A gravity meter measures the force of gravity between a mass inside the instrument and the earth. The force of gravity between two objects varies according to the mass of the objects and the distance between them: if either mass increases, the force between them increases; if the distance between them increases, the gravitational force decreases.
Dense rocks attract the mass in the gravity meter more than do those rocks that are less dense. A rock has a positive gravity reading, or anomaly, if it has a value higher than the normal regional gravity value; it has a negative gravity reading, or anomaly, if it has less than the regional gravity value. For example, ultramafic rocks or metallic ore bodies are very dense rocks that give positive gravity readings.
If gravity measurements are the same across a series of crustal blocks, it indicates the region is in isostatic equilibrium. The reverse is true if the gravity measurements are variable. Positive and negative gravity anomalies are often the result of an area's being out of isostatic equilibrium due to tectonic forces that either hold a region up (mountain range) or hold it down (ocean trenches), respectively.
Gravity anomalies on a local scale are usually associated with changes in rock type and can be used as a mapping tool, especially in areas where the bedrock is hidden by sedimentary cover. Metallic ore bodies can also result in a positive gravity feature because the metallic minerals are much denser than the surrounding rock.