Diamond Craters is an area of very recent volcanism. The craters cover an area of 23 square miles (17,000 acres). This geologic feature is unique because of its special volcanic features, lack of disturbance by man, relatively unweathered topography, and because it represents one of the most recent volcanic actions in the state of Oregon. The geologic events that have produced the Diamond craters can be divided into three relatively identifiable stages. the first was the eruption of a fluid basalt from a single vent located somewhere near the center of the present feature. The basalt spread as pahoehoe (ropy) lava over a 6 mile diameter area roughly circular in form. The lava crust later foundered into drained lava tubes producing abundant collapsed depressions. The thickness of the original flow is 75-100 feet at the center and thins out radially. This stage was followed by a relatively quiet period. Later, an intrusion of basaltic magma into underlying sediments bowed up part of the lava field into five rounded domes. The third phase of the sequence occurred during and after doming. Gas from the magmas and steam from the water saturated rocks provided enough energy for violent explosions which produced the many explosion craters that mark the surface of the lava field today. Many of these craters were later enlarged when the rim margins collapsed into the craters. The volcanic activity that formed the Diamond craters has produced one of the most diverse basaltic fields in North America. Within the complex are well preserved domes, lava lakes, cinder cones, spatter features, driblet spires (like the one pictured above), cinder fields, lava tubes, aa lava flows, pahoehoe flows, explosion craters, and collapsed structures. 

An interesting feature within the complex is Malheur Maar (pictured below), located on the western margin. The maar is an explosion crater that has filled in with rain water. Recent studied have revealed sediments over fifty feet thick containing pollen from plants living thousands of years ago. Also, ash from Mt. Mazama (6,600 years) lay near the bottom of the sedimentary sequence. So an age of at least 7,000 years has been placed on the volcanic events of Diamond Craters. 

In addition to the volcanic landscape, the Diamond Craters are rich in botanical and faunal diversity. Because of the area's unique offerings and natural history, Diamond Craters was recently set aside for permanent protection as "Diamond Craters Outstanding Natural Area".