Big Obsidian Flow in Newberry caldera, Oregon. (Credit: Jensen, Robert A.. Public domain.)

The obsidian lava flow overlies the Paulina Lake ash-flow tuff, which has been dated at ~1300 years old. The ash-flow tuff and the lava flow are parts of the same eruptive event, as is the Newberry tephra that forms a strongly east-directed narrow deposit.

The Big Obsidian eruptive period occurred in three parts.

• The eruption began with a gas-rich, explosive phase that produced a high column of tephra above the vent. The resulting pumice fall deposit, called the Newberry Pumice, covered the southern part of the caldera and was carried by the wind to mantle the eastern flank of the volcano. At about 9 km (5.5 mi) from the vent, the fall deposit is about 3.7 m (12 ft) thick, and at 64 km it decreases to about 25 cm (10 in). The Newbery tephra was lofted high into the atmosphere and strong westerly winds carried it east to Idaho.
• A less explosive eruption produced the Paulina Lake ashflow, which extends over a broad area between the Big Obsidian Flow and Paulina Lake.
• During the final stage of this eruption, magma that had lost much of its gas flowed onto the surface to form a dome at the vent. The final product was the Big Obsidian Flow. Obsidian is a natural glass with no crystalline structure.