Tides

What causes the tides

Open Ended

Reflect on what you saw in the video by answering these questions. Answer "What did you see?", "What did you think about it?", and "What did it make you wonder?".

The Hopewell Rocks in the Bay of Fundy

The spectacular time lapse video of the tides at Hopewell Rocks in the Bay of Fundy shows how dramatic this natural phenomenon can be. Water levels there can rise and fall by 14 metres and in some parts of the bay the tides flow in and out as much as 5 kilometres!

The outward flow of water is known as an ebb tide and the inward flow is called a flood tide. The area that gets periodically covered and exposed by the tides is the intertidal zone, which is usually made up of mud flats and rock ledges. Intertidal zones are home to whole communities of living things – such as birds, crabs and barnacles – that depend on the daily ebb and flow of water in different ways.

But what makes the tides in the Bay of Fundy the biggest in the world?

The Role of Gravity

As mentioned in the video, the main cause of tides on Earth is the gravitational pull of the Moon. The reason the Moon has a big effect on our planet is that it's both massive and nearby. Gravity depends on mass and distance: the more mass an object has, and the closer it is, the stronger its pull. 

The Role of Gravity

But some parts of the Earth are closer to the Moon than others. So the Moon pulls on the closer parts more than it does on the further parts. 

Imagine if the Earth was completely covered by ocean, as shown below. The strongest pull, at A, causes the ocean surface to bulge outwards.

The Role of Gravity

But notice that there's a second bulge on the side furthest from the Moon, at C. This is where the Moon's gravitational pull is weakest. The reason for this bulge is complicated, and you don't need to understand it. But it's important to know that the bulge is there because, as you'll see below, it's the reason we have two high tides a day.

Draw the following diagram of Tides into your books

The Important bits

Write the following into your Science books.

• Tides and the Moon:

  • Tides are the regular rise and fall of sea levels caused mainly by the gravitational pull of the Moon. The Sun’s gravity also plays a smaller role.

The Important bits

Write the following into your Science books.

• Gravitational Pull of the Moon:

  • The Moon’s gravity pulls on the Earth's oceans, causing water to bulge out on the side of Earth closest to the Moon, creating a high tide.

  • A second bulge (high tide) occurs on the opposite side of Earth due to inertia (water resisting the Earth’s movement as it rotates).

The Important bits

Write the following into your Science books.

• Two High and Two Low Tides:

  • As Earth rotates on its axis (every 24 hours), any given location on Earth passes through two high tides and two low tides each day.

The Important bits

Write the following into your Science books.

• Tidal Cycle:

  • The time between one high tide and the next (or one low tide and the next) is approximately 12 hours and 25 minutes.

  • The tides change gradually throughout the day as Earth rotates and the positions of the Moon and Sun shift.

The Important bits

Write the following into your Science books.

• Tidal Influences:

  • The shape of coastlines, the depth of the water, and the presence of bays can influence the height and timing of tides. For example, narrow bays can amplify tidal effects.

The Important bits

Write the following into your Science books.

• Spring and Neap Tides:

  • Spring tides occur when the Sun, Moon, and Earth are aligned (during New and Full Moons), causing higher high tides and lower low tides.

  • Neap tides occur when the Sun and Moon are at right angles relative to Earth (during First and Third Quarters), causing smaller differences between high and low tides.

Quiz time

Now head to your dashboards and do the quiz related to todays lesson.