Hundreds of meteorite craters have been identified on Earth. Small craters typically have overturned and uplifted rims and a semicircular depression or crater marking the site of the impact. Larger impact craters collapse inward to fill the crater excavated by the impact and develop a characteristic medial high and many rims of uplifted and depressed crustal rocks. Impacts are commonly identified on the basis of impact breccias and shatter cones, by impact melts, and by the presence of high-pressure mineral phases such as coesite, stishovite, and diaplectic glass. Impact craters of all sizes and shapes have been identified on Earth and the Moon.
The most famous and perhaps most consequential impact crater is the one at Chicxulub, on the Yucatán Peninsula of Mexico. Here, a six-mile (10 km) wide meteorite hit Earth 66 million years ago, generating global fires and giant tsunamis, and ejecting tremendous amounts of dust and carbon dioxide into the atmosphere. The result was a mass extinction event at the Cretaceous-Tertiary boundary, and the loss of many species including the dinosaurs.
See also asteroid; comet; mass extinctions; solar system.
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Indian geology The subcontinent of India is divided into four main geologic provinces, including two on the Indian peninsula, the mountains in the north, and the Indo-Gangetic Plain between the peninsula and the mountains. Approximately two-thirds of peninsular India consists of Precambrian rocks, including assemblages as old as 3.8 billion years. The Precambrian rocks include granites, high-grade metamorphic granulite terranes, and belts of strongly deformed and metamorphosed sedimentary and volcanic rocks called schist belts (broadly equivalent to greenstone belts of other continents). These Precam-brian rocks are covered by thick deposits of lavas known as the Deccan Plateau flood basalts, erupted in the Late Cretaceous and Early Tertiary. The Dec-can flood plateau basalts cover large sections of western and central India, especially near Mumbai. The eruption of these flood basalts was a consequence of India's rifting away from other parts of Gondwana as it started its rapid journey to crash into Asia.
Peninsular India contains two converging mountain ranges that run along the east and west coasts and form the east and west boundaries of the Dec-can Plateau. The two ranges are joined by the Nilgiri Hills in the south, and the highest point in the ranges is Anai Mudi, with an elevation of 8,841 feet (2,697 m). The Eastern Ghats have an average elevation of 2,000 feet (600 m) and generally lie at 50 to 150 miles (80-240 km) from the Coromandel coastline, and locally form steep cliffs along the coast. The Eastern Ghats are crossed by the Godavari, Krishna, and Kaveri Rivers, and are covered by many hardwood trees. The Western Ghats extend along the Malabar coast from the Tapi River to Cape Comorin at the southern tip of India and are generally very close to the coastline. Elevations in the northern part of the Western Ghats reach 4,000 feet (1,200 m) and 8,652 feet (2,637 m) at Doda Beta in the south. The western side of the Western Ghats receives heavy monsoonal rainfalls, but the eastern side of the Western Ghats is generally dry.
Geologically the Western Ghats extend from the Deccan flood basalt plateau in the north to the Pre-
cambrian basement shield including the Dharwar craton in the south. Isotopic ages of gneisses and greenstone belts in the Dharwar craton range from 2.6 to 3.4 billion years old. The Dharwar craton is well known for gold deposits associated with greenstone belts and banded-iron formations, with the best-known greenstone belt being the Chitradurga. The Dharwar craton is divided into eastern and western parts by the elongate north-northwest-striking 2.6-billion-year-old Closepet granite, probably of Andean arc affinity. Late Proterozoic metamorphism, locally to granulite grade and including large areas of charnockites, affects much of the southern part of peninsular India. Rocks of the Dharwar craton are overlain by Paleozoic sedimentary deposits of Gond-wanan affinity. The Deccan flood basalts erupted at the end of the Cretaceous and overlie Gondwana and continental margin sequences that began developing with the breakup of Gondwana. The Eastern Ghats are entirely within the Precambrian basement rocks of the Indian subcontinent and the Aravalli craton in the north. The Aravalli craton is somewhat younger than the Dharwar craton, with isotopic ages falling in the range of 3.0 billion to 450 million years.
Sedimentary assemblages of peninsular India include many deposited during the Paleozoic and Mesozoic Eras when India was part of the supercontinent of Gondwana; such assemblages are known on the subcontinent as "Gondwanas." These are mostly thin sequences except in some narrow rift valleys and are mostly absent in interior India. In late Mesozoic and Tertiary times postrifting sedimentary assemblages were deposited on the continental margins and are several miles (km) thick in some isolated basins.
The Indo-Gangetic Plain forms a broad, several-hundred-mile (several-hundred-kilometer) wide plain between the Precambrian rocks exposed in southern India, from the active ranges of the Himalayan Mountains in the north. The rocks on the surface of the Indo-Gangetic Plain are very young, and with depth they record the progressive collision of India and Asia, containing the material eroded from the Himalayas as they were thrust upward during the collision. The Precambrian rocks of peninsular India warp downward and dip under the Indo-Gangetic Plain and form the basement for the sediments deposited in the Himalayan foreland basin.
The Himalaya Mountains contain a diverse suite of rocks ranging in age from Precambrian to Tertiary, and most are strongly deformed and metamorphosed from the Tertiary collision of India with Asia. The Main Boundary Thrust dips northward under the Himalayas and separates the deformed mountain ranges from the Indian shield.
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