Download When the sea surface reflects the bottom

Extract from educational CD-ROM “The Geonauts inquire into the oceans ”, OCA/CNES © 2000
When the sea surface reflects the
bottom
Mankind has always been fascinated about the Earth’s shape and size. At first it
was thought to be disc-shaped, with some people imagining hell around its edges.
It was during Antiquity that the Greeks began drawing the Earth as a ball (or
“sphere”). Around 250 B.C., Eratosthenes calculated the Earth’s radius for the
first time, being only a few hundred kilometres out.
The Earth is more than a simple sphere
From the 17th century, the idea that this sphere might be slightly flattened
became more popular and Newton made the first calculations. In the 18th century,
the flattening of the Earth was measured for the first time. It was only in the
middle of the 20th century that this flattening could be accurately measured
using artificial satellites. The difficulty in determining this flattening lies in
the fact that the radius at the poles differs from the radius at the equator
(6,378 kilometres) by a mere 21 kilometres.
The space era has revealed that the Earth’s shape is much more complex than
simply a sphere. It appears to bulge at the equator and be flattened at the poles.
The material of which it is made is not the same everywhere, resulting in
differences in density and therefore differences in gravity from one area to the
next. What is more, the Earth’s crust is not smooth but has varied relief, with
mountains and trenches both on land and under water.
Repercussions on the surface of the oceans
The sea level varies depending on the ocean floor. This is because the surface is
sensitive to the force of gravity. The force of gravity depends on the mass, and
water acts like weighing scales as it reveals the field of gravity and thus the
distribution of mass over the Earth. When the force of gravity is stronger (more
mass, i.e. denser), the water is attracted so forms a bump. The presence of
underwater mountains (more mass) or, on the contrary, trenches (less mass)
therefore changes the force of gravity and creates bumps and hollows on the
ocean surface.
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Extract from educational CD-ROM “The Geonauts inquire into the oceans ”, OCA/CNES © 2000
The size of the bumps and hollows depends on the variations in underwater relief,
but the effect on the sea surface is much weaker. To take an example, an
underwater mountain 1,000 metres high can create a bump on the sea’s surface of
around 1 metre. Measurements taken by altimetric satellites are used to map
these bumps and hollows on the ocean surface. The map can then be used to
compute the underwater relief without having to criss-cross the seas with a
depth-finding instrument. The satellite not only provides a global view, but also a
frequency of measurement impossible to obtain with a boat (the ERS-1 satellite,
for instance, has measured sea height every 7 kilometres over all the world’s
oceans).
What can you see on the horizon of the ocean floor ?
The ocean floor is constantly being renewed. The oldest is no more than 200
million years old, which is very young compared to the Earth’s 4,5 billion years.
The movement of the ocean depths leaves numerous traces on the surface. In the
middle of the oceans, volcanic ranges testify to these movements. These ranges
are formed by columns of magma which are fixed with respect to the rotating
Earth and which break through ocean bed as they ascend up through the Earth’s
mantle. This movement gradually leads the volcanoes away from the source of
magma. They are then extinguished and are replaced by a new volcano, and so on.
This makes ranges several thousand kilometres long, like the Hawaiian Ridge in
the Pacific ocean. Working backwards, we can use these ranges to trace the
movement of the Pacific plate over time: the bend in the middle of the Hawaiian
Ridge, for example, indicates a sudden change of direction of the plate 45 million
years ago.
