TAILIEUCHUNG - BIOMES OF THE EARTH - OCEANS Phần 3

Tham khảo tài liệu 'biomes of the earth - oceans phần 3', ngoại ngữ, anh ngữ phổ thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 38 OCEANS make nearby San Francisco one of the most earthquake-prone cities in the world. Birth and death of an ocean How do ocean basins form in the first place In the 1960s the Canadian scientist Tuzo Wilson 1908-93 suggested that an ocean basin has a life cycle now called the Wilson cycle . Wilson s theory of how ocean basins are created also explains how continents might break apart as happened in the case of the ancient supercontinent Pangaea and its offspring Laurasia and Gondwana. The first stage of the Wilson cycle the embryonic ocean is a dry depression. This stage is illustrated by today s East African Rift Valley. Here a weakness in the continental crust heated from below has uplifted and split. An unsupported section of crust has sunk subsided to form a rift valley. The East African Rift Valley is not directly connected to the sea but if it were seawater would flood in to fill it. Such an event probably happened to the rift valley that formed between the African and Arabian plates some 20 million years ago. This rift valley has since widened to become the Red Sea. The second stage of the Wilson cycle the juvenile stage of an ocean occurs when a rift valley floods accompanied by seafloor spreading along its length so that the landmasses on either side move apart. The Red Sea is at this stage today. Given another hundred million years or so the Red Sea will probably have grown to become a mature ocean the third stage. The present-day Atlantic Ocean depicts the third stage of the Wilson cycle. The Atlantic began to form about 180 million years ago when a rift valley split Pangaea into Laurasia and Gondwana. The Atlantic is now a mature ocean and is still expanding. It will not however grow forever. Eventually the rate of seafloor destruction in trenches will overtake the rate of seafloor creation at mid-ocean ridges and the ocean will begin to shrink as is the case for today s Pacific Ocean. The Pacific Ocean basin illustrates the fourth stage of the .

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