"Earth" Page 2

The Earth is a solid mass, with a dense core of magnetic, metallic material. Around the "Core" is a thick mantle of heavy crystalline rock covered by a layer of solid granite and basalt which form the base of the continents and the ocean floor. The temperature of the atmosphere drops with increased height until the "Tropopause" [the upper boundary between the troposphere and the stratosphere] is reached, at 6 to 8 miles or 25,000 to 60,000 feet above the Earth were water vapor displaces other gases and varies from nearly 0% to about 4% by volume. The height of the "Ozone" layer varies from approximately 12 to 21 miles above the Earth. Traces exist as low as 6 miles and as high as 35 miles above the Earth. Atmospheric scientists commonly divide the atmosphere into five layers, or regions, on the basis of the vertical distribution of temperature. The troposphere, the region in contact with the Earth's surface and where weather occurs, is characterized by a decrease of temperature with increasing altitude. It extends in height from approximately 6 to 8 km [4 to 5 miles] at the poles to about 17 km [11 miles] at the equator. Above this region is the stratosphere, in which temperature rises with increasing altitude to about 50 km [30 miles]. The troposphere and the stratosphere show distinct circulation systems. Whereas vertical motions prevail in the former, motions in the latter are largely confined to the horizontal. Above 50 km lies the mesosphere, which is characterized by a rapid decrease of temperature. Like the troposphere, this region is subject to strong seasonal variations of temperature at high latitudes. Beyond the mesosphere is the thermosphere, where temperature rapidly rises, attaining a maximum value of more than 1,000° C [1,832° F] at about 400 km [240 miles]. The increase in temperature ceases at this height, beyond which lies the exosphere, the highest layer of the atmosphere. The density of the atmosphere is so low in this layer that molecular collisions rarely occur, and hence the concept of temperature loses its customary meaning. In the exosphere, light atoms, such as those of hydrogen and helium, may acquire sufficient velocity to escape the Earth's gravitational pull. The heat now escaping from the Earth's surface probably comes in the main form of radioactivity throughout the mantle. Some heat may still come from the dense core at the Earth's center, and some may be original heat. The question of whether the Earth began hot or cold has not been definitely settled, although majority opinion favors a cold origin with intense early heating through radioactivity and the separation of the metallic core. Ages determined by the analysis of radioactive isotopes and their daughter products provide a clue to early history. The Earth as a distinct body is known to have an age of 4.6 * 109 years, and samples of lunar material show a similar age. The oldest continental rocks currently found, however, in Canada, have ages [since the time they solidified] of approximately 3.9 * 109 years. Presumably the record at the Earth's surface of the first 700 million years was erased by the elevated temperatures that prevailed in those times.