Magma
I -INTRODUCTION
Magma, molten or partially molten rock beneath the earth’s surface. Magma is generated when rock deep underground melts due to the high temperatures and pressures inside the earth. Because magma is lighter than the surrounding rock, it tends to rise. As it moves upward, the magma encounters colder rock and begins to cool. If the temperature of the magma drops low enough, the magma will crystallize underground to form rock; rock that forms in this way is called intrusive, or plutonic igneous rock, as the magma has formed by intruding the surrounding rocks. If the crust through which the magma passes is sufficiently shallow, warm, or fractured, and if the magma is sufficiently hot and fluid, the magma will erupt at the surface of the earth, possibly forming volcanoes. Magma that erupts is called lava.
II -COMPOSITION OF MAGMA
Magmas are liquids that contain a variety of melted minerals and dissolved gases. Because magmas form deep underground, however, geologists cannot directly observe and measure their original composition. This difficulty has led to controversy over the exact chemical composition of magmas. Geologists cannot simply assume it is the same as the composition of the rock in the source region. One reason for this is that the source rock may melt only partially, releasing only the minerals with the lowest melting points. For this reason, the composition of magma produced by melting 1 percent of a rock is different from the composition of magma produced by melting 20 percent of a rock. Experiments have shown that the temperature and pressure of the location within the earth, and the amount of water present at that location affect the amount of melting. Because temperature and pressure increase as depth within the earth increases, melting an identical source rock at different depths will produce magmas of different composition. Combining these considerations with the fact that the composition of the source rock may be different in different geographic regions, there is a considerable range of possible compositions for magma.
As magma moves toward the surface, the pressure and temperature decrease, which causes partial crystallization, or the formation of mineral crystals within the magma. The compositions of the minerals that crystallize are different from the initial composition of the magma because of changes in temperature and pressure, hence the composition of the remaining liquid changes. The resultant crystals may separate from the liquid either by sinking or by a process known as filter-pressing, in which pressure compresses the liquid and causes it to move toward regions of lower pressure while leaving the crystals behind. As a result, the composition of the remaining magma is different from that of the initial magma. This process is known as magmatic differentiation, and is the principal mechanism whereby a wide variety of magmas and rocks can be produced from a single primary magma (see Igneous Rock: Formation of Igneous Rocks).
The composition of magma can also be modified by chemical interactions with, and melting of, the rocks through which it passes on its way upward. This process is known as assimilation. Magma cannot usually supply enough heat to melt a large amount of the surrounding rock, so assimilation seldom produces a significant change in the composition of magma.
Magmas also contain dissolved gases, because gases are especially soluble (easily dissolved) in liquids when the liquids are under pressure. Magma deep underground is under thousands of atmospheres (units of measure) of pressure due to the weight of the overlying rock. Gases commonly dissolved in magma are carbon dioxide, water vapor, and sulfur dioxide.
III -PHYSICAL PROPERTIES OF MAGMA
The density and viscosity, or thickness, of magma is key physical factors that affect its upward passage. Most rocks expand about 10 percent when they melt, and hence most magma has a density of about 90 percent of the equivalent solid rock. This density difference produces sufficient buoyancy in the magma to cause it to rise toward the surface.
The viscosity of a fluid is a measure of its resistance to flow. The viscosity of a magma affects how quickly the magma will rise, and it determines whether crystals of significantly different density will sink rapidly enough to change the bulk composition of the magma. Viscosity also influences the rate of release of gases from the magma when pressure is released. The viscosity of magma is closely related to the magma’s chemical composition. Magma rich in silicon and poor in magnesium and iron, called felsic magma, is very viscous, or thick (see Igneous Rock: Felsic Rocks). Magma poor in silicon and rich in magnesium and iron, called mafic magma, is quite fluid (see Igneous Rock: Mafic Rocks).
IV -GEOLOGICAL FEATURES FORMED BY MAGMA
Some magma reaches the surface of the earth and erupts from volcanoes or fissures before they solidify. Other magmas fail to reach the surface before they solidify. Magma that reaches the surface and is erupted, or extruded, forms extrusive igneous rocks. Magma that intrudes, or pushes its way into rocks deep underground and solidifies there forms intrusive igneous rock.
Volcanoes are cone-shaped mountains formed by the eruption of lava. Magma collects in a reservoir surrounded by rock, called a magma chamber, about 10 to 20 km (6 to 12 mi) below the volcano. A conduit known as a volcanic pipe provides a passage for the magma from the magma chamber to the volcano. As the magma rises in the conduit, the pressure of the overlying rock drops. Gases expand and bubble out that were kept dissolved in the magma by the pressure. The rapidly expanding gases propel the magma up the volcanic pipe, forcing the magma to the surface and leading to an eruption. The same process occurs when a shaken bottle of soda is suddenly opened.
The viscosity and dissolved-gas content of the magma control the character of the eruption. Low-viscosity magmas often have a low gas content. They flow easily from volcanic conduits and result in relatively quiet eruptions. Once the magma reaches the surface, it rapidly spreads out and over the volcano. Such fluid lava creates broad, gently sloped volcanoes called shield volcanoes, so called because they resemble giant shields lying on the ground.
Low-viscosity lava can also flow from fissures (long cracks in the rock), forming huge lava lakes. Repeated eruptions result in formations called flood basalts. The Columbia Plateau, in the states of Washington, Oregon, and Idaho, is a flood basalt that covers nearly 200,000 sq km (about 80,000 sq mi) and is more than 4000 m (13,000 ft) thick in places.
If a low-viscosity magma contains moderate amounts of dissolved gas, the released gases can eject the magma from the top of the volcano with enough force to form a lava fountain. The blobs of lava that are ejected into the air are called pyroclasts. They accumulate around the base of the fountain, forming a cinder cone.
Medium-viscosity magmas usually contain higher amounts of gases. They tend to form stratovolcanoes. The higher amounts of gases in the magma lead to very explosive eruptions that spew out large amounts of volcanic material. Stratovolcanoes have steeper sides than shield volcanoes. They are also known as composite volcanoes because they are made up of alternating layers of lava flows and deposits of pyroclasts.
High-viscosity magmas do not extrude easily though volcanic conduits. They often have a high gas content that can cause catastrophic eruptions. Both of these properties tend to promote explosive behavior, such as occurred on May 18, 1980 at Mount Saint Helens in Washington, when about 400 m (about 1300 ft) of rock was blasted off of its summit.
Intrusive bodies of rock formed from magma are classified by their size and shape. A batholith is an intrusive body that covers more than 100 sq km (nearly 40 sq mi). Lopoliths are saucer-shaped intrusions and may be up to 100 km (60 mi) in diameter and 8 km (5 mi) thick. Laccoliths have a flat base and a domed ceiling and are usually smaller than lopoliths. Sills and dikes are sheetlike intrusions that are very thin relative to their length. They can be less than one meter (about one yard) to several hundred meters thick but can be larger; the Palisades sill in the state of New York is 300 m (1000 ft) thick and 80 km (50 mi) long. Sills are formed when magma is forced between beds of layered rock; they run parallel to the layering of the surrounding rock. Dikes are formed when magma is forced into cracks in the surrounding rock; they tend to run perpendicular to the layering of the surrounding rock.