The Restless Earth: Geosciences
Volcanic tuff from Baja California, Mexico, with an age of about 13 million years. Brown glass shards with white rims compacted and welded together during cooling of the tuff. The large white grains are anorthoclase feldspar. Field of view: 2 mm.
Source: Claudia J Lewis
- What are the interactions among subsurface processes?
- Are underground resources of drinking water safe and secure?
- Can we reliably predict and control earthquakes?
- Can we make the earth "transparent" and observe underground processes in action?
Society is critically dependent on the subsurface world. Clean drinking water from underground is fundamental to civilization. Every society on earth extracts minerals from its subterranean depths. Hot subsurface rock is a potential source of enormous energy. Major structures–dams, foundations, tunnels–rely on the strength of the rock. The subsurface finds growing use as a storehouse for energy reserves, as a disposal site for toxic and hazardous waste, and as a repository for carbon dioxide sequestration. Yet, to date, the limited research on the subsurface has produced more questions than answers.
UNDERSTANDING THE UNDERGROUND WORLD
Rock, the emblem of strength in popular imagery, is no match for the relentless tectonic forces, driven by heat from the earth's interior, that have operated continuously since the birth of the planet 4.6 billion years ago. The rock bends, buckles and breaks, raising mountains and producing underground folds and faults. Usually so slow as to be imperceptible, these processes occasionally turn violent, producing earthquakes and volcanic eruptions. Underground, the rock is hot, with temperatures increasing between 10 and 30°C for each kilometer of depth. Rock becomes more deformable with heat, and the underground environment becomes progressively more challenging to engineers. Fluids flowing under pressure through porous rock and along fractures transport natural resources and form minerals. Microbial organisms live and migrate in the deep subsurface. Understanding these processes, whose mechanisms remain largely unknown, is key to the wise and effective use of the underground world.
Deformation-band faults in the Rio Grande rift, New Mexico. The Peralta Tuff is a sequence of volcanic deposits reworked by wind and water. Deformation-band faults are characterized by grain crushing, grain-boundary sliding, and pore collapse.
Source: Claudia J. Lewis
WHAT ARE THE INTERACTIONS AMONG SUBSURFACE PROCESSES?
Underground earth processes interact with and depend on each other. Tectonic forces bend and fracture rocks, altering the permeability and porosity of the rock and therefore the pressures, directions and rates of fluid movement. Changes in fluid pressures cause changes in the rock's elastic response to deforming forces, which control movement along faults and, ultimately, earthquake frequency and magnitude. Chemical dissolution and precipitation as fluids move through different thermal environments can produce mineral deposits; they can change the mechanical strength and flow properties of rock. Understanding these "coupled processes" is essential to assessing their consequences. Long-term cross-cutting experiments at great depth would lead to development of more reliable models of the earth's crust that would fully couple thermal, hydrologic, mechanical, chemical, biological-mass and energy-transport phenomena, an achievement not possible from surface-based field studies. The examples of drinking water safety and earthquake prediction illustrate the significance of coupled effects.
ARE UNDERGROUND DRINKING WATER RESOURCES SAFE AND SECURE?
Groundwater flow is currently estimated by point calibration of mathematical models using borehole data. This method is inherently inadequate, since the rock volume between the boreholes is heterogeneous, with scale-dependent properties that are unknown and hence not incorporated into models. Samples of deep rock from drill holes are small and are disturbed by the drilling process, making them of limited value for testing the factors that control fluid flow. Operating mines provide direct access to the underground but do not usually allow for long-term studies. The ability to understand fluid flow and associated chemical and physical processes is consequently severely limited. Direct testing on underground blocks of rock could overcome many of these limitations and aid in groundwater research, critical to the protection of these resources.
In a geothermal system, potentially a result of rock heated by intrusion of magma, or rock heated by storing nuclear waste, the heat induces a wide range of coupled mechanical, hydrological, chemical, and even biological processes. Open fractures act as conduits for water and steam, which accelerates chemical reactions leading to mineralization in the fractures. In addition to mineralization that can seal fractures, heating of the rock causes thermal expansion closing fractures. All of these changes act to modify the flow of fluids and the pressure distribution, transporting mass and energy through the rock. Under some environments, biofilms may develop and further modify the chemical environment and the flow of fluids in the rock.
