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 However, because these aquifers extend over large areas, large volumes of water can be withdrawn.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 However, because these rocks extend over large areas, large volumes of groundwater are withdrawn from them, and, in many places, they are the only reliable source of water supply.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.
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.
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
- Idaho, Washington, Oregon (Chapter H)
Pacific Northwest basaltic-rock aquifers
- California and Nevada (Chapter B)
- Idaho, Washington, Oregon (Chapter H)
Snake River Plain basaltic-rock aquifers (Idaho)
-
Idaho, Washington, Oregon (Chapter H)
Southern Nevada volcanic-rock aquifers
-
California and Nevada (Chapter B)
Piedmont and Blue Ridge crystalline-rock aquifers
- Arkansas, Louisiana, and Mississippi (Chapter F)
- Alabama, Florida, Georgia, and South Carolina (Chapter G)
- Illinois, Indiana, Kentucky, Ohio, and Tennessee (Chapter K)
- Delaware, Maryland, New Jersey, North Carolina, Pennsylvania, Virginia, and West Virginia (Chapter L)
Hawaiian aquifers
Below are other web pages related to principal aquifers.
Principal Aquifers of the United States
Unconsolidated and semiconsolidated sand and gravel aquifers
Sandstone aquifers
Sandstone and carbonate-rock aquifers
Carbonate-rock aquifers
Minor aquifers, confining units, and areas identified as "not a principal aquifer"
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.
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.
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
- Idaho, Washington, Oregon (Chapter H)
Pacific Northwest basaltic-rock aquifers
- California and Nevada (Chapter B)
- Idaho, Washington, Oregon (Chapter H)
Snake River Plain basaltic-rock aquifers (Idaho)
-
Idaho, Washington, Oregon (Chapter H)
Southern Nevada volcanic-rock aquifers
-
California and Nevada (Chapter B)
Piedmont and Blue Ridge crystalline-rock aquifers
- Arkansas, Louisiana, and Mississippi (Chapter F)
- Alabama, Florida, Georgia, and South Carolina (Chapter G)
- Illinois, Indiana, Kentucky, Ohio, and Tennessee (Chapter K)
- Delaware, Maryland, New Jersey, North Carolina, Pennsylvania, Virginia, and West Virginia (Chapter L)
Hawaiian aquifers
Below are other web pages related to principal aquifers.