Rockhounding: Geologic History

Central Oregon has been shaped by numerous and varied volcanic events that began millions of years ago and continue to the present. A chain of volcanoes in eastern Oregon began erupting about 50 million years ago during a time of tropical climate conditions that supported lush woodlands. The volcanic and sedimentary rocks from these volcanoes are known as the Clarno Formation. During this time period, ash on steep slopes of volcanoes frequently mixed with tropical rains and produced large-scale mudflows that swept across the landscape, entombing plants and animals, preserving them as fossils. Entombed trees and wood debris became much of the petrified wood that rockhounds in central Oregon collect today.

After the eruptive activity of the Clarno volcanoes ceased, a short period of erosion took place. Beginning about 33 million years ago, another set of volcanoes produced the lava flows and ash deposits of the John Day Formation. By that time, the climate was cooler and more temperate and the plants and animals were different from those that lived in the shadow of the Clarno volcanoes. Like the Clarno volcanoes, eruptions from the John Day volcanoes also buried and preserved the remains of plants and animals.

Steins Pillar in the Clarno Formation. © By R. Franklin Steins Pillar in the Clarno Formation. By R. Franklin

Painted Hills in the John Day Formation. © By R. FranklinPainted Hills in the John Day Formation. By R. Franklin

The earliest eruptions of the Cascade Mountain Range began about 40 million years ago. Successive lava flows began to pile high enough to form a barrier to moist Pacific storms and eventually transformed the central Oregon climate into the dry high desert conditions of today. The present day high Cascade peaks that make up the familiar backdrop of central Oregon all formed in the last million years.


The Three Sisters in the Cascade Mountains. © By R. FranklinThe Three Sisters in the Cascade Mountains. By R. Franklin

During past volcanic activity, superheated groundwater circulated through the rocks, filling the cracks and voids with quartz, calcite, cinnabar, and other minerals. Today the Clarno and John Day formations are highly eroded, exposing many different rock layers rich with semi-precious gems, creating a paradise for rockhounds. Many members of the quartz family of semiprecious gemstones are represented in these deposits including crystalline quartz, amethyst (rare), and various types of chalcedony such as agate and jasper. A particular favorite of rockhounds are the agate-filled nodules known as thunder eggs, the Oregon state rock.


Another notable series of volcanic events occurred about 4.9 million years ago in the area southeast of Hampton. Multiple flows of molten volcanic glass oozed out of the ground at Glass Buttes, producing a vast landscape covered with obsidian. Many of the hills in this area are literally solid glass! Today, rockhounds visiting the Glass Buttes area can find gold sheen, silver sheen, fire sheen, rainbow, midnight lace, double flow, and mahogany varieties of obsidian.


Glass Butte at Sunset. © By R. FranklinGlass Butte at Sunset By R. Franklin