【正文】 vapor, nitrogen, ammonia, and hydrogen sulfide. Other materials, including large amounts of water vapor, were brought in by etary bombardments. Much of the hydrogen and helium escaped。s interior. ■These lighter materials, such as the rocks we call granites, formed a layer of continental crust about 35 kilometers thick. ■Relative to Earth as a whole, this is as thin as an eggshell. ■Seafloor crust is even thinner, at about 7 kilometers。s lack of an atmosphere explains why it was bombarded with much more frequency and violence than other plaesimals. ? Continued bombardments and internal pressures made the growing Earth hotter, causing its interior to melt and the heavier elements to sink and form Earth39。s interior rose and formed the mantle, a denser layer of silicates around the core, and the crust, a thinner layer of silicates at Earth39。s core. Much of the early continental crust has remained on Earth39。s first stable atmosphere. 11. The word emitted in the passage is closest in meaning to ? Released ? Consumed ? Contained ? Heated 12. What can be inferred from paragraph 4 about Earth39。s core. Much of the early continental crust has remained on Earth39。s core was nickel ? What internal pressures caused Earth to heat up as it grew in size ? What caused Earth39。 thus, even continental crust reaches only about 1/200th of the way to Earth39。 those of others were on or a part of the body surface. One animal, Opabinia, had five eyes: two lateral pairs and a single medial eye。 they were found on a mountainside in British Columbia in a deposit known as the Burgess Shale. The Burgess Shale fossils are extraordinarily important because among them are remains of softbodied creatures, many of them lacking shells and other hard parts that fossilize easily. Consequently, their preservation is little short of miraculous (as are the delicate methods used to reconstruct threedimensional structure from these flattened fossils), and they are one of the few known repositories of early softbodied animals. Not all of the Burgess animals had eyes. However, some did. (Gross features location, size, and hemispheric shape are responsible for the designation of some structures as eyes). The reconstructed eyes of these Burgess animals look superficially like eyes of some living crustaceans, particularly those of shrimp and crabs whose eyes are mounted on stalks that improve the range of vision by raising the eyes above the surface of the head. The eyes of some Burgess anisms sat on stalks。s atmosphere, we must go back to the earliest days of the solar system, before the plas themselves were formed from a disk of rocky material spinning around the young Sun. This material gradually coalesced into lumps called plaesimals as gravity and chance smashed smaller pieces together, a chaotic and violent process that became more so as plaesimals grew in size and gravitational pull. Within each orbit, collisions between plaesimals generated immense heat and energy. How violent these processes were is suggested by the odd tilt and spin of many of the plas, which indicate that each of the plas was, like a billiard ball, struck at some stage by another large body of some kind. Visual evidence of these processes can be seen by looking at the Moon. Because the Moon has no atmosphere, its surface is not subject to erosion, so it retains the marks of its early history. Its face is deeply scarred by millions of meteoric impacts, as you can see on a clear night with a pair of binoculars. The early Earth did not have much of an atmosphere. Before it grew to full size, its gravitational pull was insufficient to prevent gases from drifting off into space, while the solar wind (the great stream of atomic particles emitted from the Sun) had already driven away much of the gaseous material from the inner orbits of the solar system. So we must imagine the early Earth as a mixture of rocky materials, metals, and trapped gases, subject to constant bombardment by smaller plaesimals and without much of an atmosphere. As it began to reach full size, Earth heated up, partly because of collisions with other plaesimals and partly because of increasing internal pressures as it grew in size. In addition, the early Earth contained abundant radioactive materials, also a source of heat. As Earth heated up, its interior melted. Within the molten interior, under the influence of gravity, different elements were sorted out by density. By about 40 million years after the formation of the solar system, most of the heavier metallic elements in the early Earth, such as iron and nickel, had sunk through the hot sludge to the center giving Earth a core dominated by iron. This metallic core gives Earth its characteristic magic field, which has played an extremely important role in the history of our pla. As heavy materials headed for the center of Earth, lighter silicates (such as the mineral quartz) drifted upward. The denser silicates formed Earth39。s interior to the surface. So we can imagine the surface of the early Earth as a massive volcanic field. And we can judge pretty well what gases bubbled up to that surface by analyzing the mixture of gases emitted by volcanoes. These include hydrogen, helium, methane, water vapor, nitrogen, ammonia, and hydrogen sulfide. Other materials, including large amounts of water vapor, were brought in by etary bombardments. Much of the hydrogen and helium escaped。s mantle, a region almost 3,000 kilometers thick between the core and the crust. With the help of bombardment by ets, whose many impacts scarred and heated Earth39。s mantle in its early stages ? is very thin relative to Earth39。s mantle, a region almost 3,000 kilometers thick between the core and the crust. With the help of bombardment by ets, whose many impacts scarred and heated Earth39。s core. ? Lighter elements from Ear