Gravity anomaly
Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"
Meaning
A gravity anomaly is the gap between the gravity we measure at a place and the gravity we expect there. Gravity (g) is not the same everywhere. It is higher at the poles than at the equator, because the equator is farther from the earth's centre. Gravity also depends on what lies below the ground. Heavy rock pulls a little more and light rock a little less. So anomalies show how mass is spread unevenly in the crust. This makes them a useful indirect clue to the interior.
Example
Under the Himalayas the continental crust is very thick, up to 70 km. This thick crust is made of light granite rock and pushes deep down into the denser mantle, so gravity over the range is lower than expected. This is a negative gravity anomaly.
Don't confuse with
- Magnetic survey: this maps where magnetic materials lie in the crust. A gravity anomaly reflects differences in mass and density.
Related concepts
- Direct sources of information about the interior
- Indirect sources of information about the interior
- Kola Superdeep Borehole
- Geothermal gradient