Another volcanic phenomenon known as a ridge, where the ocean floor is actually
formed, snakes across the ocean plates for 60,000 kilometres. Ridges are clearly
seen in the Atlantic and Indian oceans (where the rate of accretion is low, around
a few centimetres per year), but are more difficult to see in the Pacific (where
the accretion rate exceeds ten centimetres per year). Over time, the material
which arrived hot at the ridge is accreted, becoming the young ocean floor and
being pushed further out towards the centre of the basin when new material
arrives. There its heat dissipates in the water and it gradually cools down. It
becomes denser due to thermal contraction, accentuating its equilibrium depth
(which is a balance between the water above and the Earth’s mantle below). One of
the major characteristics of ocean floors is that the older they are, the deeper
they are. Ridges are segmented by furrows running perpendicular to the ridge,
allowing for expansion. The curvilinear geometry of these fracture zones
testifies to changes in the speed and direction of opening throughout the history
of each segment of ridge. In the northern part of the Pacific ocean, the ridge
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Extract from educational CD-ROM “The Geonauts inquire into the oceans ”, OCA/CNES © 2000
comes up against the Western coast of America, in California. In fact, half the
North Pacific ocean is missing, swallowed up under North America by the opening
of the Atlantic ocean. Created at the axis of the ridges, the old ocean floor
returns inside the Earth at subduction zones. Subduction zones are very deep
trenches, found in particular all around the Pacific ocean. The boundaries between
plates (subduction zones with ridges) correspond to earthquake areas.
Underwater relief also witnesses to the slow but chaotic history of the Earth’s
formation. It is possible, for instance, to trace the impact of asteroids having
collided with Earth, including the one near the gulf of Mexico which might have
caused the extinction of dinosaurs. Unfortunately, it is difficult to find the
trace of such impacts because of erosion and deposits of sediment on the ocean
floor.
Map of underwater relief based on satellite data from ERS-1, TOPEX/Poseidon
and Geosat (credits GRGS/LEGOS/CNES, Toulouse and IRD, Noumea).
The satellite’s movement reflects the field of gravity
The force of gravity causes the satellite to move around the Earth. If the
Earth’s mass was distributed evenly, the satellite’s movement would
be regular (close to an ellipse). However, the Earth’s relief both on
land and under water is far from homogeneous. Similarly, the density
of the materials of which it is composed varies greatly. These
variations in mass lead to the force of gravity being different at
different points on the Earth. When the satellite flies over regions
where the mass (and therefore the gravity) changes, its flight
changes too. By comparing the satellite’s actual trajectory with the
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Extract from educational CD-ROM “The Geonauts inquire into the oceans ”, OCA/CNES © 2000
theoretical trajectory, we can determine variations in the Earth’s field of
gravity. “The shape of the Earth” calculated in this way is known as the geoid.
Given that a satellite is only sensitive to variations in the field of gravity which
cover regions greater than or equal to its own altitude, a set of satellites flying
at different altitudes is used to reconstitute the shape of the field of gravity.
The main satellite tracking systems used in space geodesy are the French DORIS
system and laser ranging, used to compute the satellite’s trajectory. The
American Global Positioning System (GPS) can also be used experimentally.
TOPEX/Poseidon was the first satellite fitted out with all three systems. These
techniques can also be used to determine the position and movement of continents.
Around fifty DORIS beacons are spread throughout the world to determine
satellite orbits and monitor the movement of tectonic plates. After 10 years of
uninterrupted measurements, the DORIS system can give the speed at which
tectonic plates are moving to within 1 millimetre per year (did you know that in
the space of ten years, the United States have drifted 20 centimetres away from
Europe ?).
Bibliography
Satellite Altimetry and Earth Sciences, a handbook of techniques and
applications, edited by Lee-Lueng Fu and Anny Cazenave, International
Geophysics Series, Volume 69, 2001.
Comment la Terre devint ronde, by JP. Maury, Collection Gallimard, Découvertes,
no. 52.
Élément de géologie, 12th edition, by C. Pomerol, Y. Lagabrielle and M. Renard,
Collection Masson Sciences.
Formes et mouvements de la Terre, by Anny Cazenave and Kurt Le Pichon, CNRS
Editions.
La dérive des continents, la tectonique des plaques, Bibliothèque pour la Science,
Edition Belin.
Le grand Défi des pôles, by B. Imber, Collection Gallimard, Découvertes, no. 15.
Le procès des étoiles, by Florence Tristam, Editions SEGHERS.
Les planètes et leur environnement, by Daniel Benest, Claude Froeschlé, François
Barlier, Michel Blanc, Sylvestre Maurice, Jean-Paul Parisot, Jean-Pierre
Peulvast and Françoise Suagher, Hachette.
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