Engineering geology

Earth’s Gravity

Earth’s rotation is the most significant cause for the differences in the force of gravity observed at the surface. Because Earth rotates around its axis, the acceleration due to gravity is less at the equator (9.78 m/s2) than at the poles (9.83 m/s2).
Two reasons account for this phenomenon. Earth’s rotation causes a centrifugal force that is in proportion to the distance from the axis of rotation. In a manner similar to the force that throws you sideways in a vehicle going too quickly around a curve or corner, centrifugal force acts to throw objects outward at the equator, where the force is greatest.
Earth’s shape is also affected by its rotation—with the equator slightly further from Earth’s center (6378 kilometers) than the poles (6357 kilometers) (Figure 1).

Figure 1 – Earth is not a sphere but an oblate spheroid
Because Earth rotates, it bulges at the equator and flattens at the poles.

Earth, therefore, is not a perfect sphere but instead bulges at the equator a shape called an oblate ellipsoid. This difference causes the force of gravity to be slightly weaker at the equator than at the poles because gravitational attraction is less when objects are further apart. In fact, your body weight is 0.5 percent less at the equator than at the poles.

Gravity measurements show that there are other variations that cannot be explained by Earth’s rotation.
For instance, when a large body of unusually dense rock is underground, the increase in mass will cause a largerthan- average gravitational force at the surface directly above. Because metals and metal ores tend to be much denser than silicate rocks, local gravity anomalies (differences from the expected) have long been used to help prospect for ore deposits.

A map of regional gravity anomalies for the United States is shown in Figure 2. A narrow positive gravity anomaly (stronger than expected) that runs down the middle of the country is the mid-continent rift (red), where thick, dense volcanic rocks filled a rupture in the crust more than 1 billion years ago. The negative gravity anomaly (blue) in the Basin and Range region is a result of warm, low-density crust being stretched and thinned as it was intruded by hot buoyant magma bodies.
Some large-scale differences in density deep beneath the surface have been detected using satellites. These gravity anomalies result from the large upwellings and downwellings of mantle convection. Areas of upwelling are associated with hot mantle plumes, whereas downwelling occurs where cold oceanic slabs descend into the mantle.

Figure 2 – Gravity anomalies beneath the continental United States

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