Читать книгу Exploring the Solar System - Peter Bond - Страница 91

The presence of vast quantities of liquid water at the surface makes Earth unique among the bodies of the Solar System. 71% of the planet is covered by the saline oceans, equal to 97% of the world's free liquid water. In comparison, the volume of fresh water is very small. Most of this (2% of the total) is locked up in the ice sheets of Greenland and Antarctica, while the remaining 1% occurs in rivers and lakes.

Figure 3.27 (a) A false color radar image, taken from the Space Shuttle on April 13, 1994, shows the after‐effects of the 1991 eruption of Mt. Pinatubo in the Philippines. The red color shows deposits of ash on the higher slopes. The dark areas are mudflows which filled nearby river valleys after heavy rain. The area shown is approximately 45 × 68 km. (b) The eruption sent a cloud of fine particles and gases into the stratosphere and around the world, causing a noticeable cooling of the planet. A large amount of sulfur dioxide (green to red) was injected into the upper atmosphere.

(NASA‐JPL / NASA‐GSFC)

Figure 3.28 The Himalayas are the highest mountains on Earth. They are still increasing in height as the Indian plate moves north beneath the Eurasian plate. Weathering and erosion by glaciers and rivers carve deep valleys and remove large amounts of material which is eventually carried to the sea as sediment. This false color image shows the Ganges river, which rises in the snow‐covered Himalayas and flows 2,510 km to the Bay of Bengal.

(ESA)

Water plays a key role in the planet's evolution, since the temperature range on Earth enables it to exist as a gas (water vapor), a liquid, or a solid (ice). Even in the driest deserts there is likely to be some morning dew, night frost, or an occasional rainstorm.

Water plays a subtle, but key, role in breaking down Earth's rocks into small particles which can then be removed by wind or rain. This weathering process takes many forms, ranging from the expansion of ice in cracked rocks to chemical solution of rock minerals.

However, the impact of water is most noticeable through its ability to erode the land, i.e. wear away and remove surface material. In high mountains, frozen water in the form of moving glaciers slowly carves very wide and deep valleys. More commonly, innumerable streams and rivers flow from upland areas towards the ocean (or occasionally, large, landlocked lakes, such as the Caspian Sea).

The force of the flowing water removes loose material – anything from sand and silt to large boulders – from the stream's bed and banks. This in turn acts like sandpaper, further wearing away the surface. After millions of years, this process is capable of eroding mountains and forming broad valleys.

When the water slows down, it drops much of its load of sediment. This may happen during floods, or when the river enters a lake or the sea (Figure 3.34). Over thousands of years, the build‐up of sediment at river mouths may lead to growth of large deltas, such as those of the Mississippi and the Nile.

Large‐scale erosion and deposition also occur in coastal regions through the action of waves and currents. This may result in the growth of new land through the accumulation of mud, sand, and gravel, or the collapse of coastal cliffs which are undercut by wave action, especially during storms.