GEOLOGY OF EASTLAKE
NEWBERRY CALDERA CRATER
Newberry Caldera (Crater) was formed 500,000 to 250,000 years ago when Mt. Newberry collapsed leaving a crater 5 miles across and over 2,800 feet deep. Subsequent eruptions divided the caldera into two lakes, and left a pumice cone between them.
The most recent eruption was The Big Obsidian Flow 1,300 years ago and is the youngest lava flow in the Northwest. Visitors will find evidence of a wider variety of volcanic events in a more compressed area here than anywhere else in the United States and they are all easily accessible.
Cinder cones, lava flows laced with obsidian, Oregon’s longest lava tube cavern, and fields of pumice are some of the volcanic features that make this place remarkable. Obsidian is natural volcanic glass. When lava is especially rich in silica (SiO2), it has the stiff consistency of taffy or cookie dough. The lava cools fairly rapidly and hardens before its atoms have time to organize themselves into the symmetrical, closely packed structures of crystals. Native American Indians fashioned knives, arrowheads, and other sharp tools from obsidian. Because obsidian blades are sharper than steel, they cause little scarring; some doctors use them today for delicate operations, such as eye surgery.
Four lava flows abundant with obsidian exist within the Caldera. The Big Obsidian Flow is one of the geologic wonders of the Pacific Northwest, and at 1300 years old is the most recent volcanic eruption in Oregon. Created when rhyolite lava spilled to the basin floor from a high vent and formed frozen cataracts of black, volcanic glass, the Big Obsidian flow has attracted human visitors for hundreds of years. Today, forest visitors use the 0.9 mile interpretive trail on the flow to gain firsthand knowledge about the black glass and its history.
When lava flows inundated pine forests, lava casts were often formed. Eruptions along the Northwest Rift Zone leading from Newberry volcano sent Pahoehoe (smooth) lava surging to the earth’s surface, and spilling through the pine forest. Sap stored in live trees was converted to steam by super-heated lava. Steam then cooled the lava, in turn preventing some trees from being burned up completely. Trees were either turned to ash or transformed to charcoal, depending upon the availability of oxygen. Molds formed when lava flowed around tree trunks, quickly cooling to fashion a hard coating. Where the flow of lava was rapid, molds did not form. When the lava slowly receded as a result of down flow drainage, the hardened molds of burned trees stood high above the lava surface. Later, the charred wood rotted away leaving the lava molds. Roots of these vanished trees still remain 10-15 feet below the lava surface. Radiocarbon dates from these roots and recent work done in flow contact areas place the time of eruptive activity at 6,000 years ago.