What is the modern name for the area that formed after floods of highly fluid basaltic lava spread to cover an area of over 200,000 sq mi?
All rock now exposed in the area of the park is volcanic, but this has not always been the case. For hundreds of millions of years, the Lassen region underwent repeated uplifting to form mountains, only to have them worn down and submerged under encroaching seas. During the periods of submersion, sand, mud and limestone were deposited. Occasionally volcanic activity was associated with the mountain building.
About 70 million years ago, the area where the Cascade Range is now situated was under the most recent encroachment by the Pacific Ocean. The rocks that make up the modern Sierra Nevada and the Klamath Mountains were already in existence but deeply buried. Some 70 million years before (140 million years before present), the rocks that now make up the Klamaths broke away from the rocks that now make up the Sierras and moved 60 mi (97 km) west, leaving the flooded 'Lassen Strait.' This broad depression was a seaway that connected the marine basin in California with that in east central Oregon.The entire western portion of North America was being deformed from the Laramide orogeny starting around 70 million years ago. Gradually during millions of years, crustal rocks were folded and fractured and the seas driven away. This same bending and breaking of rocks relieved pressure on the hot material beneath the Earth's crust and permitted magma to rise toward the surface. Volcanoes burst into activity starting 30 million years ago from Washington southward along the Cascades and in the area now occupied by the Sierra Nevada. This activity continued until approximately 11 or 12 million years ago. Lava and ash reached a thickness of up to 10,000 ft (3,000 m) in some areas, forming what is now known as the western Cascades. These have been eroded until they are now rolling hills.
Meanwhile, toward the end of this activity, eruptions of a different kind took place on an unprecedented scale in eastern Oregon and Washington. From innumerable cracks, floods of highly fluid basaltic lava spread to cover an area of over 200,000 sq mi (520,000 km2). Now known as the Columbia Plateau, this great lava bed of flood basalt covers much of Oregon, Washington and even parts of Idaho. Northern California's Modoc Plateau is a thinner basaltic flow which some geologists associate with the Columbia Plateau, but there are technical objections to this. The High Cascades took shape as a distinct mountain belt as a result of this upheaval and the bending of the thick blanket of volcanic rocks. During the next 10 million years, a series of new basaltic volcanic cones similar to the shield volcanoes now found in Hawaii were built.
Columbia Plateau