Lava
I -INTRODUCTION
Lava, molten or partially molten rock that erupts at the earth’s surface. When lava comes to the surface, it is red-hot, reaching temperatures as high as 1200° C (2200° F). Some lava can be as thick and viscous as toothpaste, while other lava can be as thin and fluid as warm syrup and flow rapidly down the sides of a volcano. Molten rock that has not yet erupted is called magma. Once lava hardens it forms igneous rock. Volcanoes build up where lava erupts from a central vent. Flood basalt forms where lava erupts from huge fissures. The eruption of lava is the principal mechanism whereby new crust is produced (see Plate Tectonics). Since lava is generated at depth, its chemical and physical characteristics provide indirect information about the chemical composition and physical properties of the rocks 50 to 150 km (30 to 90 mi) below the surface.
II -TYPES OF LAVA
Most lava, on cooling, forms silicate rocks—rocks that contain silicon and oxygen. Lava is classified according to which silicate rocks it forms: basalt, rhyolite, or andesite. Basaltic lava is dark in color and rich in magnesium and iron, but poor in silicon. Rhyolitic lava is light colored and poor in magnesium and iron, but rich in silicon. Andesitic lava is intermediate in composition between basaltic and rhyolitic lava. While color is often sufficient to classify lava informally, formal identification requires chemical analysis in a laboratory. If silica (silicon dioxide) makes up more than 65 percent of the weight of the lava, then the lava is rhyolitic. If the silica content is between 65 percent and 50 percent by weight, then the lava is andesitic. If the silica content is less than 50 percent by weight, then the lava is basaltic.
Many other physical properties, in addition to color, follow the distinctions between basaltic, andesitic, and rhyolitic lava. For example, basaltic lava has a low viscosity, meaning it is thin and runny. Basaltic lava flows easily and spreads out. Rhyolitic lava has a high viscosity and oozes slowly like toothpaste. The viscosity of andesitic lava is intermediate between basaltic and rhyolitic lava. Similarly, basaltic lava tends to erupt at higher temperatures, typically around 1000° to 1200° C (1800° to 2200° F), while rhyolitic lava tends to erupt at temperatures of 800° to 1000° C (1500° to 1800° F). Dissolved gases make up between 1 percent and 9 percent of magma. These gases come out of solution and form gas bubbles as the magma nears the surface. Rhyolitic lava tends to contain the most gas and basaltic lava tends to contain the least.
III -ERUPTIVE STYLES
Lava can erupt in several different ways depending on the viscosity of the lava and the pressure from the overlaying rock. When lava erupts out of a vent or large crack, it may pour like water out of a large pipe. The lava flows downhill like a river and can also form large lava lakes. The rivers and lakes of lava are called lava flows. Other times, the pressure exerted by gas bubbles in the lava is so high that it shatters the overlying rock and shoots lava and rock fragments high into the air with explosive force. The fragments of hot rock and lava shot into the air are called pyroclasts (Greek pyro, “fire”; and klastos, “fragment”). At other times, the pressure may be so high that the volcano itself is destroyed in a cataclysmic explosion.
A -Lava Flows
When lava flows out of a central vent, it forms a volcano. Basaltic lava is thin and fluid so it quickly spreads out and forms gently sloping volcanoes with slopes of about 5°. The flattest slopes are nearest the top vent, where the lava is hottest and most fluid. These volcanoes are called shield volcanoes because from a distance, they look like giant shields lying on the ground. Mauna Kea and Mauna Loa, on the island of Hawaii, are classic examples of shield volcanoes. Andesitic lava is more viscous and does not travel as far, so it forms steeper volcanoes. Rhyolitic lava is so viscous it does not flow away from the vent. Instead, it forms a cap or dome over the vent.
Sometimes, huge amounts of basaltic lava flow from long cracks or fissures in the earth. These basaltic lava flows, known as flood basalts, can cover more than 100,000 sq km (40,000 sq mi) to a depth of more than 100 m (300 ft). The Columbia River plateau in the states of Washington, Oregon, and Idaho was formed by repeated fissure eruptions. The accumulated basalt deposits are more than 4000 m (13,000 ft) thick in places and cover more than 200,000 sq km (80,000 sq mi). The Parana of Brazil and Paraguay covers an area four times as large. Flood basalts occur on every continent. When basaltic lava cools, it shrinks. In thick sheets of basaltic lava, this shrinking can produce shrinkage cracks that often occur in a hexagonal pattern and create hexagonal columns of rock, a process known as columnar jointing.
Two well-known examples of columnar jointing are the Giant’s Causeway on the coast of Northern Ireland and Devil’s Tower in northeastern Wyoming.
Basaltic lava flows and rocks are classified according to their texture. Pahoehoe flows have smooth, ropy-looking surfaces. They form when the semicooled, semihard surface of a lava flow is twisted and wrinkled by the flow of hot fluid lava beneath it. Fluid lava can drain away from beneath hardened pahoehoe surfaces to form empty lava tubes and lava caves. Other basaltic lava flows, known as aa flows, have the appearance of jagged rubble. Very fast-cooling lava can form volcanic glass, such as obsidian.
