So how did this happen?
The Arabian plate is oceanic crust which is denser than the continental crust of the Eurasian plate. This weight different is what forces the oceanic plate to sink beneath the continental one. In this case, the Arabian Sea is being forced beneath Pakistan. The island has been forced up due to the movement which occurred at the place where the two plates meet. Scientists call these islands mud volcanoes.
The new island is in fact the ocean floor so has a rough surface and is covered with sand and rocks, but is also giving off gas. This gas was naturally produced beneath the ocean floor and was compressed over time because of sediment building up in layers on top of it. The sudden movement of the earthquake released the gas causing it to explode to the surface and bring up some of the sea floor with it. Unfortunately though, as time goes by the forces which pushed the island to the surface will run out and the island will gradually return to the sea floor. As well as this the power of erosion from the effect of the sea waves crashing against it will wear away the island and so it will decrease in size this way too.
The ocean floor will never look the same again at this location, and scientists in the future will be able to tell that there was a mud volcano here from the rocks that are left on the sea floor.
If you have a question you would like answered or want to know more, here are some links to the BBC and Telegraph reports of this event which can give more information: