Key Events
The Yellowstone Hotspot
The wonderful Owyhee geology was made possible by the Yellowstone hotspot that created a huge pool of magma just below the surface here about 15 to 16 million years ago and left a trail of now buried explosive volcanic calderas in its wake. The eruptions lasted for hundreds of thousands of years and covered the region with layers of rhyolite lava and tuff up to several thousand feet thick. Millions of years of erosion and weathering produced today’s scenic Owyhee canyonlands.
Today, the hotspot plume has rotated eastward under the North American plate and is now under Yellowstone National Park. Millions of years ago, hot springs, geysers, oozing lava flows, and caldera lakes like we see today at Yellowstone likely existed in this corner of southeast Oregon. So much time has passed that hotspot evidence has long since been eroded and buried by later geologic events.
Although the hot spot moved on, volcanic activity continued. Later eruptions created the Jordan Valley Volcanic Field and the rolling high lava plains that now cover the Owyhee landscape. More than once, lava flows dammed up the Owyhee River channel, altering its course. Almost everything we now see in the Owyhee Canyon is volcanic in origin and a consequence of the hot spot.
Eruptions here began about 15 to 16 million years ago. (Source: NPS)
Birth of Owyhee Canyon
The bulk of the modern narrow and deep Owyhee Canyon was carved in only the past few million years. Before that, sometime between 2 and 5 million years ago, the ancestral Owyhee River occupied a much wider and shallower canyon.
Geologists think the birth of the ancestral river was probably caused by fluctuations in the level of ancient Lake Idaho. For more than 6 million years, Lake Idaho filled the Snake River valley with a huge lake about the size of Lake Erie. Water levels fluctuated but generally stood 1,000 to 1,600 feet above today’s valley floor. The water was so high that there was little elevation difference between the lake surface and the surrounding Owyhee landscape. Because of the small elevation difference (gradient), streams emptying into the lake did not incise deep canyons.
Then sometime between 2 and 3.4 million years ago, the lake gradually overflowed to the west down what is now the Snake River, in the process carving Hells Canyon and leaving behind an empty lake basin—today’s fertile Snake River Valley.
The lake emptying took several million years, causing the ancestral Owyhee River’s base level elevation to drop by over 1,000 feet. The drop in base level triggered accelerated erosion and downward cutting of the river channel, producing today’s deep and narrow canyon.
(From Geology Underfoot in Southern Idaho by Shawn Willsey 2017)
Plateau forming lava flows
After the hotspot moved east into Idaho, volcanic activity persisted in southeast Oregon. The magma composition shifted from gummy rhyolite and andesite to highly liquid basalt. About 10 million years ago, volcanic fields consisting of shield volcanoes and fissures began erupting runny basalt lava flows that traveled miles from the vent.
Hundreds of lava flows probably occurred, each usually covering a small area. On high plateaus, newer lava covered older flows, forming an erosion-resistant “roof” over the ancient basin sediment. Thick canyon rimrocks developed when rivers eventually cut through the lava plateau. In basins, successive lava flows were interbedded with sediment layers, creating banded canyon walls.
The volcanic fields in southeast Oregon and Idaho represent some of the youngest and most extensive basalt volcanism in the western U.S. The Jordan Valley Volcanic Field alone, on the east bank of the river, has thirteen dated volcanic vents ranging in age from under two thousand years old to over five million years old. Coffeepot Crater, near the Birch Creek Historic Ranch, is 1,300 to 1,700 years old, one of the most recent dated eruptions in Oregon
It erupted 70,000 years ago and created a volcanic field that filled the canyon with lava.
River changing lava dams
The Owyhee River is well known for its many lava dams. Lava flows repeatedly entered and dammed the river canyon, forming lakes that stretched up to 45 miles upstream, often redirecting the river. During the last two million years, at least six different lava dams are known to have formed across the river, some hundreds of feet high. Earlier dams likely existed, but evidence has been erased by erosion.
Although the lava dams no longer exist, abundant evidence of their existence can be seen in today’s canyon walls. When a dam-building lava flow enters a large body of water, the lava rapidly cools and usually fragments into pieces that accumulate in slanting layers in front of the flow. The formation is called a “lava delta,” like a river delta, but formed by flowing lava instead of water. The distinctive, orange-colored dipping layers in the lava delta are called “foreset beds”. The horizontal contact line between the dipping foreset beds and overlying lava flow marks the elevation of the water’s surface. “Lava pillows” are often found intermingled with the fragmented rocks in the foreset beds. These are lavas with bulbous, spherical, or tubular-shaped structures formed when lava is extruded underwater at slow effusion rates. Geologists look for lava deltas to figure out the geological history of the Owyhee Canyon.
Source: USGS