Water and Geology
An improbable but important link
by Jon Martin, Department of Geological Sciences
This article was originally printed in the October 1999 issue of CLASnotes.
John Martin (left) with fellow geologists on deck of the French research vessel RIV Pt. Lobos.
Useful to the work of both Martin and Elizabeth Screaton, the remotely-operated vehicle MBARI is lowered from research vessel to sea floor, where it takes underwater core samples from sediment on fault lines.
Clams and other life that doesn't need sunlight thrive in mid ocean ridge areas by feeding off material spewed from the vents.
Most people probably think of a geologist as someone myopically focused on rocks, not water. But water influences the physical and chemical evolution of the earth (the essence of geology), probably making it the most important material in all geological processes.
The influence of water can be seen in many common processes, such as erosion of mountains through stream runoff, slow grinding of glaciers, and frost wedging during freeze-thaw cycles. A good Floridian example of the erosive power of water comes from the destructive force of hurricanes (I am writing this article during my forced evacuation from Turlington Hall because of Hurricane Floyd). In addition to causing erosion, however, water is important as a precious commodity similar to another valuable liquid, oil. For example, Florida's economy is essentially based on water, from tourism to agriculture. And like oil, more than 95% of all fresh water is located underground, leading to many important problems.
Most water in Florida is located in a group of limestone rocks, called the Floridan aquifer, that provides nearly all water for drinking and irrigation in the northern half of the state. Limestone rocks are made of the easily dissolved mineral calcite. Dissolution of calcite occurs when acidic surface water flows underground and leads to the widespread formation of a landform called karst. Karst is common to Florida and makes the state world-renowned for cave diving and sinkholes. Karst also leads to a series of interesting questions that I have been working on lately, such as how and at what rate does water infiltrate the aquifer, where does it flow through the subsurface, and what are the resulting chemical changes to the water and rocks during its flow?
A common belief among karst hydrogeologists (geologists interested in water flow through karst areas) is that caves act as primary reservoirs and flow paths for water in the subsurface. This belief, now almost dogma, comes about because most well-studied karst occur in old regions (more than 150 million years) of North America and Europe. Some of my recent work shows, however, that the relatively young Floridan aquifer (about 55 million years old) does not behave this way. Instead, water flows through the Floridan aquifer in both large conduits and microscopic pore spaces contained within the matrix rocks surrounding the caves. The difference between the Floridan and older karst aquifers appears to result from recrystallization and loss of porosity of the older karst rocks. The distinction between water flowing in conduits versus matrix may seem trivial, but it is very important because it controls the length of time that water remains underground, referred to as residence time. Residence time, in turn, controls the extent of dissolution and the frequency of cave and sinkhole formation. It also controls the distribution of pollutants that flow into the aquifer along with the recharged water. And ultimately, it controls where those pollutants emerge back to the surface, for example, in springs, estuaries, or perhaps your kitchen sink.
Many other important geological processes involve water, but are rarely experienced in everyday life. A good example is provided by hydrothermal vents at mid-ocean ridges. These submarine springs are sufficiently active to circulate all seawater through the crust every ten years. This process changes the chemical composition of the water, and thus controls the concentration of salts in seawater, supports non-photosynthetic biological communities, and because conditions at the vents are similar to those of the early earth, may have provided the setting for the origin of life. Less commonly known, however, is that water is also important at subduction zones, the other end of the plate tectonic conveyor belt. In these areas, high water pressure lubricates faults and allows the plates to slide freely past each other. Most free water is squeezed from the rocks by about 13 km below the surface. At this point rocks can break brittlely, causing large earthquakes with catastrophic results, such as the recent events in Turkey and Greece.
Water that is squeezed from rocks in subduction zones also carries dissolved salts and other components into the oceans. Another aspect of my research thus involves trying to understand the mechanisms that drive water from the rocks, to determine the origin of this water, and the volume of the water that is vented. It is possible that some of the water venting from subduction zones is recirculated seawater, similar to the mid-ocean ridge hydrothermal vents, in which case subduction zones could play an important role in the chemical evolution of seawater. Subduction zones also commonly contain high concentrations of carbon dioxide and methane, both greenhouse gases. Consequently, circulation through subduction zones may play a role in the greenhouse warming of the earth. The volume of water and gases venting from subduction zones is poorly known because individual vents are small, although widespread. Continued research should ultimately define the volumes of water and gas that vent from subduction zones.
Although geology can be broken into many sub-disciplines, including hydrogeology, essentially all geologists working on recent earth processes study the physical and chemical effects of water on the earth. As such, water is probably the most important, however improbable, of all geological materials.
