Tide Patterns

Engage

15 Minutes

If you live on or near salt water, use information about your local bay or body of water for this investigation. If you are inland, make a class list of saltwater bodies in Alaska that students have visited, and choose one to be the focus of the investigation. Share stories and observations of tides.

Explore​

45-90 Minutes

Students will graph local tide data, and then compare their graph to graphs of other periods of time for the same location.

Using a projector or student copies of the Marigram Information and Examples, introduce students to the idea of a marigram and show them how to make one.

Decide on the period of time for your graphs (one week, two weeks, one month), and divide students into small groups (2-4 students). Ask each group to plot the same week’s data, or assign specific weeks of a month or a specific month to each group, to cover a longer time period.
Distribute the following:

• Local tide books, or printed tide tables for your area  along with moon phase data.
• Copies of the blank graph or other graph paper. (The blank graph is for 7 days. To graph for a month on one page you will need to have graph paper with very small divisions lengthwise on the paper. 124 squares = 4 squares for each day times 31 days. A 2-week graph would need 56 divisions across the paper. Student graph paper could also be made on an overhead transparency to make it easy for them to share their work. These could be washed and re-used.
• Student worksheet copied in a size that will fit in the student science notebooks (If you print at 80%, it will fit into the science notebook).
  Give students time with their group to answer the questions and construct their own marigram.

As groups finish their marigrams, give them the Tides and the Moon handout. They will read and answer a question about the moon’s influence on tides, and then add moon phases to their marigram and label the spring and neap tides. (Slower groups can finish this part as the class discusses it, or as homework).

Explan

15 Minutes

Bring the group back together as a class to discuss the marigrams. Share graphs with the whole class to get an understanding of the pattern that occurs over time. Discuss why it is important for people to know about tides. Give examples of people who use tide data. What makes the intertidal zone (between low and high tide) such a changeable and dangerous environment for the organisms that live there?

Use the tide diagrams to learn the difference between a diurnal, semidiurnal, and mixed semidiurnal tide. Identify the type of tide that was graphed in the previous activity.

Ask students to consider: Why do tides have different patterns in different parts of the world? Ask them to write their ideas in their science notebooks.

Look at the map of tide types. Ask the students to find other areas of the world that have tides similar to the ones you graphed. Identify different types of tides that can be found in Alaska.  Then, explain the following:

If the earth were a perfect sphere without large continents, all areas on the planet would experience two equally proportioned high and low tides every lunar day. The large continents on the planet, however, block the westward passage of the tidal bulges as the earth rotates. Unable to move freely around the globe, these tides establish complex patterns within each ocean basin that often differ greatly from tidal patterns of adjacent ocean basins or other regions of the same ocean basin (J.L. Sumich 1996).

Elaborate

30-40 minutes​

Provide students with marigrams from tide patterns different from the ones they created earlier, using Four New Marigrams as a handout. Ask them to interpret the tides. The marigrams can be cut and glued into their science notebooks, and their interpretation of the tide and the answers to the following questions can be written next to them.

Ask students to answer the following in their science notebooks for each of the four marigrams:

1. What is the amplitude of the tide? (It is the difference between high and low.) How does it differ from the Alaska tide that you graphed earlier?
2. How many highs occur each day? How many lows? Is this a diurnal, semidiurnal, or mixed semidiurnal tide?
3. In what other ways does this tide pattern compare with the one that you graphed previously? Do you notice anything else interesting about it?

Notes:

1. People often believe that there are no tides in the Mediterranean Sea because the tidal range (amplitude) is only a few centimeters. The tide in the north part of the Mediterranean is mixed semidiurnal, similar to that in Southeast Alaska and the Gulf of Alaska.
2. The tide is said to be diurnal when only one high water and one low water occur during a tidal day. Tides along the Gulf Coast of Florida generally west of Apalachicola are of the diurnal type.
3. Southampton, England, has a “double high tide” caused by the flow of water around a nearby island, the Isle of Wight. At Whitby, England, in the North Sea, the spring tide occurs two days behind the new/full moon. The reason for this is that the tide originates in the southern oceans, the only place on the globe where a circumventing wave (as caused by the tidal force of the moon) can travel unimpeded by land.
4. The Bay of Fundy has the largest tidal range (amplitude) in the world.

Refer back to the map of tide types as needed (It might be helpful to print this map).

Evaluate

30-40 minutes​

Discuss the question: “Could waves and/or tides account for the location/dates of toys that were found on beaches after the spill?

These questions may help to guide the discussion:
What are waves?
How do water particles move in waves?
How do objects move in waves?
What are tides?
How do water particles move in tides?
What is a tide current?
Where would you find weak tide currents?
Where would you find strong tide currents?
Do the currents always move in the same direction?
What do we know about tide currents in the locations where the toys ended up?
What would we need to know to find out?
Which is more likely to influence the movement of the toys, waves or tides?

Following discussion as a whole class or in small groups, have students decide which factor they think has a greater effect on the movement of floating objects. Ask them to write their prediction or idea and explain it in their science notebooks. Class sharing of these predictions can lead into the next investigation.

Curricular Connections

This investigation can provides connections to math through graphing activities and measurements and calculations of wave frequencies and periods. The many opportunities to write, discuss, and present information to the class provide connections to language arts.

Ideas for adapting to different local environment or context. Investigate local cultural traditions and knowledge related to tides.