Читать книгу 101 Hikes in Northern California. Exploring Mountains, Valleys, and Seashore онлайн

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The seafloor is composed of five layers. The bottom three form a surface that is essentially a solid piece of the earth’s crust. The upper two layers are deposited underwater on this surface. At tectonic spreading ridges, liquid basalt is squeezed out onto the seafloor, piling up in distinctively shaped pillow basalts, which compose the first of the two upper layers. These are in turn covered by sediment settling from the ocean. Primarily made up of the microscopic skeletons of tiny sea creatures, this silica-rich upper layer takes millions of years to accumulate, gradually forming thin layers mixed with small bits of sand and mud. Altered by pressure and temperature, these two upper layers eventually become greenstone and chert. The Mount Diablo Ophiolite, as it is called, has been so heavily deformed and tilted that the sequence no longer lies flat. While its upper layers are visible at the summit, its lower layers are exposed northwest at the Lone Star Quarry, where diabase, a constituent rock of the lower sequence, is used for roadbeds and foundations. Exactly how this piece of 165-million-year-old seafloor became emplaced in the young sediments ringing Mount Diablo remains a mystery.