BASALTIC AND OTHER VOLCANIC-ROCK AQUIFERS
Aquifers in basaltic and other volcanic rocks are widespread in Washington, Oregon, Idaho, and Hawaii, and extend over smaller areas in California, Nevada, and Wyoming (fig. 4a and fig. 4b). Volcanic rocks have a wide range of chemical, mineralogic, structural, and hydraulic properties. The variability of these properties is due largely to rock type and the way the rock was ejected and deposited. Pyroclastic rocks, such as tuff and ash deposits, might be emplaced by flowage of a turbulent mixture of gas and pyroclastic material, or might form as windblown deposits of fine-grained ash. Where they are unaltered, pyroclastic deposits have porosity and permeability characteristics like those of poorly sorted sediments; where the rock fragments are very hot as they settle, however, the pyroclastic material might become welded and almost impermeable. Silicic lavas, such as rhyolite or dacite, tend to be extruded as thick, dense flows and have low permeability except where they are fractured. Basaltic lavas tend to be fluid and form thin flows that have a considerable amount of primary pore space at the tops and bottoms of the flows. Numerous basalt flows commonly overlap and the flows commonly are separated by soil zones or alluvial material that form permeable zones. Basalts are the most productive aquifers of all volcanic rock types.
The permeability of basaltic rocks is highly variable and depends largely on the following factors: the cooling rate of the basaltic lava flow, the number and character of interflow zones, and the thickness of the flow. The cooling rate is most rapid when a basaltic lava flow enters water. The rapid cooling results in pillow basalt (fig. 40), in which ball-shaped masses of basalt form, with numerous interconnected open spaces at the tops and bottoms of the balls. Large springs that discharge thousands of gallons per minute issue from pillow basalt in the wall of the Snake River Canyon at Thousand Springs, Idaho. Interflow zones are permeable zones that develop at the tops and bottoms of basalt flows (fig. 41). Fractures and joints develop in the upper and lower parts of each flow, as the top and bottom of the flow cool while the center of the flow remains fluid and continues to move, and some vesicles that result from escaping gases develop at the top of the flow. Few open spaces develop in the center of the flow because it cools slowly. Thus, the flow center forms a dense, low-permeability zone between two more permeable zones. Thin flows cool more quickly than thick flows, and accordingly the centers of thin flows commonly are broken and vesicular like the tops and bottoms of the flows.
The Snake River Plain regional aquifer system in southern Idaho and southeastern Oregon (fig. 42) is an example of an aquifer system in basaltic rocks. The Snake River Plain is a large graben-like structure that is filled with basalt of Miocene and younger age (fig. 43). The basalt consists of a large number of flows, the youngest of which was extruded about 2,000 years ago. The maximum thickness of the basalt, as estimated by using electrical resistivity surveys, is about 5,500 feet. The basalt is bounded at the margins of the plain by silicic volcanic and intrusive rocks that are downwarped toward the plain.
Pliocene and younger basaltic-rock aquifers are the most productive aquifers in the Snake River Plain. The saturated thickness of the Pliocene and younger basaltic rocks is locally greater than 2,500 feet in parts of the eastern Snake River Plain but is much less in the western plain (fig. 44). Aquifers in Miocene basaltic rocks underlie the Pliocene and younger basaltic-rock aquifers (fig. 43), but the Miocene basaltic-rock aquifers are used as a source of water only near the margins of the plain. Unconsolidated-deposit aquifers are interbedded with the basaltic-rock aquifers, especially near the boundaries of the plain. The unconsolidated deposits consist of alluvial material or soil that developed on basaltic rock, or both, and were subsequently covered by another basalt flow.
The Pliocene and younger basaltic-rock aquifers consist primarily of thin basalt flows with minor beds of basaltic ash, cinders, and sand. The basalts were extruded as lava flows from numerous vents and fissures which are concentrated along faults or rift zones in the Snake River Plain. Some flows spread outward for as much as 50 miles from the vent or fissure from which the flow issued. Shield volcanoes formed around some of the larger vents and fissures (fig. 45). Flows that were extruded from the volcanoes formed a thick complex of interbedded basalt.
Water in the Snake River Plain aquifer system occurs mostly under unconfined (water-table) conditions. The configuration of the regional water table of the aquifer system (fig. 46) generally parallels the configuration of the land surface of the plain. The altitude of the water table is greatest in Fremont County, Idaho, near the eastern border of the plain and least in the Hells Canyon area along the Idaho-Oregon border. Where the water-table contours bend upstream as they cross the Snake River (for example, near Twin Falls, Idaho), the aquifer system is discharging to the river. In a general way, the spacing between the contours reflects changes in the geologic and hydrologic character of the aquifer system. Widely spaced contours in the Eastern Plain indicate more permeable or thicker parts of the aquifer system, whereas closely spaced contours in the Western Plain indicate less permeable or thinner parts. Water levels in the areas where shallow aquifers or perched water bodies overlie the regional aquifer system (fig. 46) are higher than those in the aquifer system. These areas are underlain by rocks that have extremely low permeability.
Other basalt aquifers are the Hawaii volcanic-rock aquifers,
the Columbia Plateau aquifer system, the Pliocene and younger
basaltic-rock aquifers, and the Miocene basaltic-rock aquifers.
Volcanic rocks of silicic composition, volcaniclastic rocks, and
indurated sedimentary rocks compose the volcanic- and sedimentary-rock
aquifers of Washington, Oregon, Idaho, and Wyoming. The Northern
California volcanic-rock aquifers consist of basalt, silicic volcanic
rocks, and volcaniclastic rocks. The Southern Nevada volcanic-rock
aquifers consist of ash-flow tuffs, welded tuffs, and minor flows
of basalt and rhyolite.