Earth Science and Natural Science Article - Fossil Forests of Eastern Washington

Fossil Forests of Eastern Washington - Page 2 of 10
By Thomas A Dillhoff, Pasco, Washington

The Miocene of Eastern Washington

The Miocene epoch spans a time period from approximately 25 million years ago to 6 million years ago. This was the period in which volcanic eruptions laid down the flood basalts that now blanket much of eastern Washington, and created the petrified wood deposits.

According to Dr. Steve Reidel of Washington State University, the Cascade mountain range of Washington and Oregon at the beginning of the Miocene epoch was similar in height and extent to that seen today. The topography of eastern Washington was generally a gentle slope that started at the base of the Bitterroot Range in Idaho, and sloped down towards the ancestral Cascades.

Early in the Miocene, the Cascade Range stopped growing and began to weather and erode. At approximately the same time, a hotspot in the earth's mantle formed under the area that now makes up the intersection between Washington, Idaho, and Oregon. Approximately 17.5 million years ago, volcanic vents opened in this region, and very fluid lavas poured out across eastern Washington and Oregon towards the Pacific Ocean. These lava flows moved so fast that it only took two weeks to four weeks for the largest flows to make it all the way to the coastline, a distance of nearly 400 kilometers. The lavas and eruptions were similar to those currently occurring at the Kilauea volcano on the island of Hawaii. Scientists have designated a total of five major eruptive events, consisting of up to one hundred twenty individual flows. The eruptions continued intermittently for a period of eleven million years. Figure 2 shows the combined extent of the Miocene basalt flows.

Figure 2: Total extent of Miocene flood basalts (Wells, 1989, p. 114 Reprinted with permission of the Geological Society of America)

Click to read next page!