Igneous Rocks and Volcanic Landforms Igneous rocks
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Igneous Rocks and Volcanic Landforms Igneous rocks form from the solidification of once-molten rock material. When this mushy melt is found underground penetrating other rocks, it's called magma, and the solidified rock is termed intrusive. By contrast, molten material that has erupted onto the Earth's surface is named lava, which cools into what geologists call extrusive (or volcanic) rocks. Because erosion can gradually remove tens of thousands of feet of rocks overlying intrusive formations, both extrusive and intrusive rocks can be observed on the Earth's surface, sometimes in close proximity. Because different types of igneous features form under varying conditions, each offers tantalizing clues to the conditions under which it solidified. Some of the most common igneous features include: Lava Flow Lava flows are streams of lava that pour out of a volcanic vent or fissure. How quickly lava flows move, and how far they go, depends upon the type of magma that's erupting. Dark-colored magmas that contain relatively little silica (SiO2), can travel further and faster than light-colored magmas, which tend to be much stickier. Lava flows can be very destructive, burying and burning everything in their paths. Geologists also use the term lava flow to describe the rock that eventually solidifies from the flowing, molten lava. Basalt is an example of an extrusive igneous rock formed from dark-colored lava. Rhyolite is an example of an extrusive igneous rock formed from light-colored lava. Fissure A long crack on the Earth's surface from which lava pours out is called a fissure. This type of volcanic activity is called a 'fissure eruption'. It most commonly occurs in places where dark magmas with low silica contents erupt. Магматические породы и вулканические формы рельефа Магматические породы образуются в результате затвердевания некогда расплавленного горного материала. Когда этот кашеобразный расплав обнаруживается под землей, проникающий в другие породы, это называется магмой, а затвердевшая порода называется интрузивной. Напротив, расплавленный материал, извергшийся на поверхность Земли, называется лавой, которая остывает и превращается в то, что геологи называют экструзионными (или вулканическими) породами. Поскольку эрозия может постепенно разрушать десятки тысяч футов пород, перекрывающих интрузивные образования, на поверхности Земли можно наблюдать как экструзивные, так и интрузивные породы, иногда в непосредственной близости. Поскольку различные типы магматических образований формируются в различных условиях, каждый из них дает дразнящие подсказки к условиям, при которых он затвердевал. Некоторые из наиболее распространенных магматических особенностей включают: Поток лавы Лавовые потоки - это потоки лавы, которые изливаются из вулканического жерла или трещины. То, как быстро движутся потоки лавы и как далеко они заходят, зависит от типа извергающейся магмы. Темные магмы, содержащие относительно мало кремнезема (SiO2), могут перемещаться дальше и быстрее, чем светлые магмы, которые, как правило, гораздо более липкие. Потоки лавы могут быть очень разрушительными, погребая и сжигая все на своем пути. Геологи также используют термин "поток лавы" для описания породы, которая в конечном итоге затвердевает из текущей расплавленной лавы. Базальт является примером экструзионной магматической породы, образовавшейся из лавы темного цвета. Риолит является примером экструзионной магматической породы, образовавшейся из светлой лавы. Трещина Длинная трещина на поверхности Земли, из которой изливается лава, называется трещиной. Этот тип вулканической активности называется "трещинным извержением". Чаще всего это происходит в местах, где извергаются темные магмы с низким содержанием кремнезема. Volcanic Neck This landform, which is also called a volcanic plug, is created when magma solidifies inside a conduit leading to a volcano or a volcanic vent. Because the resulting rock is typically harder than the material it intrudes into, it's left standing after the surrounding, softer rock has eroded away. This feature is therefore often called the "throat" of a volcano. A classic example is Ship Rock, a volcanic neck on the Navajo Reservation that rises nearly 1,600 feet above the desert near Farmington, New Mexico. Although the use of "volcanic" in the name suggests that volcanic necks are extrusive features, they are comprised of intrusive igneous rocks. Volcanic Cone Volcanic cones are steep-sided hills or mountains built of layers of erupted lava flows and fragments of volcanic rocks that have piled up around a central vent. As the name suggests, these features tend to be conical in shape and can be light- or dark-colored. There are three types of volcanic cones: 1) cinder cones, composite cones, and shield volcanoes. Sunset Crater in northern Arizona is an example of a cinder cone so young that local farmers Volcanic Pipe A volcanic pipe is a vertical conduit beneath a volcano through which magma once passed on its journey from the magma chamber to the eruption site. Over time, volcanic pipes usually become clogged by solidified magma and other volcanic rocks, leaving a hard, cylindrical-shaped formation behind. These features can range in width from several yards to approximately half a mile. Magma Chamber A magma chamber is a pool of molten rock material located beneath the Earth's surface. Over long periods of time, magma chambers can crystallize into large intrusive igneous rock formations called batholiths. Magma chambers can be the source of both magma and lava. An inactive magma chamber will cool slowly over time. This slow cooling allows the magma to crystallize into a coarse-grained igneous rock. Granite, gabbro, and diorite are examples of rocks that can form during the crystallization of a magma chamber. Lopolith A lopolith is a large, layered igneous intrusion that is distinguished by the convex-downward bowl shape of its floor and whose top can either be flat or convex down. Sill A sill is a flat, sheet-like igneous rock mass that forms when magma intrudes into and crystallizes between preexisting rock layers. Sills can form from magmas with a range of silica contents. These features can vary from less than one inch up to hundreds of feet thick and can extend for many miles. The tabular mass of quartz trachyte near the summit of Engineer Mountain near Silverton, Colorado is a well-known example of a sill. Stock A relatively small igneous intrusion that forms when magma crystallizes underground. Although uplift and/or erosion can later unearth part of a stock, this feature is defined as having less than 40 square miles (100 square kilometers) exposed at the surface. Dike A tabular igneous intrusion that crosses through other (layered or non-layered) rocks at a steep angle. Dikes can occur alone or in sets and may be comprised of light (high-silica) or dark (low-silica) rocks, or any composition in between. One of the most-photographed examples comes from the Grand Canyon, where a dark dike slashes across thin, red shale layers above a frothing whitewater rapid. Laccolith Laccoliths are intrusive igneous rock formations distinguished by their characteristic lens shapes. These features form when the pressure of the magma intruding between preexisting layers causes the overlying rocks to dome up, creating a mushroom shape. One of the world's most spectacular examples of a laccolith is found in Chile's Torres del Paine National Park. Batholith A relatively large igneous intrusion that forms when magma crystallizes underground and is later partially exposed following uplift and/or erosion. By definition, batholiths have more than 40 square miles (100 square kilometers) of surface exposure. The heart of California's Sierra Nevada mountains is carved from a granitic batholith emplaced between about 120 and 85 million years ago. |