WEBVTT Kind: captions Language: en 00:00:03.480 --> 00:00:09.580 About 85 million years ago several oceanic plates formed the floor of the Pacific Basin. 00:00:09.760 --> 00:00:14.620 The Pacific Plate was a small southern plate with other plates spreading away from it. 00:00:14.900 --> 00:00:21.080 Over time the Pacific Plate drifted north and expanded in all directions by sea floor spreading. 00:00:21.320 --> 00:00:24.540 New sea floor was continually added to its eastern edge 00:00:24.640 --> 00:00:27.980 by spreading between the Pacific and Farallon Plates. 00:00:28.120 --> 00:00:33.240 As plate movement continued the eastern side of the oceanic Farallon Plate slid 00:00:33.300 --> 00:00:37.060 or subducted under the continental North American Plate. 00:00:37.300 --> 00:00:42.080 It subducted faster than new sea floor was being created on its western side 00:00:42.240 --> 00:00:44.960 causing the plate to gradually narrow. 00:00:45.240 --> 00:00:48.660 As the Farallon Plates width decreased do to subduction 00:00:48.660 --> 00:00:54.200 it broke into smaller plates some of which subducted entirely and disappeared. 00:00:58.100 --> 00:01:03.740 Plates subduction occurred because the cool dense oceanic Farallon Plate sank into the 00:01:03.740 --> 00:01:07.560 mantle beneath the more buoyant continental North American Plate. 00:01:07.700 --> 00:01:12.340 As the plate descended it released water causing the hot mantle to melt. 00:01:12.560 --> 00:01:16.680 The resulting buoyant magma rose and pooled beneath the surface. 00:01:17.040 --> 00:01:21.240 From time to time some of the magma broke through to the surface and erupted 00:01:21.260 --> 00:01:25.460 in explosive ash clouds and lava flows building volcanoes. 00:01:25.600 --> 00:01:31.020 This process continued along the growing volcanic mountain chain in repeating cycles. 00:01:31.020 --> 00:01:36.700 While the Farallon plate continued to subduct causing millions of years of volcanic activity along the coast. 00:01:40.920 --> 00:01:45.780 About 20 million years ago the San Andreas fault system began to form when 00:01:45.780 --> 00:01:49.740 and where the Pacific and North American plates first made contact with one another. 00:01:49.740 --> 00:01:54.210 Along this boundary the Pacific Plate continued its motion to the 00:01:54.210 --> 00:01:59.870 northwest toward the Aleutian Islands moving sideways along the rim of North America. 00:02:00.060 --> 00:02:05.880 As it slid past it scraped off slivers of the North American plate and carried them with it. 00:02:06.060 --> 00:02:09.869 Continental fragments along the borderland were captured by the Pacific Plate 00:02:09.869 --> 00:02:15.749 in this process and moved away pulling open many of the basins in Southern California. 00:02:15.880 --> 00:02:22.080 As the plate boundary lengthened southward, it first lay along the Pacific rim of Baja California. 00:02:22.160 --> 00:02:24.860 About five million years ago a new section of 00:02:24.870 --> 00:02:28.740 the San Andreas fault system broke inside the North American continent 00:02:28.740 --> 00:02:31.740 along the line that is now the Gulf of California. 00:02:32.310 --> 00:02:39.330 This broke Baja California away from the North American Plate and transferred it to the moving Pacific Plate. 00:02:39.540 --> 00:02:44.250 The northern end of Baja California began to ram into Southern California 00:02:44.250 --> 00:02:47.570 pushing up many of our local east-west trending mountains 00:02:47.760 --> 00:02:51.660 and forming the big bend in the San Andreas fault system. 00:02:51.800 --> 00:02:57.540 The Sierra Nevada Great Valley Block farther inland is currently in the early stages 00:02:57.620 --> 00:03:01.560 of being captured by the Pacific Plate and carried northwestward.