Читать книгу Afoot & Afield: Orange County. A Comprehensive Hiking Guide онлайн

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As you travel through the Santa Ana Mountains (and their distinctly named subdivisions, the Elsinore and the Santa Margarita Mountains), you’ll begin to piece together their geologic history. Starting at, say, Caspers Wilderness Park in the foothills and moving up toward the crest of the Santa Anas, you first pass among light-colored marine sedimentary rock formations that were pushed up and tilted by the rise of the mountains to the east. Next comes metamorphic rock of two basic kinds—metasedimentary and metavolcanic. These brown- or gray-colored rocks, roughly 200–150 million years old, are metamorphosed (changed by heat and pressure) forms of marine sedimentary and volcanic rock that were plastered against the core of the North American continent some tens of millions of years ago. These rocks were riding on one or more of the Earth’s tectonic plates, which collided with and were subducted (forced under) the western edge of the continent.

Near the crest of the Santa Ana Mountains, you find light-colored granitic rocks. Here’s the reason: About 100 million years ago, when the subduction process was in full swing, much of the material on the edge of the plate being subducted melted underground and accumulated in the form of vast pools of magma. Because this magma was less dense than the surrounding materials, it rose toward the surface. Some escaped through volcanoes, but most of it remained underground long enough to slowly cool and crystallize, forming coarse-grained “plutonic” (generally granitic) rocks. Erosion then nibbled away at the overlying metamorphic rocks, finally exposing—typically in high places—the granitic rocks.