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Article: The White Cliffs of Dover
The beautiful White Cliffs of Dover, England, are massive chalk deposits that are
about 400 meters thick (1,300 feet). It is the same kind of chalk used with chalkboards
found in many classrooms. Most of the chalk is made up of the shells of long extinct tiny
ocean floating organisms called coccoliths, which are about .001 millimeters in size.
Their shells are made up of calcium carbonate, which gives it its white appearance.
Calcium carbonate is also the building block of limestone. Millions of years ago, the
White Cliffs of Dover were located at the bottom of the ocean, and has since reached the
surface thanks to the uplifting forces of plate tectonics.
When alive, these coccoliths floated near the surface of the ocean as they lived
out their life. When they died, their shells settled to the ocean floor. Over millions of
years, the ocean bottom accumulated enough coccolith shells to reach a thickness of
about 400 meters. These shells are so tiny that they settle at a rate of about 2 years per
meter in very still water.
Vocabulary:
-What is the difference between chalk and coccoliths?
Content:
1. What are the White Cliffs of Dover made up of? ____________________________
2. Today, the White Cliffs are on the coastline of England. Where were they millions of
years ago? __________________________________________________________
3. How small are coccoliths? _______________________
Settling Time:
1. Assuming ocean depth is 4000 meters, how long will it take coccoliths to reach the
ocean floor as they settle at 2 years/meter? (answer in years)
2. How long will it take a clay-sized particle settling at 3 minutes/meter? (in days)
3. How long will it take sand-sized particles settling at 30 seconds/meter? (in hours)
4. According to questions 1 – 4, as particle size gets bigger, the time it takes to settle to
the ocean floor __________________ (increases or decreases).
Particle Size
Particle Size
5. Circle the graph that shows this relationship.
Settling time
Settling time