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Geologie Kostariky

18.11.2007  |  144× přečteno      vytisknout článek

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Costa Rica lies at the boundary where the Pacific's Cocos Plate--a piece of the earth's crust some 510 km wide--meets the crustal plate underlying the Caribbean. The two are converging as the Cocos Plate moves east at a rate of about 10 cm a year. It is a classic subduction zone in which the Caribbean Plate is forced under the Cocos--one of the most dynamic junctures on earth. Central America has been an isthmus, a peninsula, and even an archipelago in the not-so-distant geological past. It has therefore been both a corridor for and a barrier to landward movements, and it has been an area in which migrants have flourished, new life forms have emerged, and new ways of life have evolved. Yet a semblance of the Central America we know today became recognizable only in recent geological history. In fact, Costa Rica has one of the youngest surface areas in the Americas--only three million years old--for the volatile region has only recently been thrust from beneath the sea.

PLATE TECTONICS
Until the 1960s, when the theory of plate tectontics revolutionized the earth sciences, geologists trying to explain the distribution of earthquakes and volcanoes were at a loss. When earthquakes and volcanoes were plotted on a map, geologists realized that the planet is a puzzle--literally. The pieces of the terrestrial jigsaw are some 25 tectonic plates, interconnected pieces of the earth's crust (the lithosphere), 40-95 miles thick. Seven major plates carry the continents and ocean basins on their backs.

These plates are in continual motion, ponderously inching along on endless journeys across the surface of the earth, powered by forces originating deep within the earth. They ride on a viscous layer called the aesthenosphere, with a molten component welling up to the earth's surface on great convection currents fueled by heat from the core of our planet.

As the plates move, they pull apart or collide, unleashing titanic geological forces. When two plates slide past each other or converge--as off the Pacific coast of Central America--the geological forces generally drive one plate beneath the other, causing earthquakes. The friction created by one plate grinding beneath another melts part of both crusts, forming magma--molten rock--which wells up under pressure, erupting to form a chain of volcanoes.

Earthquakes
In its travels eastward, the Cocos Plate gradually broke into seven fragments, which today move forward at varying depths and angles. This fracturing and competitive movement causes the frequent earthquakes with which Costa Ricans contend. The forces that thrust the Cocos and Caribbean Plates together continue to build inexorably.

From insignificant tremors to catastrophic blockbusters, most earthquakes are caused by the slippage of masses of rock along earth fractures or faults. Rocks possess elastic properties, and in time this elasticity allows rocks to accumulate strain energy as tectonic plates or their component sections jostle each other. Friction can contain the strain and hold the rocks in place for years. But eventually, as with a rubber band stretched beyond its breaking point, strain overcomes frictional lock and the fault ruptures at its weakest point.

Suddenly, the pent-up energy is released in the form of an earthquake--seismic waves that radiate in all directions from the point of rupture, the "focus." This seismic activity can last for a fraction of a second to, for a major earthquake, several minutes. Pressure waves traveling at five miles per second race from the quake's epicenter through the bedrock, compressing and extending the ground like an accordion. Following in their wake come waves that thrust the earth up and down, whipping along at three miles per second.

For Costa Ricans, the bad news is that the most devastating earthquakes generally occur in subduction zones, when one tectonic plate plunges beneath another. Ocean trench quakes off the coast of Costa Rica have been recorded at 8.9 on the Richter scale and are among history's most awesome, heaving the sea floor sometimes scores of feet. These ruptures often propagate upward, touching off other, lower-magnitude tremors. This is what happened when the powerful 7.4 quake struck Costa Rica on 22 April 1991. That massive quake, which originated near the Caribbean town of Pandora (112 km southeast of San José), left at least 27 people dead, more than 400 injured, 13,000 homeless, and more than 3,260 buildings destroyed in Limón Province. The earthquake caused the Atlantic coastline to rise permanently--in parts by as much as 1.5 meters. In consequence, many of the beaches are deeper, and coral reefs have been thrust above the ocean surface and reduced to bleached calcareous skeletons.

Volcanoes
Costa Rica lies at the heart of one of the most active volcanic regions on earth. The beauty of the Costa Rican landscape has been enhanced by volcanic cones--part of the Pacific Rim of Fire--that march the length of Central America. Costa Rica is home to seven of the isthmus' 42 active volcanoes, plus 60 dormant or extinct ones. Some have the look classically associated with volcanoes--a graceful, symmetrical cone rising to a single crater. Others are sprawling, weathered mountains whose once-noble summits have collapsed into huge depressions called calderas (from the Portuguese word for "cauldron"). Still others, such as on Cocos Island, have smooth, shield-shaped outlines with rounded tops pocked by tiny craters.

Visitors seeking to peer into the bowels of a rumbling volcano can do so easily. The reward is a scene of awful grandeur. Atop Poás's crater rim, for example, you can gape down into the great well-like vent and see pools of molten lava bubbling menacingly, giving off diabolical, gut-wrenching fumes of chlorine and sulfur--and, for good effect, emitting explosive cracks, like the sound of distant artillery.

Several national parks have been created around active volcanoes, with accommodations, viewing facilities, lectures, and guided walks to assist visitors in understanding the processes at work. A descriptive map charting the volcanoes is published by the Vulcanological and Seismological Observatory of Costa Rica, at the National University in Heredia, which monitors volcanic activity throughout the nation. An excellent descriptive guide is Guillermo Alvarado's Costa Rica; Land of Volcanoes, which offers scientific explanations in layperson's terms.

In 1963, Irazú (3,412 meters) broke a 20-year silence, disgorging great clouds of smoke and ash. The eruptions triggered a bizarre storm that showered San José with 13 cm of muddy ash, snuffing out the 1964 coffee crop but enriching the Meseta Central for years to come. The binge lasted for two years, then abruptly ceased (although it began rumbling again in 1996). Poás (2,692 meters) has been particularly violent during the past 30 years. In the 1950s, the restless four-mile-wide giant awoke with a roar after a 60-year snooze, and it has been huffing and puffing ever since. Eruptions then kicked up a new cone about 100 meters tall. Two of Poás's craters now slumber under blankets of vegetation (one even cradles a lake), but the third crater belches and bubbles persistently. Volcanologists monitor the volcano constantly for impending eruptions.

Arenal (1,624 meters) gives a more spectacular light and sound show. After a four-century-long Rip van Winkle-like dormancy, this 4,000-year-young juvenile began spouting in 1968, when it laid waste to a four-square-mile area. Arenal's activity, sometimes minor and sometimes not, continues unabated; it erupted spectacularly in August 2000, killing a Tico tour guide and forcing the evacuation of Tabacón. Though more placid, Miravalles, Turrialba, and Rincón de la Vieja, among Costa Rica's coterie of coquettish volcanoes, also occasionally fling fiery fountains of lava and breccia into the air. Rincón blew in 1995, doing damage in Upala.

The type of magma that fuels most Central American volcanoes is thick, viscous, and so filled with gases that the erupting magma often blasts violently into the air. If it erupts in great quantity, it may leave a void within the volcano's interior, into which the top of the mountain crumbles to form a caldera. Irazú is a classic example. Its top fell in eons ago. Since then, however, small eruptions have built up three new volcanic cones--"like a set of nesting cups," says one writer--within the ancient caldera.

ZDROJ:centralamerica.com,kráceno


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