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mass extinction
(Encyclopedia)mass extinction, the extinction of a large percentage of the earth's species, opening ecological niches for other species to fill. There have been at least ten such events. The five greatest were thos...rock
(Encyclopedia)rock, aggregation of solid matter composed of one or more of the minerals forming the earth's crust. The scientific study of rocks is called petrology. Rocks are commonly divided, according to their o...Mexico, city, Mexico
(Encyclopedia)Mexico or Mexico City, Span. Ciudad de México (Méjico), city (1990 pop. 8,236,960; 1991 met. area est. 20,899,000), central Mexico, capital and largest city of Mexico. The city has been the met...dinosaur
(Encyclopedia)dinosaur dīˈnəsôr [key] [Gr., = terrible lizard], extinct land reptile of the Mesozoic era. The dinosaurs, which were egg-laying animals, ranged in length from 21⁄2 ft (91 cm) to about 127 f...earth, in geology and astronomy
(Encyclopedia)CE5 A. Cross section of the earth, showing its shells B. Detailed cross section of the crust and upper mantle: The lithosphere consists of the hard rock in the crust and upper mantle, lying above t...Mars, in astronomy
(Encyclopedia)Mars, in astronomy, 4th planet from the sun, with an orbit next in order beyond that of the earth. Mars has two natural satellites, discovered by Asaph Hall in 1877. The innermost of these, Phobos...Hawaii, state, United States
(Encyclopedia)CE5 Hawaii həwīˈē, häväˈē [key], 50th state of the United States, comprising a group of eight major islands and numerous islets in the central Pacific Ocean, c.2,100 mi (3,380 km) SW of San...moon
(Encyclopedia)CE5 The moon: Near side CE5 The moon: Far side moon, natural satellite of a planet (see satellite, natural) or dwarf planet, in particular, the single natural satellite of the earth. It is no...ocean
(Encyclopedia)ocean, interconnected mass of saltwater covering 70.78% of the surface of the earth, often called the world ocean. It is subdivided into four (or five) major units that are separated from each other i...Africa
(Encyclopedia)CE5 Africa ăfˈrĭkə [key], second largest continent (2015 est. pop. 1,194,370,000), c.11,677,240 sq mi (30,244,050 sq km) including adjacent islands. Broad to the north (c.4,600 mi/7,400 km wide...Browse by Subject
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