Chapter 2

MORB

Mid ocean ridge basalt

California coast ranges

The Coast Ranges of California span 400 miles from Del Norte or Humboldt County, California south to Santa Barbara County.

triple junction

is the point where the boundaries of three tectonic plates meet.

1906 San Francisco earthquake

struck the coast of Northern California at 5:12 a.m. on April 18 with an estimated moment magnitude of 7.8

paleomagnetism

The branch of geophysics concerned with the magnetism in rocks that was induced by the earth's magnetic field at the time of their formation.

divergent plate boundaries

A tectonic boundary where two plates are moving away from each other and new crust is forming from magma that rises to the Earth's surface between the two plates.

plate tectonics

A theory explaining the structure of the earth's crust and many associated phenomena as resulting from the interaction of rigid lithospheric plates that move slowly over the underlying mantle.

Panthalassa

The super ocean that surrounded the supercontinent Pangaea. During the Paleozoic—Mesozoic transition c. 250 Ma it occupied almost 70% of Earth's surface.

Juan de Fuca Ridge

a mid-ocean spreading center and divergent plate boundary located off the coast of the Pacific Northwest region of North America. The ridge separates the Pacific Plate to the west and the Juan de Fuca Plate to the east.

hot spots

are places within the mantle where rocks melt to generate magma

transform plate boundary

locations where two plates slide past one another.

1989 Loma Prieta earthquake

occurred in Northern California on October 17 at 5:04 p.m. local time. The shock was centered in The Forest of Nisene Marks State Park approximately 10 mi northeast of Santa Cruz on a section of the San Andreas Fault System

subduction zones

A geologic process in which one edge of one crustal plate is forced below the edge of another.

Pangaea

A hypothetical supercontinent that included all current land masses, believed to have been in existence before the continents broke apart during the Triassic and Jurassic Periods.

magnetic anomaly

A local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks.

mid-Atlantic ridge

A mid-ocean ridge, a divergent tectonic plate or constructive plate boundary located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world.

East Pacific rise

A mid-oceanic ridge, a divergent tectonic plate boundary located along the floor of the Pacific Ocean. It separates the Pacific Plate to the west from (north to south) the North American Plate, the Rivera Plate, the Cocos Plate, the Nazca Plate, and the Antarctic Plate.

convergent plate boundaries

A region of active deformation where two or more tectonic plates or fragments of the lithosphere near the end of their life cycle.

oceanic ridges and rises

An underwater mountain range, formed by plate tectonics.

boundary

Earth's outer layer, the crust, is divided into a set of large moving plates.

three North Poles (!)

The direction in which the north end of a compass needle or other freely suspended magnet will point in response to the earth's magnetic field, The direction from any point on Earth toward the North Pole also called true north, The point on the surface of Earth's Northern Hemisphere at which the planet's magnetic field points vertically downwards.

San Andreas Fault

a continental transform fault that extends roughly 1,200 kilometers through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip.

Cascadia subduction zone

a convergent plate boundary that stretches from northern Vancouver Island to northern California.

Hayward Fault

a geologic fault zone capable of generating significantly destructive earthquakes. This fault is about 74 mi long, situated mainly along the western base of the hills on the east side of San Francisco Bay.

Walker Lane seismic zone

a geologic trough roughly aligned with the California/Nevada border southward to where Death Valley intersects the Garlock Fault, a major strike-slip fault.

ocean trenches

a long, narrow, deep depression in the ocean floor, typically one running parallel to a plate boundary and marking a subduction zone.

Cascade Mountains

a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California

San Jacinto Fault

a major strike-slip fault zone that runs through San Bernardino, Riverside, San Diego, and Imperial Counties in Southern California. A component of the larger San Andreas transform system and is considered to be the most seismically active fault zone in the area.

Gulf of California

a marginal sea of the Pacific Ocean that separates the Baja California Peninsula from the Mexican mainland

Point Reyes

a prominent cape and popular Northern California tourist destination on the Pacific coast. It is located in Marin County

Pacific-North American plate

a tectonic plate covering most of North America, Greenland, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia.

"earthquake storm"

a theory about earthquakes, where one triggers a series of other large earthquakes—along the same plate boundary—as the stress transfers along the fault system.

San Gregorio Fault

an active fault located off the coast of Northern California. The southern end of the fault is in southern Monterey Bay, and the northern end is about 20 km northwest of San Francisco, near Bolinas Bay, where the San Gregorio intersects the San Andreas Fault.

Farallon plate

an ancient oceanic plate that began subducting under the west coast of the North American Plate—then located in modern Utah—as Pangaea broke apart during the Jurassic period.