Questions 1-9
The geology of the Earth's surface is dominated by the particular properties of
water. Present on Earth in solid, liquid, and gaseous states, water is exceptionally
reactive. It dissolves, transports, and precipitates many chemical compounds and is
constantly modifying the face of the Earth.
(5) Evaporated from the oceans, water vapor forms clouds, some of which are
transported by wind over the continents. Condensation from the clouds provides the
essential agent of continental erosion: rain. Precipitated onto the ground, the water
trickles down to form brooks, streams, and rivers, constituting what is called the
hydrographic network. This immense polarized network channels the water toward a
(10) single receptacle: an ocean. Gravity dominates this entire step in the cycle because
water tends to minimize its potential energy by running from high altitudes toward the
reference point that is sea level.
The rate at which a molecule of water passes though the cycle is not random but is
a measure of the relative size of the various reservoirs. If we define residence time as
(15) the average time for a water molecule to pass through one of the three reservoirs-
atmosphere, continent, and ocean-we see that the times are very different. A water
molecule stays, on average, eleven days in the atmosphere, one hundred years on a
continent and forty thousand years in the ocean. This last figure shows the importance
of the ocean as the principal reservoir of the hydrosphere but also the rapidity of water
(20) transport on the continents.
A vast chemical separation process takes places during the flow of water over the
continents. Soluble ions such as calcium, sodium, potassium, and some magnesium are
dissolved and transported. Insoluble ions such as aluminum, iron, and silicon stay
where they are and form the thin, fertile skin of soil on which vegetation can grow.
(25) Sometimes soils are destroyed and transported mechanically during flooding. The
erosion of the continents thus results from two closely linked and interdependent
processes, chemical erosion and mechanical erosion. Their respective interactions and
efficiency depend on different factors. 【題組】8. All of the following are example of soluble ions EXCEPT
(A) magnesium
(B) iron
(C) potassium
(D) calcium
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