Just below Earth’s surface, beneath the roots and soil, is a hard, dense layer of bedrock that is the foundation for all life on land. Cracks and fissures within the bedrock provide pathways for air and water, which chemically react to break up rock, ultimately creating soil — an essential ingredient for all terrestrial organisms. This weathering of bedrock is fundamental to life on Earth.
Accurate predictions of where open fractures are beneath the surface are valuable for additional reasons. Fractures in the bedrock affect where drinking water will flow; the paths that magma takes as it moves to the surface in volcanic eruptions; the strength of rock masses on slopes; and how severe shaking will be during earthquakes.
Now scientists at UH Mānoa, Massachusetts Institute of Technology (MIT), University of Wyoming and elsewhere have found a way to predict the extent of bedrock weathering, given a location’s topography – the shape and features on the surface. The results are published in the journal Science.
The group sought to estimate the depth to which bedrock is broken up, or fractured, using a mathematical model. This fractured rock forms the base of a layer scientists have dubbed Earth’s “critical zone,” where the interaction of rock, air and water allows life to thrive. Steve Martel, professor of Geology and Geophysics at UH Mānoa’s School of Ocean and Earth Science and Technology (SOEST), and the team developed a stress model that estimated the thickness of this critical zone, given the forces generated by topography, gravity and plate tectonics.
The model showed that if a landscape is undergoing little tectonic compression, the fractured zone should parallel the overlying topography, like layers of lasagna. If, however, a region is under high tectonic compression, the fractured zone will resemble a mirror image of the landscape — thicker beneath ridges, and thinner under valleys.
To test the model’s predictions, the researchers went to three sites in the U.S. with varying tectonic forces – Colorado, South Carolina and Maryland. In each location, they took extensive seismic and electrical conductivity measurements to gauge the extent of fracturing in the underlying bedrock. Seismic waves move faster through solid rock, and slower through rock containing many fractures filled with air, water or weathered material such as clay. The scientists also drilled boreholes to obtain photos of the bedrock at depth. The photographic data provided further confirmation that the seismic and conductivity measurements did indeed reveal fractured zones that matched well with their model’s predictions.
Said Martel, “I knew from work done for a paper I published in 2011 that the stress models could predict fracture patterns in parts of Yosemite National Park. However, everyone on the team, including me, was surprised to see how well the stress modeling results matched the geophysical results in these diverse geologic environments.”
As an extension of this work, Martel and a group of several SOEST scientists anticipate using the stress models to better understand geothermal energy resources in Hawai‘i. Geothermal energy is obtained from hot water that flows through open rock fractures underground. The model will help predict where open fractures are most likely to be, and where open fractures are unlikely to be – thereby enhancing predictions of geothermal energy resource locations.
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Portions of this release sourced from an MIT news story (used with permission).
Watch “Bedrock weathering based on topography” for an explanation of the recent findings. Credit: Melanie Gonick/MIT.