Igneous and metamorphic-rock aquifers

Science Center Objects

Igneous and metamorphic-rock aquifers can be grouped into two categories: crystalline-rock and volcanic-rock. Spaces in crystalline rocks are microscopically small, few, and generally unconnected. However, because these aquifers extend over large areas, large volumes of water can be withdrawn. Volcanic-rock aquifers have a wide range of chemical, mineralogic, structural, and hydraulic properties due to variations in rock type, ejection, and deposition.

Large areas of the eastern, northeastern, and north-central parts of the Nation are underlain by crystalline rocks. Spaces between the individual mineral crystals of crystalline rocks are microscopically small, few, and generally unconnected; therefore, porosity is insignificant. These igneous and metamorphic rocks are permeable only where they are fractured, and they generally yield only small amounts of water to wells. However, because these rocks extend over large areas, large volumes of groundwater are withdrawn from them, and, in many places, they are the only reliable source of water supply.

Although crystalline rocks are geologically complex, movement of water through the rocks is totally dependent on the presence of secondary openings; rock type has little or no effect on groundwater flow.

Diagram of groundwater movement through the regolith

Groundwater percolates downward through the regolith which is a layer of weathered rock, alluvium, colluvium, and soil to fractures in underlying bedrock. The water moves from highland recharge areas to discharge areas, such as springs and streams at lower altitudes. This example is in the Piedmont and Blue Ridge Province.

Photograph of basalt rock

A typical Pliocene and younger-age basaltic lava flow of the northwestern United States contains layers of varying permeability. Permeability is greatest near the top and the bottom of the flow and least in the dense, center part of the flow.

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 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 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. 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 are the most productive aquifers in volcanic rocks.

This map of igneous and metamorphic-rock aquifers in the United States shows the shallowest principal aquifer. In some places, other, sometimes more productive, aquifers underlie those mapped. In other places, local aquifers, such as those along stream valleys, might overlie the aquifers mapped. Local aquifers are not shown because of the scale of the map.

Map showing principal igneous and metamorphic-rock aquifers

Map showing prinicpal igneous and metamorphic-rock aquifers

Basaltic rocks form most of the volcanic-rock aquifers mapped. These flows cover extensive areas in the northwestern United States and Hawaii. In places, the basaltic-rock aquifers are extremely thick. For example, those of the Columbia Plateau aquifer system in Washington are more than 2,544 meters thick in places, and those of the Snake River Plain aquifer system in Idaho are locally more than 800 meters thick. In most places, however, the thickness of these aquifers is 100 meters or less. Groundwater flow in the basaltic-rock aquifers is local to intermediate. In Idaho, the basaltic-rock aquifers are extremely permeable, and numerous large springs discharge several tens of cubic meters per second from them.

 

Igneous and metamorphic-rock aquifers include:

Columbia Plateau basaltic-rock aquifers

 

Map of Columbia Plateau basaltic-rock aquifers

The Columbia Plateau basaltic-rock aquifers consist primarily of three basalt formations separated by confining units.

 

Pacific Northwest basaltic-rock aquifers

Map of Pacific Northwest basaltic-rock aquifers

Map of Pacific Northwest basaltic-rock aquifers. Unconsolidated-deposit aquifers, which consist primarily of sand and gravel, are the most productive and widespread aquifers in Idaho, Oregon, and Washington.

 

Snake River Plain basaltic-rock aquifers (Idaho)

Map showing Snake River Plain basaltic-rock aquifers

Map of Snake River Plain basaltic-rock aquifers. Unconsolidated-deposit aquifers predominate in the western plain.

 

Southern Nevada volcanic-rock aquifers

Map showing Southern Nevada volcanic-rock aquifers

Map of Southern Nevada volcanic-rock aquifers. The volcanic-rock aquifers can be separated into three catagories - welded tuffs, bedded tuffs, and lava flows. The different characteristics for the storage and trasmission of water in each catagory depend on the presence of primary and secondary porosity.

 

Piedmont and Blue Ridge crystalline-rock aquifers

Map showing Piedmont and Blue Ridge crystalline-rock aquifers

Map of Piedmont and Blue Ridge crystalline-rock aquifers.

 

Hawaiian aquifers

Map of Hawaiian volcanic rock aquifers

Volcanic-rock aquifers extend throughout the eight major Hawaiian Islands and are the most important sources of potable ground water. Sedimentary deposits overlie the volcanic-rock aquifers in some areas. Because the island of Hawaii has not been extensively eroded, sedimentary deposits there are not widespread. The isthmus of Maui contains sedimentary deposits overlying the volcanic-rock aquifers. Sedimentary deposits exist in some coastal areas on the islands of Kahoolawe, Lanai, and Molokai. On Oahu and Kauai, sedimentary deposits act as caprock to the underlying volcanic-rock aquifers. Sedimentary deposits on Niihau occur mainly in the southeastern and northwestern parts of the island.