Index fossils are used to identify geologic periods or faunal stages in rock layers. These fossils must have a wide geographic distribution and manifest obvious evolutionary trends to help geologists and others who need this geological data to better understand the time scale and to pinpoint a time period. For example, ammonites were common during the Mesozoic Era, but they were extinct by the Cretaceous period. Geologists would use ammonites to help determine this time frame if present.
This is happening because of the plate tectonics. The Pacific Plate is moving a few centimeters in a direction of northwest each year. There is a hot spot under the Pacific plate that keeps creating the islands, new islands are being formed in the southeast as the plates move, at the older islands in the northwest are being gradually destroying away.
As the crust of the San Andrea is continuously moving due to the tectonic activity and is the main manifest of the tectonic boundary of the North America and the pacific plate. This fault along with the pacific plate will move towards the Los angles and eastern San Francisco as it moves about 3.5 cm per year.
As 1 km equals to about 100 000 cm
, then the distance of 620 km = 62, 000,000 cm and 62,000,000 will be divided by 3.5 equal to 17. 71 years to collide with the eastern side.
