GRADE 7
Name:
The tides of the oceans and seas are the predictable, repeated rising and falling of
their levels every 24 hours. In a 24-hour period, the ocean will rise twice (two high
tides) and then it will fall twice (two low tides).
High Tide
Low Tide
Tides and the Moon
The tides are created by the pull of the Moon on the Earth. Remember that gravity
is the force exerted on one body or object by another body or object. The Moon,
though smaller than the Earth, exerts a gravitational pull on the surface of the Earth.
It exerts a force on both the land and on the ocean. We can’t notice the pull on the
land but we can notice the pull on the oceans. As the Moon revolves around the
Earth, it causes the high and low tides.
When the Moon is in this position, A and B experience high tides and C and D
experience low tides.
Moon
At A the water is pulled toward the Moon creating a tidal bulge resulting in a high
tide. At B, the Moon is pulling the land towards itself and the water “left behind” so
that there is a tidal bulge, and high tide, on that side as well.
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Grade 7 Natural Science: Planet Earth and Beyond: Relationship of the Moon to the Earth: Tides
Tides
Over a 24-hour period, in any particular area, there will be a tidal bulge and high tide
twice a day, as well as a low tide. The high tides are approximately 12 hours apart
and the low tides are about the same.
Spring Tides
Tides are not only influenced by the gravitational pull of the Moon. The Sun affects
them too.
Can you remember the two factors that determine the gravitational pull of one
object by another?
1)
2)
If we compare the Sun and Moon with regard to their gravitational effect, we must
consider these two factors.
The Sun is much bigger than the Moon, but the Sun is very far away.
The Moon is much smaller than the Sun, but the Moon is very close to the Earth,
compared to the Sun!
Therefore, the effect of the Moon’s gravitational pull on the surface of the Earth
(including the oceans) is greater than the effect of the Sun’s pull on the Earth’s
surface.
However, at certain times of the year and month, the Sun and Moon work together
to create Spring tides!
New Moon
The Sun, Moon and Earth are all aligned, and the Moon is in its New Moon phase.
As usual, there is a tidal bulge at A and B, because of the Moon’s position. But now,
that tidal bulge is enhanced, because the Sun, along with the Moon is “pulling” at the
water and the land. This means that high tide (A and B) is extra high and low tide (C
and D) is extra low. These very high tides are known as Spring Tides.
© e-classroom 2014 www.e-classroom.co.za
GRADE 7
Grade 7 Natural Science: Planet Earth and Beyond: Relationship of the Moon to the Earth: Tides
At C and D, the pull of the Moon on the water is not that strong, and so the oceans
there experience a low tide.
Full Moon
Activity: Ecosystems in Intertidal zones
Background:
Most organisms exist in ecosystems where there are not drastic changes in short
periods of time. For the organisms living in the intertidal zone, the conditions of their
ecosystems change continually through the day.
The intertidal zone is the area on the rocks or on the beach where the water meets
the land. This zone is affected by changes in the tides every six hours. At high tide,
the area is often covered in water or experiences more water than at low tide. At
low tide, these areas can be completely dry, exposed to the Sun and dry air. The
organisms living in this environment need to adapt to survive.
Your task:
Conduct some research and then create a poster about one of these intertidal
ecosystems. Use the questions to guide you and remember to include all the
information and some pictures on your poster!
Choose either the rocky shore ecosystem OR the sandy shore (beach) ecosystem.
Include this information:
1) Describe the shoreline of each of these ecosystems. (What does it look like? What is it made of? Is it sloped or level?)
2) List a few of the common animal and plants that are found there.
3) What challenges do these organisms face with changes in tidal activity?
4) How have some of these organisms adapted to cope with the extreme changes?
© e-classroom 2014 www.e-classroom.co.za
GRADE 7
Grade 7 Natural Science: Planet Earth and Beyond: Relationship of the Moon to the Earth: Tides
The same occurs at Full Moon as you can see in the diagram below. The only
difference is that the Sun and Moon and pulling in opposite directions. The Sun pulls
the water and the land in one direction and the Moon pulls in the other direction.
Thus, the tidal bulge at A and B is enhanced to create Spring Tides.
