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 TidesThe main causes of land tides are the gravitational fields of the sun and moon and the Earth's elasticity. The Earth is not a completely rigid body and it is made up of several layers with varying consistency (diagrams). The Earth has a solid inner core that is surrounded by a liquid outer core. The outer core is surrounded by the mantle which consists of molten rock closest to the outer core and stiff rock closer to the Earth's crust, which is its outermost layer. It is because of these flowing liquid and molten rock layers that the Earth has elasticity and thus, 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 TidesLand tides occur in four measurable cycles based on the Earth's rotation. These cycles are the lunar diurnal, the lunar semidiurnal, the solar diurnal and the solar semidiurnal. Diurnal tides last around 24 hours and semidiurnal tides last about 12 hours.
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 TidesAlthough land tides are not noticeable to people on a daily basis like ocean tides, they are still very important to understand because they can have significant impacts on the Earth's geological processes and especially volcanic eruptions. As a result, volcanologists are very interested in studying land tides. Scientists are mainly interested in them on a daily basis because they are "cyclical, small, and slow ground movements that [they] use to calibrate and test sensitive volcano deformation monitoring instruments" (USGS).
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." (PDF)