The example shown is based on observations during the 8-year duration Heated Drift Experiment in unsaturated high permeability volcanic tuff at Yucca Mountain. Rock temperatures exceeded 200C. Together with smaller scale experiments in various low permeability saturated rocks, this provides a valuable basis for studies of coupled processes in DUSEL.
Source: Eric Sonnenthal, LBNL
CAN WE RELIABLY PREDICT AND CONTROL EARTHQUAKES?
Earthquakes occur when a fault or fault region can no longer accommodate the forces applied to it and dynamic slip takes place. This may result from an increase in tectonic forces, a decrease in the fault resistance due to thermal, hydrological, chemical and other changes along the fault surfaces, or to some combination of both. "Precursor" events indicate that an earthquake is a progressive phenomenon, but the detailed processes are far from clear. The spatial distribution of rock deformation deep in the subsurface is also unknown. Are the forces on faults in a state of critical equilibrium, as some scientists suggest?
A deep underground laboratory would permit continuous, direct measurements of rock strain as a function of position and sampled volume at depth, both in the immediate vicinity of active faults and in the rock mass. These data would elucidate the influence of geology and human activity on tectonic strain and stress distribution in rock, allow direct observation of how energy accumulates near faults and fractures, and provide insights into how fault slip processes can be scaled to larger events. The understanding gained from this research would be a vital step toward reliable prediction of earthquakes.
Landsat image superimposed on a digital elevation model with a view to the southeast within the Owens Valley of California. The Sierra Nevada mountains are on the right, and the White Mountains on the left. The trace of the 1872 Owens Valley earthquake scarp is evident in several places, most obviously on the left-front flank of the small shield volcano in the foreground.
Source: William Bowen
Surveying in a paleoseismic trench. The trench was excavated across a part of the Pajarito fault system in the Rio Grande rift, New Mexico. Trenching on this and other nearby faults revealed that three magnitude 6-7 earthquakes have occurred here in Holocene time.
Source: Claudia J. Lewis
CAN WE MAKE THE EARTH "TRANSPARENT" AND OBSERVE UNDERGROUND PROCESSES IN ACTION?
Rock is opaque; scientists can't "see" into the system they are exploring. Just as medical imaging techniques have revolutionized the practice of medicine, so would accurate sub-surface imaging benefit the entire spectrum of basic and applied geoscience. Computer modeling advances now allow scientists to postulate how subsurface processes unfold, but until researchers can delve into the opaque rock, they cannot verify computer predictions. Advances in geophysical techniques are gradually revealing more details of the structure of the crust and the processes operating within it, but a concerted effort could provide needed levels of precision on a variety of scales.
Seismic surveying from the surface is currently the main approach for imaging the deep earth, and some impressive advances offer promise that more can be achieved. Subsurface geology is typically only inferred from the study of surface outcrops and samples from boreholes and mines. A deep underground laboratory would allow direct verification or "ground-truthing" of seismic imaging. Researchers could verify surface-based predictions of underground structures directly from a deep, three-dimensional volume of rock. Directly accessible underground, this rock volume would also serve as a test bed for imaging research and the development of new imaging techniques. Detailed views just 10 to 20 meters into the rock can illuminate coupled processes, follow slip development on faults, and help prevent catastrophic collapse in tunnel borings.
GEOSCIENCE AT A DEEP UNDERGROUND LABORATORY
A Deep Underground Science and Engineering Laboratory dedicated to long-term basic and applied research would provide a unique window on the unseen subterranean processes that so profoundly affect humanity's life on the planet's surface. Geoscientists, working with scientists from biology, physics and other disciplines would conduct fundamental experiments on research themes including transparent earth, groundwater, rock deformation, coupled processes, and dark life.
DUSEL would also provide an opportunity for very deep drilling into the upper crust. Depending on the site chosen, deep holes drilled from the lowest levels could provide valuable information on geophysics of the earth's crust, thermal structure at depth, tectonic stresses, and formation of ore deposits. Sites now under consideration for DUSEL are mines, so research on the genesis of ore deposits is a logical component of studies in the underground laboratory. Drilling of deep boreholes involves high temperatures (200-300°C) and would provide valuable opportunities for technological innovation of particular interest to the petroleum industry. In summary, a deep underground laboratory would allow geoscientists to peel back the earth's surface and discover the mysteries of the deep underground.
EarthScope is funded by the National Science Foundation and conducted in partnership with the US Geological Survey and NASA.
Source: EarthScope
