Land Tides or Earth Tides

Land tides, also called Earth tides, are very small deformations or movements in the Earth's lithosphere (surface) caused by the gravitational fields of the sun and moon as the Earth rotates within their fields. Land tides are similar to ocean tides in how they are formed but they have very different impacts on the physical environment.

Unlike ocean tides, land tides only change the Earth's surface by around 12 inches (30 cm) or so twice a day. The movements caused by land tides are so small that most people are not even aware that they exist. They are very important to scientists like volcanologists and geologists however because it is believed that these small movements may be able to trigger volcanic eruptions.

Causes of Land Tides

Like ocean tides, the moon has the greatest effect on land tides because it is closer to the Earth than the sun. The sun does have an effect on land tides as well because of its very large size and strong gravitational field. As the Earth rotates around the sun and the moon each of their gravitational fields pull on the Earth. Because of this pull there are small deformations or bulges on the Earth's surface or land tides. These bulges face the moon and the sun as the Earth rotates.

Like ocean tides where water rises in some areas and it is also forced down in others, the same is true of land tides. Land tides are small though and the actual movement of the Earth's surface is usually no greater than 12 inches (30 cm).

Monitoring Land Tides

Due to these cycles, it is relatively easy for scientists to monitor land tides. Geologists monitor the tides with seismometers, tiltmeters, and strainmeters. All of these instruments are tools that measure the motion of the ground but tiltmeters and strainmeters are capable of measuring slow ground movements. The measurements taken by these instruments are then transferred to a graph where scientists can view the distortion of the Earth. These graphs often look like undulating curves or bulges indicating the upward and downward movement of land tides.

The Oklahoma Geological Survey's website provides an example of graphs created with measurements from a seismometer for an area near Leonard, Oklahoma. The graphs show smooth undulations indicating small distortions in the Earth's surface. Like ocean tides, the largest distortions for land tides appears to be when there is a new or full moon because this is when the sun and moon are aligned and the lunar and solar distortions combine.

Importance of Land Tides

In addition to using land tides to test their equipment, scientists are interested in studying their effect on volcanic eruptions and earthquakes. They have found that although the forces causing land tides and the deformations in the Earth's surface are very small they do have the power to trigger geologic events because they are causing changes in the Earth's surface. Scientists have not yet found any correlations between land tides and earthquakes but they have found a relationship between the tides and volcanic eruptions because of the movement of magma or molten rock inside volcanoes (USGS). To view an in-depth discussion about land tides, read D.C. Agnew's 2007 article, "Earth Tides."