Basalt is lava that has cooled and solidified. Where I live we have 6,000 to 12,000 feet of basalt above the underlying sedimentary formations. The basalt flow predominantly came out of vents rather than volcanic eruptions. The lava was very liquid rather than the lava oozing like you see in Hawaii.
Geologists estimate that the flows were moving in the 30 mph area to spread the 100+ miles that they extend from the vents. Each individual flow is anywhere from 50 to 250 feet thick.
Individual flows have been named and can be identified by their differing chemical composition. I regularly send chips sample the the WSU Geology lab for identification thru XRF analysis . Some of the wells I drill are required to be cased past a specific flow or to not extend past a specific flow.
When a new lava flow was deposited the bottom cooled very fast. Lots of fractures and seams. The interior of the flow can form columnar basalt. It has a polygonal vertical form about 1 to 3 feet in diameter. If it cooled very slowly it might be just a massivive sold rock with no joints, seams or fractures. The top part of an individual flow cooled faster being exposed to air. Any gasses in the flow rose to the top and formed bubbles, called vesicular basalt. If basalt flows into standing water, it forms blobs, called pillow basalt.
There was time in between individual flows. It may have been days, years, or thousands of years. The earth still rotated around the sun. We had seasons, we had rain and snow. We had lakes form. We had trees and vegetation grow. I have had wood chips come out from a clay bed 400-750 below the top layer of basalt. Sometimes the wood is charred. Sometimes it looks like wood on top of the ground. A friend of mine has a rock shop. He specializes in petrified wood.
Most water bearing zones in basalt formations are found in the interbeds. They are the fractured and rubbley part of the bottom on one flow and the vesicular pourous top of the next lower flow. There can be a cinder bed in between the flows from a volcanic eruption, or a layer of volcanic ash, or a layer of soil or even clay or lake bottom muck. I have drilled thru clay beds in between basalt flows that are any where from a few feet thick to 50 feet or more thick. Usually the brown, tan and yellow clays aren’t much of a problem. The blue and green clays have a tendency to be highly reactive to water and swell and slough in.
I have wells that will produce over 2000 gpm from just a 2 foot thick zone.