Between five and six million years old, the field reaches north from Jefferson to Olallie Butte, and it covers an area of . Scientists think that the setup of this field, where various vents have erupted lava, explains why the otherwise similar Cascades volcano at Mount Hood is three times as voluminous as Jefferson, because Hood has concentrated most of the eruptions from its magma chambers. The field is also likely underlain by a batholith, a large mass of intrusive igneous rock (also called a pluton) that forms from cooled magma deep in the Earth's crust.