Rocks From Reefs Sheet - Reading Comprehension Worksheets


Rocks From Reefs
Notes Column
Coral animals are tiny relatives of jellyfish that live together in vast
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numbers. They produce skeletons that grow together to form a structure
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called a coral reef.
Tiny algae grow within the body of each coral animal. The algae provide
substances that the coral animals need to live. Like plants, algae need
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sunlight. For this reason, reefs form only in the warm, shallow water of
tropical oceans. Almost all growth in a coral reef occurs within 40 meters
of the water’s surface.
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A coral reef is like a living rock built by tiny creatures. How is the rock
built? Coral animals absorb the element calcium from the ocean water.
Their bodies combine the calcium with carbon and oxygen to form an
exo-skeleton made out of calcium carbonate (CaCO
). When coral
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animals die, their skeletons remain. More corals build on top of them,
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gradually forming a coral reef. Over thousands of years, reefs may grow
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to be hundreds of kilometers long and hundreds of meters thick. The
type of rock that the coral skeletons build is called limestone.
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Limestone has been forming in Earth’s oceans for more than 400 million
years. Limestone deposits that began as coral reefs provide evidence of
how Earth’s surface and environment has changed. Limestone that
formed when shallow seas covered the low-lying parts of the continents
may have become exposed when the seas retreated. When geologists
find fossils of an ancient coral reef, they know that the location once was
an area that had a warm climate and shallow ocean water.
Identifying fossilized reefs is fairly easy to do. When a drop of dilute
hydrochloric acid is placed on a piece of limestone, the acid reacts with
the calcium carbonate (CaCO
) to form bubbles of carbon dioxide. This
#1 and #2
“fizz” reaction is so characteristic of limestone than many geologists
using complete
carry a small bottle of dilute hydrochloric acid into the field for a rapid and
easy identification of limestone. We can watch the same reaction here in
(of course!).
Reflection Questions:
1. How do coral reefs form? _______________________________________________
watch the
2. What evidence do limestone deposits from coral reefs provide about Earth’s history?
#3 and #4.
3. Describe the reaction between coral (CaCO
) and hydrochloric acid (HCl):
4. What can you ASSUME will happen any time that CaCO
and HCl are combined?


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