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Tham khảo tài liệu 'mastering skills for the toefl ibt part 3', ngoại ngữ, toefl - ielts - toeic phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Mastering-Books_5 2006.5.29 2 13 PM Page 713 W M the other hand occur in warmer waters that average around 18 C and can reach into the high 20s. These reefs are built by the animals that live there especially algae and corals. Over time the reef grows by building on top of the calcium carbonate skeletons of polyps. Excuse me sir what exactly are polyps Polyps Polyps are small simple tube-shaped water animals. The polyps divide and grow on top of their old skeletons or houses. Now as I said earlier we can use coral reef ecosystems as indicators of larger global changes. All sorts of information about the past can be obtained from rocks and coral cores. Coral reefs can tell us things such as which times the Earth experienced a rise or fall in sea levels. They can even inform us about events such as ice ages huge volcanic eruptions and meteors falling to Earth causing mass extinctions by wiping out whole species. Modern corals grow in warm clear well-lighted and shallow water. Since coral reefs grow best from the low tide line to about a 20-meter depth even relatively small changes in sea level can have very dramatic effects on coral reef growth. Reefs will always grow to the level of low tide. The location of fossils in a reef is very important because by observing fossils in a reef we can accurately estimate sea level at the time the reef was formed. Therefore fossils help us chart the various changes in sea level that have occurred over the years. Thanks to research by marine scientists we know that sea level change has been going on continuously during the evolution of corals and reef organisms. There have been 17 cycles of sea level rise and fall in the last 2 million years. So how do marine scientists get their information One method they use is called coral coring. By drilling a vertical core through the reef it is possible to see periods of growth and decline in the reef. It is possible to date these periods of growth and decline by looking at fossils in the .
