UNIT: EARTH`S EXTERNAL GEOMORPHOLOGY
Session 4 - Worksheet 4
THE ACTION OF SEAWATER
Activity 1
Read and translate this text about the action of seawater.
THE ACTION OF SEA WATER
The ocean is always moving although it moves in different ways.
There are three kinds of movement: waves, currents and tides.
WAVES
A wave is a rhythmic movement of the surface of the sea normally caused by the wind. It
can carry a lot of energy.
In a wave, we can distinguish the crest or highest point, the trough or lowest point, the
height, which is the vertical distance between crest and trough, and the wavelength or
horizontal distance between the crest of two waves.
Off-shore waves only move floating objects up and down, but waves break in shallow
water near the shore. Coastal waves are formed when the ocean floor slows the water
at the bottom of the wave and the top moves faster, making the wave break against the
shore.
When waves hit the rocks on the coastline, they break away, transport and deposit
materials. They shape the coastal landscape.
CURRENTS
Ocean currents are mass movements of ocean water. Currents are like rivers in the
ocean, they move water and materials from place to place. Sailors use them to sail faster
from place to place.
Surface currents move water horizontally. They are powered by the wind. They can be
warm or cold. They are important because they distribute heat from equatorial regions
to other areas of the Earth, and therefore have an important influence on the climate.
The Gulf Stream current flows from West to East in the Atlantic Ocean. It brings warm
waters from the Equator to Northern Europe. It releases heat to the atmosphere and
makes the climate warmer.
In some sea places, currents are produced by differences in temperature or salinity.
These are called density currents. They push water from the surface to the bottom or the
other way round.
Upwelling is the name we give to a density current that brings deep water to the surface.
It drags high concentrations of nutrients making the plankton grow and attracting fish to
the area.
TIDES
Tides are a regular rise and fall of the sea level. A daily cycle of two high tides and two
low tides takes 24 hours and 50 minutes. Tides are caused by the gravitational attraction
of the Moon on the ocean water. When the moon is located opposite a mass of water, it
pulls strongly on it and the water level rises (high tide on the shoreline). Approximately
six hours later, the Earth and Moon have changed place so the water is not pulled
anymore. Its level now falls (low tide on the shoreline).
The sun also affects tides. When the Moon, the Earth and the Sun are lined up, they pull
together and their combined effect on water is stronger so they cause spring tides. At
spring tides, the high tides are higher and the low tides are lower than normal. When the
Moon, the Earth and the Sun form a right angle, their effect is weaker and they cause
neap tides. At neap tides, high tides are lower and low tides are higher than normal.
THE ACTION OF THE OCEAN ON THE COAST
The coastline or seashore is the line where the sea meets the land. Its features vary
depending on climate, wind, sea and the type of rocks of which it is composed.
Headlands are formed when the sea attacks a section of coast with alternating bands of
hard and soft rock.
The bands of soft rock, such as sand and clay, erode more quickly than those of more
resistant rock, such as chalk. This leaves a section of land jutting out into the sea called a
headland. The areas where the soft rock has eroded away, next to the headland, are
called bays.
The most important coastal features are:
1. CLIFFS: a cliff is a steep high rock wall along the seashore usually with layers of
sediments exposed. Sea cliffs retreat landward because of the constant action of
waves.
2. WAVE-CUT PLATAFORM: a sloping rocky platform left behind as the cliff retreats
due to wave erosion. A wave-cut platform can be seen at low tide.
3. HEADLAND: a point of high land extending out into the sea, a promontory. The
erosion caused by waves on a headland can build caves, arches and stacks.
4. BAYS: Headlands and bays are often found on the same coastline. A bay is
surrounded by land on three sides, whereas a headland is surrounded by water
on three sides. Headlands are characterized by high, wave-breaking, rocky shores,
intense erosion, and steep sea cliffs. Bays generally have less wave (and often
wind) activity than the water outside the bay, and typically have sandy beaches.
5. NATURAL CAVE: a cavity that results from the slow dissolution and erosion of
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rock by water. The weakest areas of a cliff are eroded faster creating first a hole
and then a cave.
NATURAL ARCH: an arch which is hollowed from a headland by the action of sea.
STACK: a needle-shaped column resulting from the collapse of an arch. When it is
eroded, it is called stump.
BEACH: an accumulation of sand and pebbles along the coast which is usually
mixed with particles originating from living things.
TOMBOLO: a ridge of sand joining an island to the shore line.
Activity 2
Loop activity
a) Work with your partner/group to do this loop activity.
b) Cut out the "loop cards" and divide them into two piles of "Terminology" cards
(short) and "Definition" cards (long).
c) Put the "Terminology" pile face down on the table.
d) Give each person in your group 3 "Definition" cards.
e) Take it in turns to pick up a terminology card and ask for the definition:
e.g.
What does RIDGE mean?
Can you define RIDGE?
f) The person who has the correct definition matches the pair and says:
e.g.
A RIDGE is the long and narrow upper edge, angle or crest of
something, like a hill, wave, or roof.
WORK UNTIL ALL YOUR CARDS ARE FINISHED
Activity 3A
Label the coastal features that you can see in this drawing.
Activity 3B THE PROCESS OF CLIFF EROSION
Identify in the drawing the following process of cliff erosion:
 The sea attacks the base of the cliff forming a wave-cut notch.
 The process is repeated and the cliff continues to retreat.
 Eventually the cliff takes its current shape.
 The notch increases in size causing the cliff to collapse.
 Weather weakens the top of the cliff.
Activity 4
Search for images on the Internet.
Look for some seashore images online and show them to your classmates,
explaining the most important geological features.
Activity 5
Field trip.
Apply your knowledge to the
environment!
Carranques Beach