2

SURFACE OF THE EARTH

The surface of the earth can be divided into oceans and continents. They have significance beyond the obvious fact that one is wet and the other dry.

The rocks and topography of the seafloor are different from those on land.  Bathymetry in oceanography is anologous to topography in geography.  As investigations into the depth of the ocean have proceeded, it has become apparent that a broad shelf like feature that steepens and slopes off into deep basins has generally developed around the continents.  The mean elevation of the continents above the sea (840 m) and mean depth of the oceans (3800 m) is the result of different densities of the continental and oceanic crustal rocks. You need to understand the nature of major sea-floor features such as mid-oceanic ridge, trench, and fracture zone, as well as the surprisingly young seafloor rocks and marine data gathering.  An understanding of general bathymetric features of the ocean is important because these features will be related ultimately to the origin of the ocean basins as well as physical and biological phenomena.

The Hydrologic Cycle

When volcanic activities and earthquakes work on a portion of the Earth’s surface above the sea-level, rocks are exposed to the atmosphere.  The Earth’s external processes then come to into play.  The external engine, driven by solar power and gravity, is exemplified by the hydrologic cycle.  When rain or snow falls on the land surface, more than half the water returns, rather rapidly, to the atmosphere by evaporation or by transpiration from plants.  The remainder flows over the land surface as runoff in streams, or it trickles or percolates down into the ground to become groundwater.  Our weather patterns are largely a product of these processes.  Hot air rises, near the equator, for instance, sinks in cold regions nearer the poles.  Ocean waves and currents are largely caused by wind, generated by solar heating.  The interaction of surface waters and the atmosphere with the lithosphere is important to the Earth System.

These various changes and movements are linked together into a grand scheme called the hydrologic cycle. Water evaporates in huge quantities from the ocean and falls as precipitation, either on the ocean or land. That which falls onto the land evaporates, runs off, or infiltrates. In the end, all of the water ends up back in the ocean again, but it can take a really long time.

Elements of the hydrologic cycle are
          evaporation from the ocean, from the land
          precipitation
          transpiration
          sublimation
          transportation of water: in the atmosphere, by rivers, ground water, also winds transport moisture.

Earth is unique among the planets in having abundant water in all its forms: ice and liquid water on the surface and interior and water vapor in the atmosphere. The study of this water and its movements on the surface and inside the Earth is called hydrology.

Water is more or less constantly moving and changing from one form to another. Here are some of the more common changes and movements:

     Water (liquid) to water vapor (gas)
     Three main ways in which water is converted to water vapor in the atmosphere:

          Evaporation
          Heat water and it becomes water vapor.

          Transpiration
          Plants take up water in growth and release water vapor through their leaves.

          Sublimation
          Ice goes directly from solid to water vapor without going through a liquid state. You may have seen dry ice used to   generate ``smoke''. This is sublimation - the solid carbon dioxide becomes carbon dioxide gas without ever melting into liquid carbon dioxide.

     Water vapor to water
     Air can hold a certain amount of water vapor. The amount depends on (among other things) temperature; hot air can hold
     more water vapor than colder air. When air is saturated with water vapor, the water vapor condenses into droplets of water,
     forming clouds. When the droplets get big enough, they fall to the ground as rain or snow.

     Runoff/Infiltration
     Water which falls as snow or rain either evaporates, runs off the ground surface (runoff) or soaks into the ground (infiltration).
     Whether the water runs off or infiltrates is controlled by a number of factors, such as

          Precipitation rate
          If rain falls more quickly than it can be absorbed into the ground, it will run off. The faster it falls, the more will run off. If
          it falls very rapidly, as in cloudbursts in the desert, you get flash floods.

          Soil water content
          More water will infiltrate into dry soil than into soil which is already soaked.

          Slope and vegetation
          Clearly, more water will run off steeper slopes than off gentler slopse. Vegetation counteracts this by helping to hold
          water in. Hillsides which have been denuded by wood-cutting or fires often suffer badly from mudslides and flooding.

During the various stages of the hydrologic cycle, Earth's water moves between different temporary storage areas called reservoirs.
Earth has six major natural water reservoirs:

oceans
glacial/polar ice
ground water
lakes
rivers
the atmosphere
the biosphere.

The oceans contain about 96% of Earth's water, with glacial/polar ice accounting for just under 3%. Only 1% of Earth's
water, that stored in lakes, rivers, and underground, is immediately useful to humans. Of these sources, groundwater is the most
important for drinking.