Grade 6 - Science - Chapter 6 - Section 4

The Cascadia Subduction Zone

- Today the Juan de Fuca plate is subducting beneath the North American plate. This area is known as the Cascadia subduction zone. • A chain of active volcanoes is present in this zone, in the Cascade Mountains of California, Oregon, and Washington.

The San Andreas Fault System

• California is home to the most famous transform plate boundary in the world, the San Andreas fault system. • The San Andreas fault system extends for about 1,000 km. • The San Andreas fault forms the boundary between the Pacific and North American plates. • Most of California is on the North American plate. A small part of California, west of the San Andreas fault, lies on the Pacific plate. • The Pacific plate is moving to the northwest relative to the North American plate.

Subduction and Accretion

• During subduction, pieces of the plate that subducts may be scraped off and attached to the overriding plate. This process, called accretion, forms mountain chains. These mountain chains are parallel to the plate boundary. • The rocks in California's Coast Ranges and Transverse Ranges are though to have been formed by accretion. • The Central Valley, Los Angeles Basin, and Ventura Basin separate some of these mountain ranges.

Compression in Southern California

• In southern California, the San Andreas fault makes a huge bend as it passes east of Los Angeles. • Because of this bend, the Pacific and North American plates collide as they move past each other. As a result, the motion along this boundary is partly convergent. • Because southern California is being compressed, areas near the bend are being uplifted or dropped down by active faults. • The San Bernardino Mountains and the San Gabriel Mountains are tectonically created mountain ranges. • The Los Angeles basin is a large depression bordered by active faults.

Plate Tectonics and the California Landscape

• Much of California's landscape has been formed by plate tectonics. • Compression has recently uplifted California's rugged mountains. • The steep, rocky coastlines have been formed by uplift along the plate boundary. • Major river valleys, mountain ranges, and the coastline are oriented in a northwesterly direction. A northwesterly orientation is parallel to the faults of the plate boundary. • The Transverse Ranges are oriented east-west, due to motion from the San Andreas fault system.

Plate Motion on the San Andreas Fault System

• Not all plate movement takes place on the San Andreas fault itself. In the San Francisco area and southern California, motion takes place on other faults of the system. These faults lie west and east of the San Andreas fault. • In these areas, it is best to think of the boundary between the North American and Pacific plates as a zone, not a line.

Offset on the San Andreas Fault System

• The Pacific and North American plates have been moving along the San Andreas fault system for about 25 million years. • During the last 16 million years, the separation, or offset, along the fault has been 315 km. • Geologists use rocks to estimate the amount and rate of movement along the fault. Geologists determine offset by matching unusual rocks that have been separated by the fault. • They date these rocks to determine when they formed. Then geologists use that date to determine when the areas were not separated.

The Transform Boundary is Born

• The ancient Farallon plate lay between the North American and Pacific plates. • About 25 million years ago, the entire Farallon plate was subducted at one part of the boundary. The Pacific Plate touched North America for the first time, forming a transform boundary. • As the Farallon plate continues to subduct, the transform boundary continues to grow longer. Today, it is about 2,600 km long. • The Juan de Fuca plate off northern California is part of what remains of the ancient Farallon plate.

Accreted Terranes

• The chunks of lithosphere that are scraped off of subduction plates and added to the edge of a continent are called accreted terranes. • The rocks of a terrane differ from the surrounding rocks by age or composition.

California Gold

• The foothills along the western side of the Sierra Nevadas contain rocks filled with gold. • These rocks are thought to be accreted terranes. This rock formed near submarine volcanic vents. After the terranes were accreted, the gold became concentrated in the quartz veins of the Mother Lode.

Building California by Plate Tectonics

• The region that we know as California has been at an active plate boundary for the past 225 million years. As a result, plate tectonics has been the most important force shaping California's geologic history. • Before about 225 million years ago, North America's western edge was much farther east than it is now. • The area where Nevada and the eastern deserts of California are today was the west coast of North America. Most of what is now California was either part of a distant oceanic plate or did not exist. • When Pangaea began to break up, the North American plate moved west. The continent's western edge became an active convergent plate boundary. • A long period of subduction began, which was an important period of geologic "building" in California.

Subduction and Volcanism

• The subduction of the Farallon plate caused rocks to melt and caused chunks of rock to collide with the North American continent. • Subduction caused a great deal of magma, or molten rock, to form in the lithosphere.

The Sierra Nevada Batholith

• This magma solidified to form a huge mass of granite called the Sierra Nevada batholith. • A batholith is a large mass of igneous rock that forms deep below the surface. Batholiths are the "roots" of subduction zone volcanoes. • A chain of huge volcanoes must have formed above the giant magma chamber. These volcanoes probably stood twice as tall as today's Sierra Nevadas. • The granite batholith is exposed in the Sierra Nevada mountain range.

Ancient Plate Boundaries What are the three major tectonic plates that influenced California's geologic history?

• Three major tectonic plates influenced California's geologic history: the North American plate, the Farallon plate, and the Pacific plate. • A convergent boundary existed between the North American and Farallon plates. The Farallon plate subducted beneath the North American plate.