# Equatorial bulge and tectonic plates

It is well known that the Earth is not a sphere, but rather it bulges at the equator. Also it is well known that the Earth's crust is composed of 7 or 8 (depending on definition) major tectonic plates, which are able to move on top of the asthenosphere, the upper layer of the Earth's mantle.

Due to the equatorial bulge, it would seem as though plates near the equator should not be able to drift away from the equator, and plates away from the equator should not be able to drift near the equator, since they will not be of the right shape to fit over these portions of the Earth. So how are the plates able to drift to and from the equator when the surface of the Earth is shaped differently there?

## 2 Answers

The plates are not as rigid as you think. You seem to be imagining the situation as something like this: I boil an egg and take the shell off in pieces, but I can't take a piece of shell from the end and make it lay flat on the side of the egg. However, rock is not that rigid on scales of thousands of kilometres and millions of years (I don't think there exists any material which would be that rigid). Also, Earth's equatorial bulge is tiny relative to its diameter -- less than 50km. Tectonic plates move very slowly, and there is plenty of time for them to deform as they move.

Pont is correct. It is only the uppermost supracrustal rocks that seem brittle to us, and even then they are somewhat plastic when considered on a long enough timescale. Most of the crust is about 10 km thick (oceans) to >40 km thick (continents), almost all of which is sufficiently hot and pressurized to be somewhat plastic. To give just one example of lateral compression caused by the non-spheroidal world, the Indian tectonic plate is rapidly moving north and crashing into the Tibetan plate (hence the Nepalese earthquake earlier this year). The Indian plate is being squeezed laterally, i.e. sub-parallel to lines of longitude, partly due to relative displacement of tectonic plates, and partly due to the geometric adjustment caused by the plate movement pole-wards from the tropics. That is, to a narrower Earth-radius geometry. The resulting deformation has caused a slight bulge over north India which has reversed the drainage of the Ganges over the past 25 million years or so.

I should point out that the geometry is not the only reason for the bulge - there is also the little matter of Himalayan compression, the two processes resulting in both north-south and east-west compression.