The Hawaiian Islands are a volcanic chain, with large, active volcanic islands at the eastern end, and older, subsided seamounts and atolls at the western end of the chain. They are a continuation of the Emperor seamount chain, created by the same hot spot beneath the sea bed as the Pacific Plate first moved north, and then west-northwest. This change in direction of plate motion about 40 million years ago accounts for the different alignments of the Emperor and Hawaii chains. Hawaii’s Mauna Loa is a broad shield volcano, the largest on Earth. From its base on the sea floor it rises about 55,800 ft (17,000 m)—much taller than Mount Everest. Its weight has caused the underlying ocean crust to sag, producing the Hawaiian Trough to the north and east of the main island. The seas around Hawaii are tropical, and warm enough for coral reefs to grow. The prevailing winds in this region are the trade winds from the northeast, making the northeast coasts of the islands considerably wetter and greener.