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.
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 a 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 a 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.
PHYSICAL PROPERTIES OF MAGMA
The density and viscosity, or thickness, of magma are key physical factors that affect its upward passage. Most rocks expand about 10 percent when they melt, and hence most magmas have 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 . Magma poor in silicon and rich in magnesium and iron, called mafic magma, is quite fluid
Sunday, June 27, 2010
Friday, June 18, 2010
NATURE: THE ETHOLOGISTS
In contrast, ethology—a discipline that developed in Europe but that now dominates United States studies as well—holds that much of what animals know is innate (instinctive). A particular species of digger wasp, for example, finds and captures only honey bees. With no previous experience a female wasp will excavate an elaborate burrow, find a bee, paralyze it with a careful and precise sting to the neck, navigate back to her inconspicuous home, and, when the larder has been stocked with the correct number of bees, lay an egg on one of them and seal the chamber. The female wasp’s entire behavior is designed so that she can function in a single specialized way. Ethologists believe that this entire behavioral sequence has been programmed into the wasp by its genes at birth and that, in varying degrees, such patterns of innate guidance may be seen throughout the animal world. Extreme ethologists have even held that all novel behaviors result from maturation—flying in birds, for example, which requires no learning but is delayed until the chick is strong enough—or imprinting, a kind of automatic memorization discussed below.
The three Nobel Prize-winning founders of ethology—Konrad Lorenz of Austria, Nikolaas Tinbergen of the Netherlands, and Karl von Frisch of West Germany (now part of the united Federal Republic of Germany)—uncovered four basic strategies by which genetic programming helps direct the lives of animals: sign stimuli (frequently called releasers), motor programs, drive, and programmed learning (including imprinting).
Sign Stimuli (Releasers)
Sign stimuli are cues that enable animals to recognize important objects or individuals when they encounter them for the first time. Baby herring gulls, for example, must know from the outset to whom they should direct their begging calls and pecks in order to be fed. An adult returning to the nest with food holds its bill downward and swings it back and forth in front of the chicks. The baby gulls peck at the red spot on the tip of the bill, causing the parent to regurgitate a meal. The young chick’s recognition of a parent is based entirely on the sign stimulus of the bill’s vertical line and red spot moving horizontally. A wooden model of the bill works as well as the real parent; a knitting needle with a spot is more effective than either in getting the chicks to respond.
Sign stimuli need not be visual. The begging call that a chick produces is a releaser for its parents’ feeding behavior. The special scent, or pheromone, emitted by female moths is a sign stimulus that attracts males. Tactile (touch) and even electrical sign stimuli are also known.
The most widespread uses of sign stimuli in the animal world are in communication, hunting, and predator avoidance. The young of most species of snake-hunting birds, for instance, innately recognize and avoid deadly coral snakes; young fowl and ducklings are born able to recognize and flee from the silhouette of hawks. Similar sign stimuli are often used in food gathering. The bee-hunting wasp recognizes honey bees by means of a series of releasers: The odor of the bee attracts the wasp upwind; the sight of any small, dark object guides it to the attack; and, finally, the odor of the object as the wasp prepares to sting determines whether the attack will be completed.
This use of a series of releasers, one after the other, greatly increases the specificity of what are individually crude and schematic cues; it is a strategy frequently employed in communication. Most animal species are solitary except when courting and rearing young. To avoid confusion, the signals that identify the sex and species of an animal’s potential mate must be clear and unambiguous. For example, a minnowlike fish called the stickleback uses a system of interlocking releasers to orchestrate its mating. When its breeding season arrives, the underside of each male turns bright red. This color attracts females but also provokes attacks by other males; red objects of almost any description will trigger male stickleback aggression. A female responds to the male’s red signal with a curious approach posture that displays her swollen belly full of eggs. This incites the male to perform a zigzag dance that leads the female to the tunnel-like nest he has built. The female struggles into the nest, whereupon the male touches her tail with his nose and quivers. The resulting vibration causes the female to release her eggs for the male to fertilize. If the male fails to perform the last part of the ballet, the female will not lay her eggs; vibrating the female with a pencil, however, which she can plainly see is not a male stickleback, works perfectly well, although the male in this case, not having gone through the last stage of the ritual, refuses to fertilize the eggs and eats them instead.
NATURE: THE ETHOLOGISTS
In contrast, ethology—a discipline that developed in Europe but that now dominates United States studies as well—holds that much of what animals know is innate (instinctive). A particular species of digger wasp, for example, finds and captures only honey bees. With no previous experience a female wasp will excavate an elaborate burrow, find a bee, paralyze it with a careful and precise sting to the neck, navigate back to her inconspicuous home, and, when the larder has been stocked with the correct number of bees, lay an egg on one of them and seal the chamber. The female wasp’s entire behavior is designed so that she can function in a single specialized way. Ethologists believe that this entire behavioral sequence has been programmed into the wasp by its genes at birth and that, in varying degrees, such patterns of innate guidance may be seen throughout the animal world. Extreme ethologists have even held that all novel behaviors result from maturation—flying in birds, for example, which requires no learning but is delayed until the chick is strong enough—or imprinting, a kind of automatic memorization discussed below.
The three Nobel Prize-winning founders of ethology—Konrad Lorenz of Austria, Nikolaas Tinbergen of the Netherlands, and Karl von Frisch of West Germany (now part of the united Federal Republic of Germany)—uncovered four basic strategies by which genetic programming helps direct the lives of animals: sign stimuli (frequently called releasers), motor programs, drive, and programmed learning (including imprinting).
