Igneous and Metamorphic-Rock Aquifers
Igneous and metamorphic rocks seem unlikely candidates for storing and moving water, and, from a primary porosity and permeability perspective, little groundwater moves in these rock types. There are, however, other factors (e.g., depositional modes) that complicate the general perception that these rock types are improbable aquifers.
Most rocks in this category have spaces between the individual mineral crystals of crystalline rocks that are microscopically small and generally unconnected; therefore, primary porosity and permeability is very low. These igneous and metamorphic rocks are permeable only where they are fractured or where they have been weathered. They generally yield only small amounts of water to wells. However, because these rocks may extend over large areas, large volumes of groundwater are withdrawn from them, and, in many places, they are the only reliable source of water supply for human and ecosystem use.
Discussion Questions:
After going through the following text and graphics, answer the following questions:
- Briefly explain, in your own words, how aquifers can develop/exist in otherwise low permeability igneous and metamorphic rocks.
- Briefly describe the challenges associated with modeling groundwater flow in igneous and metamorphic rock aquifers. What are some approaches put forth for handling these challenges/uncertainties?
- Research an igneous/metamorphic rock aquifer in your country/region of the world and provide a brief characterization of its role as a water resource (used for public water supply? Irrigation? Broad use or localized? Etc.)
Igneous Rock Aquifers
The two basic types of igneous rocks are intrusive and extrusive. Intrusive igneous rocks start as molten material but cool and crystalize underground which means that the crystal formation is slower and results in larger individual crystals. The crystal formation takes place according to the melting point of each crystal structure in what is referred to as the Bowen Reaction Series of fractional crystallization. Thus, in many intrusive rocks, the heavier minerals with high melting temperatures, such as olivine and peridotite, form first and the lighter minerals with lower melting temperatures, such as granite, form last. This is the reason that gold, which has a very low melting point, is usually associated with the last intrusive rocks to solidify. Intrusive rocks rarely transmit groundwater unless they are fractured or faulted.
Extrusive igneous rocks, also known as volcanic rocks, have a wide range of chemical, mineralogic, structural, and hydraulic properties, due mostly to variations in rock type and the way the rock was ejected and deposited. Unaltered pyroclastic (extruded into the air) rocks, for example, might have porosity and permeability similar to poorly sorted sediments. Hot pyroclastic material, however, might become welded as it settles, and, thus, be almost impermeable. Silicic lavas (lava with high silica content) tend to be extruded as thick, dense flows, and they have low permeability except where they are fractured. Basaltic lavas tend to be fluid, and they form thin flows that have considerable pore space at the tops and bottoms of the flows. On the continents, numerous basalt flows commonly overlap, and the flows are separated by soil zones or alluvial material that form permeable zones. Columnar joints that develop in the central parts of basalt flows create passages that allow water to move vertically through the basalt. Basaltic rocks, particularly flood basalts, are the most productive aquifers of the volcanic rocks.
Flood Basalts
Flood basalts are also among the most curious rock types on earth and they are important aquifers on a worldwide scale. Most basalts, which have relatively high density, occur as part of seafloor spreading from mid-ocean ridges that move toward continental crust where they dive under lower density rocks. As part of this lesson, we are not very concerned about aquifers on the ocean floor, but, surprisingly, there are a number of places on the continents where large-scale basalt flows dominate. These almost always occur in places where a mantle plume (also known as a “hot spot”) reaches the surface. Some of these currently underlie the ocean floor (e.g. Hawaii, Iceland, Reunion Island) but some underlie the continental crust. The most prominent “hot spot” in the United States is the Yellowstone hot spot that is the driving force for Old Faithful geyser. It was also the driving force for the Columbia River and the Snake River Basalt Flows that cover about 70,000 square miles of the Northwestern U.S. These flows, which mostly took place between 17 and 14 million years ago, were also influenced by tectonic forces of the subducting plate that created the Sierra Nevada Mountains. Other similar basalt flows are the Emeishan Traps in southwestern China which are about the same size as the Columbia River Basalts, the Deccan Traps that cover about one fourth of India (about 200,000 square miles), and the Siberian Traps in Russia (about 3,000,000 square miles). A “trap” is a commonly used term for “flow.” The Deccan Traps are associated with the Reunion hot spot and the Siberian Traps are associated with the hot spot under lying Iceland today. Both the Deccan and Siberian Traps are older than the Columbia River Basalt Flows and, therefore, have different hydrologic properties and mineral development due to age.
Figure 1 – Active basalt flow in the Hawaiian Islands.
Most of the Columbia River Basalt flows occurred in five main flow events with many individual lava flows in each event (Figure 2). The average flow is about 110 feet thick with some flows up to 300 feet but the flows are thicker near the source and thin as the flow hardens at its toe. Today, the rubbly top of each flow, where outgassing occurred most, generally will readily transmit groundwater. The central part of each flow is usually massive and contains some fractures but transmits water less readily and impedes the vertical flow of water between flow tops. As one flow overlaps another, the initial rock buildup usually covered a soil horizon which makes it fairly permeable. Median hydraulic conductivity is 1.5 feet per day for a series of flows (Figure 3) (Bauer and Hanson, 1980).
