae and ferns. The plants are first established in craks and deep hollows in the flow, because that's were moisture is trapped. In Hawaii, lava flow as young as four years old bear small, red-flowering shrubs that belong to the same species (Metrosideros polymorpha) as much larger trees in the surrounding forests. The intense sunlight and relatively dry surface of the flows have favored growth of the shrubs, whose hairy leaves trap moisture.
The lack of large shrubs and trees on young lava flows means that they are often windy places, and plant and animal debris from neighboring areas is easily swept into craks and holes in the highly irregular surface. This debris provides food for several types of insects, which are often the first organisms to colonize a new lava flow, even before plants.
For instance, the wingless dark lava flow cricket of Hawaii, discovered as recently as 1973, can colonize new flows within a month after a eruption. It hides in craks during the day and emerges at night to feed on the windblown debris. The cricket is adapted to survive on only very young lava flows that have no significant vegetation. Vegetation would use up the moisture that the crickets require to prevent desiccation in the harsh sun.
Pahoehoe lava flows often travel in tubes. A lava tube can be formed by several mechanisms, but they all involve the building of a static, chilled crust over a molten interior that continues to flow until it is drained out an tubelike cave remains. The tube becomes a habitat for cave-adapted fauna, mainly insects.
In Hawaii, various insects that have no wings, no pigments, and little or no vision have adapted to living in lava tubes. They include wolf spiders, earwigs, millipedes, thread-legged bugs, and crickets. Interestingly, some of these insects have adaptations--such as increased tolerance to certain volcanic gases--that could only have developed in lava tubes, rather than in other types of caves. Lava caves also provide important habitat for bats and birds such as swiflets.
The birdlife of Hawaiian lava flows is rather sparse, but it was much greater in the past. The most famous of the resident birds is probably the nene or Hawaiian goose. It had become nearly extinct by 1951, but projects involving captive breeding and release of the birds have raised their numbers dramatically. Yet the nene's survival in the wild is by no means assured. At least nine other species of geese--seven of which were flightles--have become extinct since the arrival of Polynesian people on the Hawaiian Islands. Some of these geese may have had a lifestyle similar to that of the nene, grazing on grass and the berries of fruiting bushes on the flows.
Occasionally , the lava completely surrounds an area of land without destroying it, creating an island ecosystem called a kipuka in Hawaiian. Such a place, which is difficult for alien flora and fauna to reach, becomes a critical refuge for the dwinling population of native birds and plants. It is also an important area for research into the adaptation and evolution of species. Knowledge of the sequence of colonization and the developing ecology of a new lava flows is vital in unraveling the complex ecosystems that have been established on older flows.
Lava flows are testimony to the internal heat source of planets. They are both detroyers and sustainers of life and provide vital information for ecologists trying to understand how life colonizes new land.
Ian Skilling lectures in vulcanology at Rhodes University, Grahamstown, South Africa.