planet. The crust’s relative rigidity (brittle nature) corresponds to a relatively
cooler region, and is involved in the mechanisms of tectonic plate motion. The
lithosphere’s crust has broken up into several rigid plates, which move with
respect to each other according to a common pole of rotation. Each rigid plate is
like a ‘‘raft’’ that is floating on the more ductile layer of the lithosphere, i.e. the
asthenosphere.
The lithosphere-crust has changed continuously over geological time. This is
due to the slow convective flows carrying mantle material and heat, which are
responsible for the renewal of the Earth’s outer layer (See Chap. 5). These changes
have also been called the Wilson Cycle in honor of Tuzo Wilson (1972) who
developed the concept that continental masses have been formed by the opening
and closing of the Earth’s oceans as a natural consequence of plate motions. How
many times have the oceans and continents been recycled? How far back in
Earth‘s history can the surface evidence of this convective cycle be extended? This
has certainly occurred many times, but it is hard to be more specific on the basis of
our present geological record.
Water on Earth
Water abounds in the Universe, however the origin of water on Earth is speculative. On Earth, 96.5 % of the planet’s surface water is found in the oceans, 1.7 %
in groundwater, 1.7 % in glaciers and the ice caps of Antarctica and Greenland, a
small fraction in other large water bodies, and 0.001 % is found in the air as vapor
or clouds, formed of solid and liquid water particles suspended in air (Fig. 2.4) as
well as in precipitation. Only 2.5 % of the Earth’s water is fresh water, and 98.8 %
of that water is found in ice and in groundwater. \0.3 % of all fresh water is
located in rivers, lakes, and the atmosphere, and an even smaller amount of the
Earth’s freshwater (0.003 %) is contained within biological bodies and manufactured products.
How this water formed the oceans is not easy to grasp. Convay’s hypothesis
(1943) claims that water condensed from the primeval atmosphere. Thus, the
genesis of the air and ocean is due to planetary gas release during the capture,
concentration and accumulation of particles forming a layered Earth composed of
solid, liquid and gas. The blue color of the ocean is due to the fact that the red
wavelengths of sunlight are absorbed quickly while the blue rays are transmitted
through the water column.
Another hypothesis is that the hydrosphere was formed at the beginning of the
Earth’s formation from degassing during volcanic activity and then by gravitational differentiation, when volatiles concentrated on the surface. It is also evidenced that the release of volatiles from the Earth’s mantle is another source of
gas and liquid in the hydrosphere and atmosphere. Furthermore, it must be noted
that the volatile release, which could condense and form the oceans, will react
32
2 Our Haven, Planet Earth
cooler region, and is involved in the mechanisms of tectonic plate motion. The
lithosphere’s crust has broken up into several rigid plates, which move with
respect to each other according to a common pole of rotation. Each rigid plate is
like a ‘‘raft’’ that is floating on the more ductile layer of the lithosphere, i.e. the
asthenosphere.
The lithosphere-crust has changed continuously over geological time. This is
due to the slow convective flows carrying mantle material and heat, which are
responsible for the renewal of the Earth’s outer layer (See Chap. 5). These changes
have also been called the Wilson Cycle in honor of Tuzo Wilson (1972) who
developed the concept that continental masses have been formed by the opening
and closing of the Earth’s oceans as a natural consequence of plate motions. How
many times have the oceans and continents been recycled? How far back in
Earth‘s history can the surface evidence of this convective cycle be extended? This
has certainly occurred many times, but it is hard to be more specific on the basis of
our present geological record.
Water on Earth
Water abounds in the Universe, however the origin of water on Earth is speculative. On Earth, 96.5 % of the planet’s surface water is found in the oceans, 1.7 %
in groundwater, 1.7 % in glaciers and the ice caps of Antarctica and Greenland, a
small fraction in other large water bodies, and 0.001 % is found in the air as vapor
or clouds, formed of solid and liquid water particles suspended in air (Fig. 2.4) as
well as in precipitation. Only 2.5 % of the Earth’s water is fresh water, and 98.8 %
of that water is found in ice and in groundwater. \0.3 % of all fresh water is
located in rivers, lakes, and the atmosphere, and an even smaller amount of the
Earth’s freshwater (0.003 %) is contained within biological bodies and manufactured products.
How this water formed the oceans is not easy to grasp. Convay’s hypothesis
(1943) claims that water condensed from the primeval atmosphere. Thus, the
genesis of the air and ocean is due to planetary gas release during the capture,
concentration and accumulation of particles forming a layered Earth composed of
solid, liquid and gas. The blue color of the ocean is due to the fact that the red
wavelengths of sunlight are absorbed quickly while the blue rays are transmitted
through the water column.
Another hypothesis is that the hydrosphere was formed at the beginning of the
Earth’s formation from degassing during volcanic activity and then by gravitational differentiation, when volatiles concentrated on the surface. It is also evidenced that the release of volatiles from the Earth’s mantle is another source of
gas and liquid in the hydrosphere and atmosphere. Furthermore, it must be noted
that the volatile release, which could condense and form the oceans, will react
32
2 Our Haven, Planet Earth