Figure 2 – Columbia River Basalt flows at the Columbia River (Photo from Kahle and Vaccaro, 2015).
Figure 3 – Extent of the Columbia River Basalt Flows in the northwestern United States and the trace of the Yellowstone hot spot (Bauer and Hanson, 1980).
Columbia River Basalt Flows were exposed in the River valley due to intense floods of the Columbia River between 15,000 and 13,000 years ago when glacial ice dams ruptured in Montana. The floods sculpted the Columbia River valley and left excellent exposure of the individual basalt flows (Figure 2). Similar exposure of basalt flows of the Deccan Traps, which were deposited about 65 million years ago and exists in a wetter climate, is less distinct (Figure 4). Older basalt flows such as the Siberian Traps (about 250 million years old) and the Keweenaw Basalts of the United States (about 1,100 million years old) have less flow distinction and lower hydraulic conductivities at the flow tops and bottoms (Grannemann and Twenter, 1981).
Figure 4 – Basalt flows in the Deccan Traps, India (NASA photo).
Metamorphic-Rock Aquifers
An aquifer is a geologic formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant amounts of groundwater to wells and springs. (Lohman, 1972)
Metamorphic rocks have negligible matrix porosity and permeability. To be aquifers, these rocks require fractures and fissures to transmit groundwater. Sometimes these rocks are called “hard rocks” and they principally occur in large areas of the world called “shields” which are mostly Precambrian-aged metamorphosed magmatic rocks that form the most stable parts of continents. Some examples include the Canadian, Baltic, Amazonian, African, Australian, and Siberian shields. Smaller hard rock regions also exist such as the Massif Central in France.
Hard rock outcrops and subcrops cover about 20 percent of the continental crust (Gustafson and Krasny, 1994). If no surface water supply is available, these fractured rocks become important, though usually small, sources of water. To understand the limits of these rocks as a source of water, it is necessary to understand the fractures that dictate their hydraulic properties which can vary greatly within short distances. Such variability makes modeling flow in these aquifer systems very difficult.
Figure 5 – The difference in the yield of rock types is substantial in places. Well A, which is completed in schist, yielded about 1 gallon per minute; well B, which is open to granitic gneiss and the contact zone between the two rock types, yielded 100 gallons per minute. (U.S. Geological Survey, 1990).
In spite of the variability illustrated in Figure 5, there are some generalities about the hydraulic properties of crystalline rocks. Some are as follows:
- Contact zones between rock types have potential for high well production rates.
- Hydraulic conductivity usually decreases with depth because fractures close due to lithostatic pressure.Openings commonly are present along relict bedding planes and zones of rock weakness.Weathering near the land surface creates a zone of higher porosity and permeability (Figure 6).
- Topographically high areas and ridges generally have less weathering and overlying soil which reduces hydraulic conductivity.Conversely, lower lying areas, especially broad valleys, have more weathered rock and greater groundwater storage capacity and hydraulic conductivity.
- Acidic waters can enlarge fracture zones, especially if the rock is calcite-rich.
- Large faults may not be groundwater conduits if they are filled with fine-grained fault-related material such as mylonite or clay.
- Rocks of granitic (felsic) composition usually have higher hydraulic conductivity fractures than rocks of basic (mafic) composition.
Figure 6 -- Groundwater percolates downward through the regolith – which is a layer of weathered rock, alluvium, colluvium, and soil – to fractures in underlying bedrock. (U.S. Geological Survey, 1990)
References:
Bauer, H.H. and Hansen, A.J., Jr., 1980, Hydrology of the Columbia Plateau Regional Aquifer System, Washington, Oregon, and Idaho: U.S. Geological Survey Water-Resources Investigations Report 96-4106, 61 p.
Grannemann, N.G. and Twenter, F.R., 1981, Groundwater for Public Water Supply at Windigo, Isle Royale National Park, Michigan: U.S. Geological Survey Open-File Report 82-567, 16 p.
Gustafson, G. and Krasny, J., 1994, Crystalline Rock Aquifers: Their Occurrence, Use, and Importance: Applied Hydrogeology, Vol. 2, 1994, pp. 64-75.
Kahle, S.C. and Vaccaro, J.J., 2015, Groundwater Resources of the Columbia Regional Aquifer System: U.S. Geological Survey Fact Sheet 2015-3063, 6 p.
Lohman, S.W., 1972, Ground Water Hydraulics: U.S. Geological Survey, Professional Paper 708, 65 p.
U.S. Geological Survey, 1990, Piedmont and Blue Ridge Aquifers: Hydrologic Atlas 730-G.