During the late Miocene and Pliocene epochs, one of the largest flood basalts ever to appear on the Earth's surface engulfed about 63,000 square miles of the Pacific Northwest, spreading over Idaho, Oregon and Washington. Eruptions were most vigorous from 17 to 14 million years ago, when over 99% of the basalt was released, although flows continued until six million years ago. Perhaps "vigorous" isn't the right word since, for the most part, the lava flowed out of cracks in the ground, rather than erupting with the explosiveness of Hollywood movies. Flows were much like you see in Hawaii today, which are still dramatic, instead of the cataclysmic eruption of a volcano like Mt. Hood.
One result of the area being formed by flood lavas are the large columnar basalts that can be found on the Monument. When thick lava cools slowly, contractional joints or fractures form. While the lava can shrink in the vertical dimension without fracturing, it can't easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns. These structures are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed. The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (½-inch diameter) columns, while slow cooling is more likely to produce large columns.
Under the Monument, these basaltic lava deposits (called the Columbia River Basalt Group) are over 13,000 feet thick. Although the Columbia River Basalt Group is one of the smallest flood-basalt areas, the individual lava flows that comprise the area are some of the largest found on Earth. Within the area covered by the Columbia River Basalt Group are two distinct subbasins, each having its own geologic character—the Columbia Basin and Pasco Basin.
This basin encloses the Columbia River Basalt Group, which is surrounded by the Cascade Mountains to the west, Rocky Mountains to the northeast, and Blue Mountains to the southeast. The Columbia Basin lies mostly in southeastern Washington but does extend into western Idaho and northern Oregon.
The Pasco Basin is a depression in the lower part of the Columbia Basin. Geographically, the ridges surrounding the Monument and vicinity define this smaller basin; the basin is bounded by the Saddle Mountains to the north, Naneum Ridge to the west, the Rattlesnake Hills to the south, and the Palouse Slope to the east—generally the area north of where the Snake River flows into the Columbia River. These ridges surrounding the Pasco Basin are the result of tectonic forces. The Pasco Basin is filled with Ringold sediment from the ancestral Columbia River and sediment left by the Missoula Floods. The Hanford Site comprises the southern portion of the Pasco Basin, occupying over 586 square miles.
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Once a national wildlife refuge itself, Saddle Mountain National Wildlife Refuge still exists, but as part of the much larger Hanford Reach National Monument.