Chapter three (Basic Geology) The Blue Herons: Danielle Gobert, Stacy Holland , Ann Mason, and Eugenie Steinman

Geology
Is the study of the planet earth, its rocky exterior, its history and the processes that act upon it.

Igneous Rock

Is rock formed when molten or partially molten material called Magma, cools and solidifies. Igneous rocks are one of three main types of rocks, Sedimentary rocks and Metamorphic rocks.

The two most common types of igneous rocks are granite and basalt. Granite is light colored and is composed of large crystals of the minerals quartz, feldspar, and mica. Basalt is dark and contains minute crystals of the minerals olivine, pyroxene, and feldspar.

Sedimentary Rock

Which is formed when wind or water deposit sediments and the sediments become compacted. These rocks are classified according to their manner of origin into mechanical or chemical sedimentary rocks. Mechanical rock or also called fragmental rocks are composed of mineral particles produced by the mechanical disintegration of other rocks and transported without chemical deterioration by flowing water. They are carried into larger bodies of water, where they are deposited in layers. Examples are shale, sandstone, and conglomerate. Chemical sedimentary rocks consist of the remains of microscopic marine organisms precipitated on the ocean floor. Example of this is limestone. Or they may also have been dissolved in water circulating through the parent rock formation and the deposited in a sea or lake by precipitation from the solution. Examples are halite, gypsum, and anhydrite.

Metamorphic Rock

Is a type of rock formed when rocky material experiences intense heat and pressure in the crust of the Earth. Many of the oldest rocks on the continent are metamorphic and some of these are found in California. Some examples are slate, quartzite, schist, gneiss, marble, and serpentine.

Tectonic Plates

Are made of either oceanic or continental crust and the very top part of the mantle. The crust and the upper mantle form what is called the lithosphere. Under the lithosphere lies the fluid rock layer called the asthenosphere. Tectonic plates are able to float upon the fluid asthenosphere because they are made of rigid lithosphere.

Geology

Study of nonliving components of the earth, more specifically, rocks. Rocks are composed of several

minerals and can be divided into three categories: igneous, sedimentary, and metamorphic:

Igneous Rocks

Igneous rocks are one of three main types of rocks formed from cooling and solidification of molten rock.

Two most common types of igneous rocks are granite and basalt.

Sedimentary Rocks

Make up only 5% of earths curst, but 75% of rocks on the surface. Formed when wind or water deposit

sediments and the sediments become compacted (become rock). They are classified as mechanical or

chemical sedimentary rocks depending on their origin.

Metamorphic Rocks

Rocks that have been altered from original form by heat and pressure. Some examples are slate,

quartzite, schist, gneiss, marble, and serpentine. Many of the oldest rocks on the continent are

metamorphic.

Geologic Time Scale

A geologic time scale combines absolute yearly age determinations, with relative age based on rocks.

Many different techniques have been discovered for making estimations of absolute age and relative age.

These techniques can produce a picture of events that have occurred during the earth's history.

Plate Tectonics

Plate tectonics are plates sliding about on the surface of the earth. They are made up of oceanic or

continental crust and the very top part of the mantle. Two or more plates may converge on each other and

depending on the motion produce different topography, California is one example.

Earthquakes and Faults

California has many faults including San Andreas and Garlock. Areas that experience several small

earthquakes are known as active areas or creep zones. Areas that are bounded by hard rock materials

that do not move often are known as locked zones. When they do move a large earthquake happen.

Tension in a creep zone is released a little at a time, but in a locked zone it builds up for many years.

Earthquakes do not always occur on faults with a recent history of movement.

specifics to the study of rocks. Moreover the study of nonliving components of the earth. While living organisms consist of primarily four

elements; hydrogen (H), oxygen (O), nitrogen (N), and carbon (C) nonliving organisms consist or are composed of some combination of eight

elements: silicon (Si), oxygen (O), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K) and magnesium (Mg).

A mineral is composed of a single element or compound. By definition, a mineral is a naturally occurring inorganic substance with a definite

chemical composition and ordered atomic structure. Rocks are usually composed of several minerals. Rocks can be divided into three primary

categories: igneous, sedimentary, and metamorphic.

