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Our transcription: Seismic studies aren't the only way of unlocking the secrets of the deep Earth. Geophysics provides other techniques as well. Gravity is another type of indirect evidence that geophysicists use to learn about the interior of the Earth. The force of gravity between two objects depends on the mass of the object and the distance between them. The greater the mass of either object or the closer they are together, the stronger the gravitational attraction. For example, the ocean tides are produced by the gravitational pull of the sun and the moon. However, even though the sun is 25 million times more massive than the moon, the effect of the moon on tides is more than twice as great because it's so much closer to the Earth. The instrument that geophysicists use to measure the force of gravity is called a "gravimeter." This device is incredibly sensitive, capable of detecting variations in the force of gravity as tiny as one part in 100 million. On Earth, changes in gravity are often due to variations in the mass of the rock in the Earth's interior. For example, the Earth's mantle rock is denser and therefore more massive than the crustal rock of the Earth; so in areas where the mantle is unusually thick, there are unusually high gravity readings.
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