Tides Science 8: Water Systems

Addressed Curriculum Outcomes: •Explain how waves and tides are generated (311-10a)•Explain how waves and tides interact with shorelines (311-10b)

What are Tides?

• Tides are the rising and falling of the ocean surface

• The highest tides occur in Canada’s Bay of Fundy

• Average of 14 meters » The average tide globally is only about 2.5 meters

• Why? • Tides are often greatest where the coast funnels

the water, forcing it into a narrow inlet

The Bay of Fundy

The Bay of Fundy is known for its high tidal range. It is believed to have the highest vertical tidal range in the world (i.e. highest tides)

High Tide and Low Tide at Bay of Fundy (NS/NB)

High and Low Tides at Bay of Fundy

“Flower pots” caused by tidal erosion

Low Tide and High Tide at Hopewell Rocks (NB)

What is a Tidal Bore?• A rare natural phenomenon occurring on several rivers emptying

into the Bay of Fundy.

• Bores occur in relatively few locations worldwide, usually in areas with a large tidal range and where incoming tides are funneled into a shallow, narrowing river via a broad bay.

• The surge of the incoming Fundy tide temporarily reverses the flow of these rivers and appears as a crest of water traveling upriver.

• On the Shubenacadie River, the tidal bore and rapidly rising tide results in extremely turbulent waters. This is great for adventures like tidal bore rafting!

Tidal Bore Rafting in Shubenacadie (NS)

Shubenacadie River 2009

Tidal Bore Rafting in Shubenacadie (NS)

September 20th 2009

There are two main causes for the rising and lowering of tides…

Any ideas?

What Causes Tides?

What Causes Tides?

1. Centrifugal Force due to the rotation of the Earth

2. The gravitational forces exerted by the Moon and the Sun on the earth’s ocean waters

1. Centrifugal Force• The rotation of the earth on its own axis results in a centrifugal

force (outward force on a body moving in a curved path around another body).

• Swinging around a bucket of water or Gravitron Amusement Park Ride

• A “tidal bulge” (high tide) forms at the opposite side of the Earth’s centre of gravity (axis) due to the centrifugal force created by the Earth spinning on its own axis.

– “Center of gravity” = The point around which two bodies move as each revolves around the other (dependent on mass – the earth is not consistent of mass throughout)

Gravitron/Round-Up RidesAn example of centrifugal force

2. The Gravitational Forces Exerted by the Moon & Sun

• The moon exerts a gravitational force on the Earth, pulling the “oceans” toward it. This creates a tidal bulge (high tide) at the side of the Earth that is closest to the moon.

• The sun also exerts a gravitational force on the Earth’s oceans, but even though it is much larger than the moon, its “pull” on the oceans is much smaller because it is much farther away from the Earth than the moon is.

Why are there 2 sets of tides per day?

Why are these tides 6 hours apart?

Areas such as the Bay of Fundy look very different over time. The tides are complete

opposites (low vs. high) over only a few hours.

There are 2 “low” tides and 2 “high” tides per day. These tides change approximately

every 6 hours. Why might this be?

Why do we have 2 sets of Tides per Day?

• The moon’s gravitational pull and the centrifugal force caused by the spinning of the Earth are in opposite directions

• Both of these forces cause tidal bulges; this (and because the earth takes 24 hours to make one full rotation) is why we have 2 high tides and 2 low tides per day (over the course of 24 hours)

Sea Kayaking at Hopewell Rocks (July 1st 2011)

“Not Since Moses” 5km Run at Five Islands, New Brunswick

July 3rd 2011

Why do the tides occur a little later each day?

• Even though low and high tides are about 6 hours apart, they come a little later each day

• Why might this be?

Why do the tides occur a little later each day?

• The moon rotates around the earth a little faster than the earth rotates around its own axis.

• This causes the tides to come about 50 minutes later each day.

• However, low and high tides are still said to be approximately 6 hours apart.

RECAP: The 2 Main Causes of Tides

Recap: Centrifugal Force

• The centrifugal force of the Earth spinning around its center of gravity causes a tidal bulge (high tide) at the opposite side of its center of gravity.

Recap: Gravitational Force by Moon

• The gravitational force exerted by the moon on the Earth causes a tidal bulge on the side of the Earth closest to the moon because that is where this gravitational force is the strongest.

Tides Simulation: http://www.pbs.org/wgbh/nova/venice/tides.html