Igneous Rocks: These rocks are rormed from cooling and solidification of molten rock. The definition of igneous refers to fire while also coming

from the same root as ignite. The internal core of the earth is high temperature therefore equalling heat energy, which melts rock to produce

magma. When the magma cools it becomes igneous rock. Dependent upon the time it takes it to cool, determines the size of the crystals of

each mineral. The texture of the rock is dependent upon the length of time it took to cool. There are many types of igneous rock, they differ in

color and composition. Light colored minerals= quartz and feldspar. Dark colored= minerals that contain iron and magnesium and are also

called ferromagnesian. In relation to California, granite and diorite are the most common rocks that the Sierra Nevada consist of.

Sedimentary Rocks: These rocks make up only 5% of the earth's crust, but they make up 75% of the rocks on the surface of the earth. The

make up of sedimentary rocks differs from Igneous rock. Sedimentary rock deals with mechanical and chemical weathering which causes the

rock materials to break up into smaller particles, where these particles are transported from their source and when they come to rest they

represent sediement. Most sedimentary rock contains particles or clasts which are known as clastic. In California there are many geographical

regions that consist of sediment. Through wind, water, erosion and the forces of nature, we have had the formation of sediment resulting in

familiar areas such as: the Basin - Range Province, Rocky mountains, Sierra Nevada, the Great Basin, the Grand Canyon and parts of Arizona.

Metamorphic Rocks: These rocks are made by the alteration of their original form by heat and pressure.Heat and pressure are generated by

the movements of the earth, by the shifting of plates. This shifting is known as plate tectonics. When the plates move the force creates the rock

to become plastic resulting in metamorphic rock, by changing the original form. Some of these rocks are the oldest on the continent and are

found in the San Gabriel mountains in California. Some types of metamorphic rock are; Slate, Quartzite, Schist, Gneiss, Marble and

Serpentinite.

A geologic time scale is what is used by geologists which determines dates to show a geologic time scale. This time scale is an estimation of

absolute age which serves as a reliable and universal scale. The eras represented are Cenozoic, Mesozoic, Paleozoic and Precambrian these

terms in order of previous are, "recent or new animals," "middle animals," "ancient animals," and "first animals."

Plate Tectonics: This is what made up the diversity of California. Plate tectonics is the sliding of plates about the earth's surface. When two

plates slide they may converge with each other with a certain degree of force creating them to converge on each other or diverge on each

other thus creating the diverse surface of California. The activity of these plates have been changing the surface of the earth for millions of

years creating our basins and coast ranges along with others geographical features.

Earthquakes and Faults: California is known as a rifted borderland. The reason for this are the types of faults located in California. One in

particular is the San Andreas faults, this is only one among many, this fault is known as a right-lateral strike-slip. This means that the land is

moving predominently sideways. The effects of earthquakes and the types of faults and severity of the earthquakes changes the landscape

over time.There are two ways to measure the size of an earthquake, one is the Richter scale and the other is a seismograph. The Richter scale

is a mathematic measure of magnitude based on a logarithmic scale. A seismograph measures the vibrations in the earth.

Based upon most of these factors mentioned here in our summary, this begins to explain the geologic and geographic diversity of California

and much of the earth in general. This chapter really discusses the basis of our regions and how they have been formed. Understanding the

fundamentals of how our earth was formed and the changes within gives us a better understanding of the nature of our geological and

geographical environment.

Geology is the study of Rocks

Igneous rock:

When magma cools and solidifies if forms igneous rock.

Basalt is the most common igneous rock. Intrusive or

Plutonic rocks are cooled slowly and deeply in the

earth. Three types of fine-grained igneous rocks are

Rhyolite, Andesite, and Basalt. Three types of course

igneous rocks are Granite, Diorite, and Gabbro.

Granite is most common is California. Obsidian is

glassy and smooth, it is black because of the

non-oxidized iron and the mahogany obsidian has it

iron oxidized so it is brown. Pumice is also another

example of igneous rock its lightweight and if the

wind carries small pebbles of it into a lake it floats

because of the oxygen trapped inside.

