Hot Topics: Biogeography of deep-sea hydrothermal vent faunas

We all know that if you drop a ball, it will fall towards the ground. We also know that the higher you are when you drop the ball, the faster and harder it will hit the Earth. Balls drop because of gravity -- the force of the Earth's gravity field. Actually all objects pull on (or exert a force on) each other. The reason that the ball falls towards the ground is that the Earth is very much bigger than anything else nearby it in space, and therefore Earth’s gravity is much greater than that of any other object nearby, even the Moon. The reason that objects dropped from a great distance above the ground hit the ground harder is that they are accelerated (their speed changes with time) because of gravity. The more time they spend falling the faster they go.

gravity graphic
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.
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 Earth’s crust and the distribution of different rock types.

During Cruise 2, scientists are using a gravimeter, a sensitive instrument that determines the force of gravity, mounted in the sphere of Alvin, to measure very small changes in Earth’s gravity at the mid-ocean ridge on the East Pacific Rise crest near 9° 37’N. Using data collected by the instruments on Alvin, it is hoped that we can determine the pattern of gravity anomalies, or changes from what the gravity should be at this latitude, that are associated with the different types of rocks underneath the East Pacific Rise crest. The two main types of rocks in this area are lava and dikes. Because the volcanic terrain in this area has many collapse features, scientists think that there is a considerable volume of the shallow ocean crust that is not solid rock, rather it comprises lava tubes and collapse features that are filled with sea water. The hypothesis that we are testing is whether small changes and different patterns in the gravity anomalies measured by Alvin can be used to see where the feeder dikes, which contain denser rocks, are beneath the ridge axis at a fast spreading center. If we can find this out it will help scientists better understand how the ocean crust is formed by volcanic eruptions.

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