Mount Pelée in 1902) wiped out several hundred thousand lives as well, and have even altered world climate.
accretionary wedge, forearc basin, and magmatic arc of the now inactive subduction zone.
|The Golden Gate Bridge and the entrance to San Francisco Bay. The Marin Headlands are on the left, and San Francisco is on the right. The San Andreas would cross the scene just below the bottom margin of the photograph, underwater.|
|The East Bay hills, the Diablo Range, and the Carquinez Strait, where the Sacramento River flows into the bay. These hills are underlain by the accretionary wedge deposits of the Franciscan Complex.|
|The eastern margin of the Diablo Range in the Coast Ranges near Patterson. The parallel stripes of rock are the sedimentary rocks of the Great Valley Sequence, which were deposited in a forearc basin|
|The San Joaquin portion of the Great Valley near Patterson and Modesto. The floodplain of the San Joaquin River is the gray streak across the middle of the photograph.|
|The town of Colusa in the northern Sacramento Valley. The Sacramento River winds through the area.|
We then reach the foothills of the Sierra Nevada. The Sierra is a huge westward tilted block of granite and metamorphic rock 400 miles long, and 50-60 miles wide. The mountains start gently enough, an almost imperceptible change of slope, but the bedrock is close to the surface so groundwater is not available for irrigation purposes. The prairie is mostly used as cattle range, and remains much as it was hundreds of years ago, aside from barbed wire fences and exotic European grasses that have crowded out the native bunchgrasses. Thousands of acres have recently been converted to almond groves with uncertain water sources.
|The Sierra Nevada foothills near Sonora and Oakdale. Highway 108 crosses the middle of the photograph. The rocks are mostly composed of volcanic lahars (mudflows) of the Mehrten formation, dating to around 10 million years.|
We finally reach the alpine landscape of the high Sierra Nevada. The rocks exposed here are the granitic plutons that once fed the volcanoes of the magmatic arc of the subduction zone. More than a hundred individual intrusions have been mapped, ranging in age from about 200 to 80 million years. Each of the intrusions probably fed a volcanic field miles above, but erosion has stripped away those miles of overlying rock. The remnants of the ancient subduction zone volcanoes can be seen as cobbles in the rocks of the Great Valley Sequence.
|The high country of the Sierra Nevada at the headwaters of the San Joaquin River.|