How big can cracks in the Earth be?

Answer 1

If you're asking how long cracks (fault lines) can be in the earth, we don't think there's a limit. Fault lines we've found on the earth's crust (upper layer) are many thousands of miles long (e.g. in the Pacific Ocean).

If you're asking how wide they can get, that depends on how they are formed. The earth's upper layer/crust is composed of many materials, but together they all behave as a semi-solid/semi-liquid. They are constantly moving, though slowly and slightly. Also some places have pressure building up under it, inside the earth, for different reasons.

When the pressure becomes too great - either due to plate-tectonic shifts (e.g. in an earthquate) or a volcano, the upper crust can be fractured and "cracks" will develop. Their width and length depend on the amount of energy released, the softness or brittleness of the crust (types of rocks that the area is made of), what is beneath the crust in that area (e.g. internal fissures), the directionality of the energy release (along a line or radially) etc.

Fractures are known to be as wide as half a mile wide, though ones wider end up collapsing into something like a sink-hole. They can be thousands of miles, but new ones usually run for less than a few miles.

There are also natural formations some people think of as cracks - such as the Grand Canyon in the U.S. - but these are not true cracks in the earth. The Grand Canyon was formed through constant erosion of the earth by the meandering Colorado River, over millions of years.