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1. (b) wind and water erosion Yes. Millions of years of surface erosion shaped the spectacular stone sculptures of Arches National Park in eastern Utah. Delicate Arch, perched precariously on the edge of a canyon, is just one of 1,500 arches in the park, with spans reaching up to 300 feet. Most of the arches likely began as solid walls, or fins. Wind and water dug holes in the fins' porous sandstone—holes that were further enlarged by rockfalls and weathering. Many structures collapsed, but some with the right hardness and balance survived. 2. (c) slow-cooling lava Correct. The Devil's Postpile was indeed made by a fiery force—a massive lava flow that scalded the land 100,000 years ago. When the thick volcanic flow slowly cooled, it began to shrink. Cracks formed, first at the surface and then propagating downward, fracturing the solidified lava into columns. While some of the columns have three, four, five, or seven sides, most are hexagonal—the pattern that allows for the most shrinkage with the fewest cracks. Similar six-sided columns exist elsewhere, including the Palisades Sill north of New York City and the Giant's Causeway in Northern Ireland. 3. (b) a meteorite Yup. Its actual name is a giveaway—The Barringer Meterorite Crater, or simply Meteor Crater. If you picked "an explosion of volcanic steam," don't feel bad. At the turn of the 20th century, many scientists thought the same. A mining engineer named Daniel Barringer, though, was convinced that iron fragments around the crater were evidence of a meteor impact, and he assumed the meteor's iron remnants would be a mining bonanza. He was right about the impact, but wrong to launch a costly mining venture. Most of the meteor, which likely measured 150 feet across and struck the Earth at 40,000 miles per hour, vaporized on impact 50,000 years ago. 4. (c) the environment of an ancient salty lagoon That's right. Roughly 155 million years ago, a warm shallow sea covered what is now southern Germany. Coral reefs rose to cordon off the sea into isolated lagoons, which became increasingly salty and devoid of oxygen. Most creatures that fell into the lagoons couldn't survive long, and when they sank to the bottom, they were quickly buried in fine-grained carbonate mud. The low oxygen level and lack of strong currents in the lagoons delayed the decay of even delicate organic materials like feathers until long after burial. When the encasing mud turned to stone, the intricate details of the skeleton were essentially cast in stone. 5. a) underwater vortexes Indeed. Scientists reconstructing the events of cataclysmic Ice Age floods suspect that powerful kolks—vortexes akin to underwater tornadoes—gouged out the potholes. Some kolks likely were generated when two distinct flows of raging water, travelling at slightly different speeds, met and sheared past each other. Other kolks could have formed when a deep and fast flow passed over a large outcrop of rock. The whirling kolks acted like monstrous sledgehammers, pounding out huge chunks of bedrock that were then swept away in the flood.
6. (b) minerals that precipitated around objects Good pick. Each "pumpkin" is technically known as a concretion—a compact mass of minerals that forms around a core object such as a shell or piece of bone. These pumpkins formed underground within a bed of Diablo sandstone. Over many centuries, rainwater dissolved so-called "cementing" minerals at the surface and carried them downward. The minerals latched onto buried organic objects, encasing them in newly hardened sandstone. Layer upon layer, the pumpkins grew. They eventually made it to the surface, as wind and water eroded the softer sandstone in which they were buried. 7. (a) the same forces that carve caves Yes. And the towers are themselves riddled with caves. Limestone is a porous sedimentary rock created ages ago from the remains of marine life. Weak acids that naturally occur in groundwater dissolve the limestone and, together with erosion and rockfalls, carve caves, karst valleys, and a host of other fantastic landforms. Dramatic karst towers form in tropical regions like Guilin when thick limestone dissolves rapidly. The columns and spires are left behind after surrounding sections of limestone collapse or erode away. 8. (c) glaciers and ice wedging Correct. Roughly 12,000 years ago, a retreating ice sheet sheered the sides of Profile Mountain. After the glaciers melted, ice continued to play a role in chiseling the features of the face: water seeped into cracks in the granite and then froze, expanding by about 9 percent. This so-called ice wedging helped cut the five distinct ledges that made the Old Man's forehead, nose, mouth, and chin. Eventually, such ice wedging, along with changes in the granite called chemical weathering, led to the Old Man's collapse. He remains the state symbol for New Hampshire, but his passing makes clear the ephemeral nature of every structure on Earth—even those "carved in stone."
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