Although volcanic mountains and islands usually form where two crustal plates meet and crush against one another they also form over 'hot spots' on the earth's crust - areas lying above intense activity deep in the earth's mantle. Directly over the hot spot a volcano is formed. When the crust passes away from the center of activity the volcano becomes extinct and a fresh one then erupts alongside it, producing in time a chain of progressively older volcanic islands in the middle of the ocean. During the Neogene to Quaternary (starting around 5 million BC), a hot spot was responsible for producing the Hawaiian island chain, and in the Pacific during the Posthomic a hot spot is in the process of generating the Batavian Islands.