Province of the western United States highly porous scoria formed by volcanic eruptions.
Explanation:
Aquifer refers to the layer of water that is present inside the soil at a certain depth below rock layer. Rocks that are present in Volcano makes a good aquifer. Volcanic rocks are good aquifer because they are porous i. e. tiny holes are present between rocks through which water can pass through these rock layers and this water collect there and forms good aquifer.
