WEBVTT Kind: captions Language: en 00:00:04.000 --> 00:00:09.380 About 20 million years ago when the Pacific and North American plates first made contact with each other 00:00:09.690 --> 00:00:13.490 the San Andreas fault system began to form at their common boundary. 00:00:13.600 --> 00:00:17.860 The Pacific Plate was moving Northwest in relation to the North American Plate. 00:00:18.420 --> 00:00:23.310 The block of crust that currently forms the Western transverse ranges and 00:00:23.310 --> 00:00:30.290 the Channel Islands was submerged and lined up in a north-south direction along the coast of present-day San Diego. 00:00:30.960 --> 00:00:36.980 Between about 20 million in 5 million years ago this block broke loose and pivoted along the edge of the 00:00:36.980 --> 00:00:41.100 North American plate rotating clockwise about 60 degrees. 00:00:41.620 --> 00:00:47.720 Beginning about five million years ago, Baja California broke away from the North American plate and moved 00:00:47.720 --> 00:00:50.400 Northwest ward as part of the Pacific plate. 00:00:50.720 --> 00:00:55.660 As it did so it compressed against the North American plate in Southern California. 00:00:55.980 --> 00:01:02.860 This compression caused folding, faulting and uplift within the Western transverse ranges Channel Islands block. 00:01:03.220 --> 00:01:08.480 During this time the block underwent as much as 30 degrees of additional clockwise rotation. 00:01:09.740 --> 00:01:14.940 Similar deformation and rotation has continued in the block up to the present time. 00:01:19.400 --> 00:01:23.480 The rotation of the western transverse ranges Channel Islands block 00:01:23.490 --> 00:01:28.590 stretched thin and fractured the crust, drawing magma to fill the gaps. 00:01:28.820 --> 00:01:34.020 In some places the magma erupted on the sea floor as volcanic lava that accumulated 00:01:34.020 --> 00:01:36.510 to as much as 10,000 feet thick. 00:01:36.920 --> 00:01:42.800 Marine basins that formed in subsided areas of the extended crusts collected sediments. 00:01:43.600 --> 00:01:48.260 Rivers on the mainland that flowed into the sea deposited some of these sediments. 00:01:48.820 --> 00:01:52.480 Submarine landslides were also deposited on the sea floor. 00:01:53.400 --> 00:01:58.220 Over time diatoms and other marine organisms shells accumulated along with 00:01:58.230 --> 00:02:02.970 the other sediments creating sedimentary layers of diatomaceous mudstone on the 00:02:02.970 --> 00:02:06.090 seafloor top the underlying volcanic rock.