Diamond Craters is a basalt lava field that covers about 70 km2 (26 mi2) in southeastern Oregon. The lava flows and associated tephra, 1–2 km3 (about 0.25 to 0.5 mi3) in total, were erupted sometime between about 7,790 and 7,320 years ago. The Diamond Craters lava field is unique among young basalt fields in Oregon because of its eruptive progression. An early eruptive phase was typical enough, beginning with lava flows fed from a central fissure or vent. At some point, magma was injected into the shallow subsurface, which caused the lava field to arch up as much as 150 m (500 ft) creating six elongate dome-shaped structures. Largest of these, Graben dome is about 2–3 km long and 1.5 km wide (nearly 2 mi long and 1 mi wide). It takes its name from the prominent 40-m-deep (130-ft-deep) axial crack along its crest.
Basaltic magma also intercepted shallow ground water, which led to substantial hydrothermal explosions. The explosions shattered underlying volcanic and sedimentary rocks and then showered the fragments across the landscape. The resulting tephra blanket, as thick as 20 m (65 ft) near the field’s central crater complex, thins abruptly toward the margin of the lava field. One of the smaller explosion craters hosts a shallow pond. Coring of this pond in the 1970s yielded a 15-m-thick (50-ft-thick) sequence of sedimentary beds that date back nearly 6,000 years from which has come a rich history of the region's paleobotany and fluctuations in water-table elevation.