Vesicular basalt, or scoria, is a solidified froth formed when bubbles of gas trapped in the basaltic lava rise to the surface and cool. Some gas-rich andesitic or rhyolitic lava produces rock, called pumice, that has so many gas bubbles that it will float in water.
Pillow lava is made up of interconnected pillow-shaped and pillow-sized blocks of basalt. It forms when lava erupts underwater. The surface of the lava solidifies rapidly on contact with the water, forming a pillow-shaped object. Pressure of erupting lava beneath the pillow causes the lava to break through the surface and flow out into the water, forming another pillow. Repetition of this process gives rise to piles of pillows. Pillow basalts cover much of the ocean floor.
B -Pyroclastic Eruptions
Pyroclasts are fragments of hot lava or rock shot into the air when gas-rich lava erupts. Gases easily dissolve in liquids under pressure and come out of solution when the pressure is released. Magma deep underground is under many tons of pressure from the overlying rock. As the magma rises, the pressure from the overlying rocks drops because less weight is pressing down on the magma. Just as the rapid release of bubbles can force a fountain of soda to be ejected from a shaken soda bottle, the rapid release of gas can propel the explosive release of lava.
Pyroclasts come in a wide range of sizes, shapes, and textures. Pieces smaller than peas are called ash. Cinders are pea sized to walnut sized, and anything larger are lava bombs.
Cinders and bombs tend to fall to earth fairly close to where they are ejected, but in very strong eruptions they can travel farther. Lava bombs as large as 100 tons have been found 10 km (6 mi) from the volcano that ejected them. When cinders and bombs accumulate around a volcanic vent, they form a cinder cone. Although the fragments of lava cool rapidly during their brief flight through the air, they are usually still hot and sticky when they land. The sticky cinders weld together to form a rock called tuff.
Ash, because it is so much smaller than cinders, can stay suspended in the air for hours or weeks and travel great distances. The ash from the 1980 eruption of Mount Saint Helens in the state of Washington circled the earth twice.
Many volcanoes have both lava eruptions and pyroclastic eruptions. The resulting volcano is composed of alternating layers of lava and pyroclastic material. These volcanoes are called composite volcanoes or stratovolcanoes. With slopes of 15° to 20°, they are steeper than the gently sloped shield volcanoes. Many stratovolcanoes, such as the picturesque Mount Fuji in Japan, have convex slopes that get steeper closer to the top.
Pyroclastic materials that accumulate on the steep upper slopes of stratovolcanoes often slide down the mountain in huge landslides. If the volcano is still erupting and the loose pyroclastic material is still hot, the resulting slide is called a pyroclastic flow or nuée ardente (French for "glowing cloud"). The flow contains trapped hot gases that suspend the ash and cinders, enabling the flow to travel at great speed. Such flows have temperatures of 800° C (1500° F) and often travel in excess of 150 km/h (100 mph). One such pyroclastic flow killed 30,000 people in the city of Saint-Pierre on the Caribbean island of Martinique in 1902. Only one person in the whole town survived. He was in a basement jail cell.
Loose accumulations of pyroclastic material on steep slopes pose a danger long after the eruption is over. Heavy rains or melting snows can turn the material into mud and set off a catastrophic mudflow called a lahar. In 1985 a small pyroclastic eruption on Nevado del Ruiz, a volcano in Colombia, melted snowfields near the summit. The melted snow, mixed with new and old pyroclastic material, rushed down the mountain as a wall of mud 40 m (140 ft) tall. One hour later, it smashed into the town of Armero 55 km (35 mi) away, killing 23,000 people.
C -Explosive Eruptions
Rhyolitic lava, because it is so viscous, and because it contains so much gas, is prone to cataclysmic eruption. The small amount of lava that does emerge from the vent is too thick to spread. Instead it forms a dome that often caps the vent and prevents the further release of lava or gas. Gas and pressure can build up inside the volcano until the mountaintop blows apart. Such an eruption occurred on Mount Saint Helens in 1980, blowing off the top 400 m (1,300 ft) of the mountain.
Other catastrophic eruptions, called phreatic explosions, occur when rising magma reaches underground water. The water rapidly turns to steam which powers the explosion. One of the most destructive phreatic explosions of recorded history was the 1883 explosion of Krakatau, in the strait between the Indonesian islands of Java and Sumatra . It destroyed most of the island of Krakatau. The island was uninhabited, so no one died in the actual explosion. However, the explosion caused tsunamis (giant ocean waves) that reached an estimated height of 30 m (100 ft) and hit the nearby islands of Sumatra and Java, destroying 295 coastal towns and killing about 34,000 people. The noise from the explosion was heard nearly 2,000 km (1,200 mi) away in Australia.