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Tides, coastal ecosystems and gravity

Gravity, why there are two tidal bulges, spring and neap tides, and life in the intertidal zone. Read one part at a time and tap “I know this”. Then test yourself in the quiz.

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Gravity

Gravity pulls all objects with mass towards each other. Two large masses close together pull the strongest. An astronaut weighs less on the Moon, because the Moon’s gravity is weaker. But a 6 kg rock still has a mass of 6 kg on the Moon: mass does not change, only weight.

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Two tidal bulges

MoonEarthhigh tidehigh tidelow tidelow tidegravity
The sea bulges out towards the Moon and on the far side.
  • The near bulge: the Moon pulls the near water the most.
  • The far bulge: the Earth is pulled more than the far water, so the water stays behind.
  • The Earth spins, so a beach moves in and out of the bulges: high and low tide at different times.
  • High tide is later each day, because the Moon moves on in its orbit and the Earth must turn further.
Rule

Spring tide: the Sun and Moon pull together in a line. Neap tide: the Sun pulls sideways and partly cancels.

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Life between the tides

  • The intertidal zone lies between the high- and low-tide marks; each zone is dry for a different time, so different organisms live there.
  • Mussels and limpets shut tight or clamp on; sea anemones pull in their tentacles; crabs hide under stones.
  • Seaweed’s holdfast anchors it to the rock; incoming tides bring plankton to filter feeders.
  • African black oystercatchers feed at low tide, when their food is uncovered.
  • Spring tides are dangerous for people fishing on rocks: the sea comes much higher than usual.
African black oystercatchers at low tide on the rocky shore.
African black oystercatchers at low tide on the rocky shore.
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Remember this and test yourself

Remember this: your summary
  • Gravity is a pull between all objects with mass: bigger mass = stronger; further apart = weaker.
  • The Moon affects the tides more than the Sun, because it is much closer.
  • There are two tidal bulges: one towards the Moon and one on the far side. Halfway between them it is low tide.
  • Two high tides are about 12 hours 25 minutes apart; high tide comes about 50 minutes later each day.
  • Spring tide: Sun, Earth and Moon in a straight line. Neap tide: at first and last quarter.

Write these points in your book. Read them again the night before the test.

Memory trick

The Earth wears two sea bulges, like a rugby ball: one faces the Moon, one faces away. The beach turns through both: two high tides a day.

Flash cards · Tap to turn over

Important words

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Test yourself · Part 2

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