of deep-sea hydrothermal vent faunas
The force of gravity is not the same everywhere on the Earth. One reason for
the difference in gravitational force is that the Earth is not a perfect sphere.
Its radius is larger at the Equator than at the North or South Pole. Therefore
someone at the Equator is further from the center of the Earth and feels slightly
less force. The average acceleration due to gravity at the Earth's surface is
about 9.8 meters per second per second (m/s2). However, at the Equator, it is
9.78 m/s2 and at the North Pole it is 9.83 m/s2. In addition, there are much
smaller, local variations in gravity due to the fact that the Earth is not uniform,
meaning that different types of rocks affect the gravity field of the earth.
The force of gravity is slightly greater over a area of very dense rock at the
surface than over an area where there is a thick layer of less dense sediments.
Scientists have developed extremely sensitive instruments to measure very small
variations in gravity. Information, or data, from these instruments can be used
by geologists and geophysicists to study the internal structure of the Earths
crust and the distribution of different rock types.
Dikes are more compact and dense than lava flows, so the force of gravity is very slightly greater above zones where numerous dikes, or swarms as geologists call them, are concentrated. The gravimeter mounted in Alvin
can measure these small changes in gravity and map out the pattern of shallow dikes under the axis.