Time To Read
Date Last Modified
Place 02 · Monterey Bay · Chapter 5 of 7
The Tide Comes In
Twice a day the ocean at Monterey pulls back, and a whole world of animals has to survive being left out in the air.
About 15 minutes · 1 video · 1 simulation · 3 quick activities
Start here
The Moon runs the shoreline
The Moon’s gravity pulls the ocean into two bulges, and Earth spins through both, so the coast gets two high tides a day. At Monterey the two highs are different heights and so are the two lows, and on an average day the water rises and falls about 1.6 meters (5 feet). Every animal on the rocks lives by that schedule: some get a few hours of air a day, others a few hours of sea.
Watch this
See it move
BrainStuff — How Do Tides Work? Under four minutes on how a rock 384,000 kilometers away keeps moving your beach towel.
While you watch, look for
- What is pulling on the ocean to make the tides
- Why most beaches get two high tides a day, not one
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The pull
Two bulges, one Moon
The Moon’s gravity tugs the ocean into a bulge on the side facing it, and a second bulge forms on the far side. Earth spins through both, and because the Moon moves along its orbit too, each day’s tides arrive about 50 minutes later than yesterday’s. The Sun pulls too, but only about half as hard.
The rhythm
Spring and neap
When the Sun and Moon line up, at new moon and full moon, their pulls add together: spring tides, with the highest highs and lowest lows. At the quarter moons they pull at right angles: neap tides, with a small tidal range. Spring tides have nothing to do with spring; they come twice a month, all year.
The shore
Four zones, four lifestyles
The splash zone only gets spray, and periwinkle snails hide in its cracks; the high zone belongs to barnacles and limpets that clamp shut through hours of sun. The mid zone is crowded with mussels, ochre sea stars and anemones, and the low zone, uncovered only by the lowest tides, is thick with surfgrass, urchins and nudibranchs. The higher you live, the harder your day.
Photos: Miguel Rivera, Ray Bilcliff, David J. Boozer / Pexels
Play with it
Line up the Sun, Earth and Moon
This is a real gravity simulator. Open the Model screen, choose the Sun, Earth and Moon, press play, and pause when the Moon is between Earth and the Sun, then again when it sits off to the side at a right angle.
Simulation: Gravity and Orbits, PhET Interactive Simulations, University of Colorado Boulder (CC BY 4.0).
Two questions to answer while you are in there
- With the Moon lined up between Earth and the Sun, what phase is the Moon in, and would Monterey get spring tides or neap tides that week?
- With the Moon at a right angle to the Sun, why would that week’s tides be smaller?
Try it yourself
Check yourself
Try it
Uncovered or Underwater?
It is low tide at Monterey: decide which spots are out in the air and which are still under the sea.
Try it · interactive
Tide Clock
Slide through a day at Monterey, switch the Moon between new, full and quarter, and watch the lopsided double tide uncover the splash, high, middle and low zones one at a time.
Open the Tide Clock in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Tidal range
Height difference between high and low tide
Spring tide
Extra-big tide at new and full moon
Neap tide
Small tide at the quarter moons
Intertidal zone
Shore between high and low tide
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: The Tide Comes In
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Our World Ocean 14.6 — Spring and Neap Tides — LibreTexts · the friendliest version of this · 10 minutes
- Webb 11.3 — Tide Classification — LibreTexts · diurnal, semidiurnal and mixed tides · 10 minutes
- MiraCosta 15.11 — Rocky Intertidal Zonation — LibreTexts · short, with a Point Lobos tidepool · 5 minutes
Photo credits (Pexels): Robert So, Howard Herdi, Miguel Rivera, Ray Bilcliff, David J. Boozer.