How Do Black Holes Form from Space-Time Curvature?

[Gravitonion]

If gravity is a distortion between space and time, then how come black holes are formed, only the surface of an object should then exert gravity, right?

 

[Simon Bridge]

In general relativity, gravity is represented as curvature in space-time. Space-time exists everywhere, even inside solids. Every atom has its own gravitational field, it's own little dimple in space-time, and they all add-up to make the object's gravity. Black holes are formed when enough little bits gather together so that the escape velocity some distance from the objects center of mass is greater than the speed of light.