Until about 70 million years ago, most of what is now Oregon was covered by warm seas that supported a rich variety of sea life including brachiopods, corals, sponges, and ammonites. On low-lying land, ferns, cycads, ginkgoes, and conifers grew in a warm, temperate climate. Fossils of these and many other plants and animals have been found in the older rocks of Oregon.

From about 70 million years ago to the present, tectonic forces pushed land upward from the tropical warm seas and volcanism dominated the geologic history of Oregon. The Cascade Range, a great north-south chain of volcanoes, has been growing in episodes for the past 40 million years. Fifteen million years ago they had grown high enough to affect the climate of central and eastern Oregon. The Cascades began to block moisture carried eastward by winds from the Pacific Ocean. Today, the climate east of the Cascades is relatively dry (mostly 10 to 25 inches of precipitation annually), and west of the Cascades it is wet (40 to 100 inches).

Part of the Pacific Rim "Ring of Fire", the Cascade Range of mountains 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 7,000 years ago by a catastrophic eruption that left a deep, six-mile wide caldera.

Today, the calm beauty of Crater Lake belies the violent earth forces that formed the lake. Crater Lake lies inside the top of the ancient volcano known as Mount Mazama. For half a million years this mighty volcano produced massive eruptions interrupting long periods of quiet. Ash, cinders, and pumice exploded upward, building the mountain to a height of about 12,000 feet. About 7,000 years ago the climatic eruptions of Mount Mazama occurred. Ash from these eruptions lies scattered over eight states and three Canadian provinces, some 5,000 square miles were covered with 6 inches of Mazama's ash. In the park's Pumice Desert ash lies 50 feet deep. The eruptions were 42 times greater than those of Mount St. Helens in 1980. The Mazama magma chamber was emptied and the volcano collapsed, leaving a huge bowl-shaped caldera. The collapse of Mount Mazama marked the beginning of the formation of Crater Lake. Snow and rain fell into the 3,000-foot deep hole, filling the collapsed structure. Eventually, the lake reached a relatively constant level. Precipitation entering the lake was offset by evaporation and seepage. Today, the lake level only varies about three feet each year. Crater Lake, at 1,958 feet (597 meters) deep, is the seventh deepest lake in the world and the deepest in the United States.

Individual stratovolcanoes and shields make up Mount Mazama. The oldest Mazama lavas dated are flows near lake level at Phantom Ship and the lavas of Mount Scott (around 400,000 years). The youngest stratovolcano is Hillman Peak (around 70,000 years). Local andesite flows on the north rim are 50-40,000 years old. A small volcanic island, Wizard Island, appears on the west side of the lake. This cinder cone rises 760 feet (233 meters) above the lake and is surrounded by black volcanic lava blocks. A small crater, 300 feet (90 meters) across and 90 feet (27 meters) deep, rests on the summit. The crater is filled by snow during the winter months, but remains dry during the summer.