Tides and Tidal Forces

Anchorage High and Low Tides

Historical Tide Marker, Anchorage, AK

Tides are caused by interactions between the sun, the moon, and coastal geography.

  • Newton's Laws of Gravitation say that any two bodies will have an attraction for each other.  The larger the bodies the larger the attraction and the closer the bodies the larger the attraction.  Therefore even though the sun is much larger than the moon, the moons influence on the tides is greater, because it is so much closer. 
  • Tides come and go as waves.  The shape of a bay or basin will affect the speed of that wave.
  • Bore Tides - When the tidal wave must move faster than the shallow water wave speed, it can break, forming a spilling wave front.

Spring (Flood) Tides

  • Highest high tides and the lowest low tides
  • Caused when the forces of the moon and sun are pulling together, occurs on new moons and full moons.

Neap Tides

  • More moderate tides
  • Caused by the moon and the sun pulling at right angles to each other, occur on ¼ and ¾ moons.
  • The graph beow shows 30 days of tides in Bridgeport, Conneticut. Can you find two flood tides and two neap tides?

The Effect of Parallax

  • Since the moon follows an elliptical path, the distance between the earth and moon will vary throughout the month by about 31,000 miles. The moon's tide-producing force acting on the earth's waters will change in inverse proportion to the third power of the distance between the earth and moon, in accordance with the previously mentioned variation of Newton's Law of Gravitation.
  • Once each month, when the moon is closest to the earth (perigee), the tide-generating forces will be higher than usual, thus producing above average ranges in the tides. Approximately two weeks later, when the moon (at apogee) is farthest from the earth, the lunar tide-raising force will be smaller, and the tidal ranges will be less than average.
  • Similarly, in the earth-sun system, when the earth is closest to the sun (perihelion), about January 2 of each year, the tidal ranges will be enhanced, and when the earth is farthest from the sun (aphelion), around July 2, the tidal ranges will be reduced.

Types of Tides

  • Some places have two high and two low tides a day, these are known as semi-diurnal tides.
  • Some places have one high and one low tide a day, these places are called diurnal tides. 
  • Some places that has two tides a day but of uneven size is known as a mixed tidal area.
Tidal Types

Image From: http://tidesandcurrents.noaa.gov/restles3.html

Tidal Type Maps


Life in Tidal Communities

  • Few other places on earth experiences such drastic changes in living conditions on a daily basis as do high latitude tide pools.
  • Temperatures can vary from extremely hot to well below freezing. During a sunny day the water temperature in a tide pool can increase significantly, raising salinity by the process of evaporation, while the oxygen content decreases to dangerous levels.
  • Because of these conditions, the creatures that live in the tidal zone have to be very adaptable. This is the most common characteristic shared by intertidal organisms.
  • When the concentration of salt in the pool becomes too high, the organisms become very stressed. They lose their body fluid and many of them die.   This can also happen if an area is exposed to the air for too great a time.
  • On the other hand, after a heavy rain the salt concentrations decrease. Too little salt is just as much of a problem for the intertidal organisms as too much. The freshwater soaks into the organisms tissues and inflates them, causing the animal or algae to swell and possibly die.

Tide Pool LIfe

Tidal Zonation

  • Animation of Tide Coming In
  • Intertidal organisms tend to group themselves by zones based on certain physical limitations (abiotic factors that affect their ability to thrive). A single intertidal area can be divided into several zones where environmental conditions differ.
  1. The ‘splash’ zone: organisms in this zone are adapted for long periods of exposure to air and the possibility of desiccation.
  2. The upper intertidal: organisms here are exposed to desiccation as well as the force of breaking waves.
  3. The mid-intertidal: in this zone, periods of exposure to air and the threat of desiccation are reduced, but living in prime real estate has its price as competition for space and predation becomes increasingly important.
  4. The lower intertidal: this area is not subject to severe wave or tidal forces except for minus tides, therefore more delicate sessile species and motile species not adapted for high energy live here.

Questions to Research

  1. Explain how the moon, the sun, and the shape of the shoreline act together to create "tides"
  2. What is a spring tide?  What conditions cause a spring tide?
  3. What is a neap tide?  What conditions cause them?
  4. Go to http://www.alaskaoutdoorjournal.com/ select tides on the left, and then Clam Gulch.  What were the best days in August to go clamming at clam gulch. (Hint: clamming is best done on days with minus tides).
  5. Using the same link, go back and select (the tides for) Anchorage.  Using the graph paper you were given in class create a graph showing the tidal heights (lows and highs) for Anchorage this week (Monday-Friday).   
  6. Explain the difference between diurnal, semi-diurnal, and mixed tides (http://www.physicalgeography.net/fundamentals/8r.html may help).  Look at your graph from number 5.  What "type of tides" does Anchorage have - Diurnal, Semi-diurnal, or mixed tides?
  7. Go to http://en.wikipedia.org/wiki/Bay_of_Fundy and describe why the Bay of Fundy has the highest tides in the world.  Examine the shape of this Bay, what are its similarities to Cook Inlet?
  8. Why do tides follow a 24 hour and 50 minute cycle instead of the expected 24 hour cycle?
  9. Go to Intertidal Ecosystem Description  or Monterey Bay Aquariums - guide to interdial animals  and give two or three types of organisms that might be found in each of the different tidal zones along the Alaskan coast.  Also describe what the conditions are like in each of these zones.
    a.    Spray zone:

    b.    Upper tidal zone:

    c.    Middle tidal zone

    d.    Low or Sub-tidal zone
  1. Color the handout on intertidal-zonation.