GRADE 7
Can you remember the two factors that determine the gravitational pull of one
object by another?
1) Mass of the objects
2) Distance of the objects from each other
Activity: Ecosystems in Intertidal zones
Your task:
Conduct some research and then create a poster about one of these intertidal
ecosystems. Use the questions to guide you and remember to include all the
information and some pictures on your poster!
Choose either the rocky shore ecosystem OR the sandy shore (beach) ecosystem.
ROCKY SHORE ECOSYSTEM
Include this information:
1) Describe the shoreline of each of these ecosystems. (What does it look like? What is it made of? Is it sloped or level?)
The shoreline is made of rocks of different sizes which are all grouped together.
They form rock pools. The ground generally slopes toward the ocean but the
rocks provide flat surfaces as well as steeply sloped areas.
2) List a few of the common animal and plants that are found there.
The rocky shore ecosystem has incredibly high biodiversity, with seaweed and
phytoplankton, crabs and other crustaceans, snails and limpets (molluscs),
small fish, zooplankton, sea sponges, sea cucumbers, and birds that use the
ecosystems for food.
3) What challenges do these organisms face with changes in tidal activity?
As the tide comes in there is a lot more water than at low tide. This water is
saline (salty) and cold. As the tide goes out, the rocks and the organisms living
on or around them (eg. in rock pools) are exposed to dry air and the heat of
the Sun. Changes in temperature, salinity as well as the impact of the waves
crashing against the rocks are just some of the challenges organisms in these
areas face. Another challenge is being exposed to predators when the tide goes
out – there is nowhere to hide!
4) How have some of these organisms adapted to cope with the extreme changes?
Waterproof shells that seal tightly onto the rock surface – this seals the
organisms fleshy body inside the shell and locks in the moisture until the tide
comes in again (eg: limpets, periwinkles, etc.).
© e-classroom 2014 www.e-classroom.co.za
Grade 7 Natural Science: Planet Earth and Beyond: Relationship of the Moon to the Earth: Tides
Answer sheet
Organisms in high impact zones (where the waves beat against the rocks) are
often enclosed in strong shells and are securely attached to the rocks (e.g.
muscles).
Some organisms live in the rock pools – they hide under rocky outcrops to be
protected from the Sun and from predators (e.g. small fish, crabs).
Sea cucumbers have a slimy layer over their “skin” which prevents dehydration.
SANDY SHORE ECOSYSTEM
Include this information:
1) Describe the shoreline of each of these ecosystems. (What does it look like? What is it made of? Is it sloped or level?)
The Sandy Shore is a very harsh environment. It is sloped toward the ocean with
little protection. It is basically made of sand, that shifts as freely as the ocean
moves it.
2) List a few of the common animal and plants that are found there.
Shelled creatures (crabs and other crustaceans), mollusks and worms and
birds such as seagulls, oyster-catchers, sand-pipers, etc. Phytoplankton and
zooplankton that is brought in as the water rises.
3) What challenges do these organisms face with changes in tidal activity?
The Sandy Shore is a particularly hostile environment. Its inhabitants need the
water to survive, but are only exposed to it at high tide. If they live closer to the
water, they have to endure the constant moving of the waves and the pounding
of the sand. There is not a lot of food available – most food is brought in with
the tide and many of the organisms are scavengers. The ecosystem is exposed
to the Sun and offers little protection from the heat and from predators.
4) How have some of these organisms adapted to cope with the extreme changes?
Most organisms living in this environment live under the surface of the sand.
Buried in the cool damp sand they wait for the water to rise (e.g. crabs).
Others bury themselves under the ground at the water’s edge so that they can
be protected from the heat and the pounding of the waves and stay wet (e.g.
mollusks).
The birds (like oyster-catchers and sandpipers) have long thin legs which allow
them to move quickly across the hot sand.
Many of these organisms have adapted to live as scavengers, because that is
the most readily available source of food.
© e-classroom 2014 www.e-classroom.co.za
GRADE 7
Grade 7 Natural Science: Planet Earth and Beyond: Relationship of the Moon to the Earth: Tides
Sea sponges are attached to the rocks and as the name suggests, they soak in
the water at high tide and “hold” it there through low tide.