Every six hours and twelve minutes, or thereabouts, our planet undergoes extraordinary changes. To most of us, busily going about our daily lives, these changes go unnoticed, but for one of the largest habitats on Earth, they are a matter of life and death.

The gravitational forces exerted on the Earth by the moon's elliptical orbit combined with the centrifugal force of the Earth's own rotation, causes the masses of water that make up the world's oceans to shift, producing the daily cycle of two unequal high tides and two unequal low tides. The sun also affects the tides but since the moon is closer, it exerts the major force.

It all seems simple enough but the currents, the varying height of tides and speed of tides are dependent on a complex relation ship between the Earth and the moon's axis variations, their position in relation to the Sun, closeness or distance (perigee and apogee) of the sun, moon and other planets, geography of a given area and its location on the planet.

The intertidal zone bordering the world's oceans supports an extraordinarily rich assortment of unique plant and animal life that must be able to adapt to the cadence of the oceans and a variety of harsh and ever changing conditions. During the course of any given day, the intertidal zone may be exposed to pounding surf, freezing cold, inundations of fresh water or baking sun. Very few animals on the planet need to adapt to such environmental extremes, but for survival along the shore it is a basic necessity.

For the complete article "Life on the Rocks" pick up a copy of the December issue of DIVER Magazine.