Sign Stimuli (Releasers)
Sign stimuli are cues that enable animals to recognize important objects or individuals when they encounter them for the first time. Baby herring gulls, for example, must know from the outset to whom they should direct their begging calls and pecks in order to be fed. An adult returning to the nest with food holds its bill downward and swings it back and forth in front of the chicks. The baby gulls peck at the red spot on the tip of the bill, causing the parent to regurgitate a meal. The young chick’s recognition of a parent is based entirely on the sign stimulus of the bill’s vertical line and red spot moving horizontally. A wooden model of the bill works as well as the real parent; a knitting needle with a spot is more effective than either in getting the chicks to respond.
Sign stimuli need not be visual. The begging call that a chick produces is a releaser for its parents’ feeding behavior. The special scent, or pheromone, emitted by female moths is a sign stimulus that attracts males. Tactile (touch) and even electrical sign stimuli are also known.
The most widespread uses of sign stimuli in the animal world are in communication, hunting, and predator avoidance. The young of most species of snake-hunting birds, for instance, innately recognize and avoid deadly coral snakes; young fowl and ducklings are born able to recognize and flee from the silhouette of hawks. Similar sign stimuli are often used in food gathering. The bee-hunting wasp recognizes honey bees by means of a series of releasers: The odor of the bee attracts the wasp upwind; the sight of any small, dark object guides it to the attack; and, finally, the odor of the object as the wasp prepares to sting determines whether the attack will be completed.
This use of a series of releasers, one after the other, greatly increases the specificity of what are individually crude and schematic cues; it is a strategy frequently employed in communication. Most animal species are solitary except when courting and rearing young. To avoid confusion, the signals that identify the sex and species of an animal’s potential mate must be clear and unambiguous. For example, a minnowlike fish called the stickleback uses a system of interlocking releasers to orchestrate its mating. When its breeding season arrives, the underside of each male turns bright red. This color attracts females but also provokes attacks by other males; red objects of almost any description will trigger male stickleback aggression. A female responds to the male’s red signal with a curious approach posture that displays her swollen belly full of eggs. This incites the male to perform a zigzag dance that leads the female to the tunnel-like nest he has built. The female struggles into the nest, whereupon the male touches her tail with his nose and quivers. The resulting vibration causes the female to release her eggs for the male to fertilize. If the male fails to perform the last part of the ballet, the female will not lay her eggs; vibrating the female with a pencil, however, which she can plainly see is not a male stickleback, works perfectly well, although the male in this case, not having gone through the last stage of the ritual, refuses to fertilize the eggs and eats them instead.
The three Nobel Prize-winning founders of ethology—Konrad Lorenz of Austria, Nikolaas Tinbergen of the Netherlands, and Karl von Frisch of West Germany (now part of the united Federal Republic of Germany)—uncovered four basic strategies by which genetic programming helps direct the lives of animals: sign stimuli (frequently called releasers), motor programs, drive, and programmed learning (including imprinting).
Sign Stimuli (Releasers)
Sign stimuli are cues that enable animals to recognize important objects or individuals when they encounter them for the first time. Baby herring gulls, for example, must know from the outset to whom they should direct their begging calls and pecks in order to be fed. An adult returning to the nest with food holds its bill downward and swings it back and forth in front of the chicks. The baby gulls peck at the red spot on the tip of the bill, causing the parent to regurgitate a meal. The young chick’s recognition of a parent is based entirely on the sign stimulus of the bill’s vertical line and red spot moving horizontally. A wooden model of the bill works as well as the real parent; a knitting needle with a spot is more effective than either in getting the chicks to respond.
Sign stimuli need not be visual. The begging call that a chick produces is a releaser for its parents’ feeding behavior. The special scent, or pheromone, emitted by female moths is a sign stimulus that attracts males. Tactile (touch) and even electrical sign stimuli are also known.
The most widespread uses of sign stimuli in the animal world are in communication, hunting, and predator avoidance. The young of most species of snake-hunting birds, for instance, innately recognize and avoid deadly coral snakes; young fowl and ducklings are born able to recognize and flee from the silhouette of hawks. Similar sign stimuli are often used in food gathering. The bee-hunting wasp recognizes honey bees by means of a series of releasers: The odor of the bee attracts the wasp upwind; the sight of any small, dark object guides it to the attack; and, finally, the odor of the object as the wasp prepares to sting determines whether the attack will be completed.
This use of a series of releasers, one after the other, greatly increases the specificity of what are individually crude and schematic cues; it is a strategy frequently employed in communication. Most animal species are solitary except when courting and rearing young. To avoid confusion, the signals that identify the sex and species of an animal’s potential mate must be clear and unambiguous. For example, a minnowlike fish called the stickleback uses a system of interlocking releasers to orchestrate its mating. When its breeding season arrives, the underside of each male turns bright red. This color attracts females but also provokes attacks by other males; red objects of almost any description will trigger male stickleback aggression. A female responds to the male’s red signal with a curious approach posture that displays her swollen belly full of eggs. This incites the male to perform a zigzag dance that leads the female to the tunnel-like nest he has built. The female struggles into the nest, whereupon the male touches her tail with his nose and quivers. The resulting vibration causes the female to release her eggs for the male to fertilize. If the male fails to perform the last part of the ballet, the female will not lay her eggs; vibrating the female with a pencil, however, which she can plainly see is not a male stickleback, works perfectly well, although the male in this case, not having gone through the last stage of the ritual, refuses to fertilize the eggs and eats them instead.
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