The Oregon landscape has been shaped by plate tectonics and millions of years of volcanic activity. Oregon can be divided into the following nine geologic provinces, each with outstanding geologic scenery.
The heavily vegetated, elongated Coast Range province has a varied geologic history. Its basement was formed by a volcanic island chain that collided with North America about 50 million years ago. The ancient volcanoes form many of the scenic headlands along the coast, and the sediments that have accumulated around them contain marine fossils that help unravel the area’s complicated geologic story.
This province consists of four north-south-trending belts of metamorphic and igneous rocks that formed in an oceanic setting and subsequently collided with the North American continent about 150 million years ago. Complexly folded and faulted rocks are bounded by belts of sparsely vegetated bands of serpentinite. Oregon Caves National Monument lies within an enormous fault-bounded block of marble. The historic gold-rush town of Jacksonville remains today as evidence of the area’s colorful gold-mining history.
Between 14 and 16 million years ago, "fissure" volcanic eruptions in eastern Washington, eastern Oregon, and western Idaho produced enormous volumes of molten Columbia River basalt that flowed like water west into the Deschutes-Columbia Plateau province in eastern Washington and northeastern Oregon, with some lava continuing to flow as far west as the Pacific Ocean via the ancestral Columbia River valley. As the basalt cooled and congealed, it formed the columnar cliffs that dominate the landscape today. Erosions by the Columbia River has exposed a particularly spectacular sequence of these rocks in the Columbia River Gorge on Oregon’s northern boundary.
The Willamette Valley province forms a catch basin between the Coast Range and the Cascades. The sediments collected into this basin record multiple Ice Age floods that originated in Montana, poured through the Cascades (via the Columbia River), and backed up in the valley before eventually draining to the Pacific Ocean..
This area is made up of separate "exotic terranes," areas that were created elsewhere and scooped up by North America as it moved west toward the Pacific. Fossils found in this province reveal their foreign origins. Placer and lode gold mines were active here in the past, and towns such as John Day and Baker City, together with the Sumpter gold dredge, are vivid reminders of the Blue Mountains gold mining heritage.
Basin & Range and Owyhee Uplands
Both lie in a region that has been stretched, or extended, almost 100 percent from its original width in the last 10 million years. Evidences of this extension are massive fault block mountains such as Steens and Hart Mountains, with intervening basins containing such features as the Alvord Desert and Lake Abert. Intense volcanic and hot spring activity over millions of years has produced fine-grained gold deposits and jasperoids that are prized by rock hounds. The Owyhee Uplands gets its strange name from a group of Hawaiian trappers who got lost and died in the region in 1818.
High Lava Plains
This area has some of the most recent faulting and youngest volcanic activity in Oregon. Well-preserved in a high desert climate, volcanic features stand out about the plains.
The Cascade province is actually made up of two volcanic regions, the older, broader, and deeply eroded Western Cascades and the dominating, snow-capped peaks of the younger, more easterly volcanoes of the High Cascades, such as Mount Hood, Mount Jefferson, and the Three Sisters (North, Middle, and South Sister). Another High Cascade peak, Mount Mazama, was destroyed about 6,800 years ago by a catastrophic eruption that left a deep caldera later filled by what is now Crater Lake.