Volcanologists study terrestrial tides because this movement in the Earth’s crust can sometimes trigger a volcanic eruption. Terrestrial tides are important for radio astronomy and calculating coordinates on a global positioning system (GPS). Terrestrial tides can change an object’s precise location. These movements are called terrestrial tides. Land surfaces do move, however, up to 55 centimeters (22 inches) a day. This has little effect on Earth’s land surfaces, because they are less flexible. The moon exerts a tidal force on the whole planet. The moon’s ability to raise tides on the Earth is an example of a tidal force. Although the sun and moon both exert gravitational force on the Earth, the moon’s pull is stronger because the moon is much closer to the Earth than the sun is. The closer objects are, the greater the gravitational force is between them. The major tidal constituent is the moon’s gravitational pull on the Earth. The Earth’s rotation is a tidal constituent. Causes of Tides Forces that contribute to tides are called tidal constituents. Some lakes and rivers can also have tides. When the water falls to its lowest level, it is at low tide. When the water has risen to its highest level, covering much of the shore, it is at high tide.

Along coasts, the water slowly rises up over the shore and then slowly falls back again. The regular rise and fall of the ocean’s waters are known as tides.

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