Sedimentary rocks:

Sedimentary rocks are clastic, which is made up of

particles from other rocks. A conglomerate was

deposited in an area like a river bottom or a beach.

Breccia is a conglomerate with sharp or angular rocks

in it. Sandstone is deposited close to the shore;

shale was deposited in deep water. Sedimentary

rocks make up 5% of the earth's crust and 75% of the

rocks on the surface.

Metamorphic rocks:

Metamorphic rocks change from their beginning

structure by heat and pressure, but they generally

tend to keep part of their original characteristics.

Some examples of metamorphic rocks were from Slate

from shale, quartzite from quartz sandstone, schist

from slate, gneiss from granite, marble from

limestone, serpentine from basalt gabbro.

Geologic Time Scale:

The geologic time scale was generally decided by a

group of geologists in 1983. Certain fossils are

known for certain time periods; the deeper they are

the older they are. There is a argument between

geologists about the determination of absolute ages of

events for example the beginning of the Miocene, which

is thought to have started 24 million years ago.

Plate tectonics:

Plates slide around on the surface of the earth.

Earthquakes occur with the movement of the plates.

Faults are deep cracks in the crust of the earth.

Plates generally move frontal and when they change to

a sideway motion then it is called a transform system.

Earthquakes and Faults:

The San Andreas Fault is a right lateral strike slip

fault and an example of a left &endash;lateral strike slip

fault is Garlock. AN interesting fact to know is that

the town Hollister in the bay area is on the Calveras

fault; the west part of the town gets moved a little

more north each year. Earthquakes do not always have

to happen along a fault that has been active. Scarp

is an elevated face along in a fault, most scarps

stays visible but some submerges. Earthquakes are

measured as "1 I only observed instrumentally" "2 II

Is barely felt near epicenter" "4.5 VII Slight damage

at epicenter, felt 20 miles (36 km) away" "6+ VIII

moderately destructive" "7+ IX Considerable damage- a

major earthquake" "8+ X+ All buildings suffer some

damage, panic is genera- a great earthquake" (pg 65)

the number is magnitude and the roman numeral is

Intensity and the comment is the effect of the

earthquake. Mercalli scale measures intensity and

the Richter scale measures the magnitude of an

earthquake.

Geology is the study of rocks, which is the nonliving components of the earth. There are 102 inorganic substances known as elements.

Living organisms consist primarily of four elements yet 99 percent of all rocks are composed of some combination of eight elements.

(SI)-silicone, (O)-oxygen, (Al)-aluminum, (Fe)-iron, (Ca)-calcium, (Na)-sodium, (K)-potassium, (Mg)-magnesium.

A mineral is composed of a single element of compound. By definition, a mineral is a naturally occurring inorganic substance with a

definite chemical composition and ordered atomic structure. (ex. NaCl. It occurs in crystal shapes like small cubes. Gold and silver are

composed of single elements.)

Granite is composed of several minerals, which gives it it's speckled appearance. Limestone is a rock composed of a single mineral.

On the basis of their origin on earth, rocks may be divided into three primary categories: igneous, sedimentary and metamorphic.

(1) Igneous Rocks

-formed from cooling and solidification of molten rock.

-refers to fire (from the root ignite)

-the earths high core temp. caused convection of heat energy which melts rock to produce magma

-upon cooling, magma becomes igneous rock

-length of time it takes the rock to cool determines the size of the crystals

-coarse-grained rocks were cooled deep in the earths crust and will have minerals visible to the eye

-Granite is the most common of these coarse -grained rocks.

-When magma comes to the surface and cools rapidly, as in a volcano, it will be fined grained

because there had been too little time for large crystals to form.

-Basalt is the most fined grained igneous rock (a heavy , black volcanic rock)

-Intrusive or plutonic rocks have been cooled slowly, deep beneath the earth.

-Extrusive or volcanic rocks have been formed by molten material that flowed upon the surface

of the earth

-Igneous rocks have different colors and composition depending on the minerals in them.

-Granite and diorite make up the Sierra Nevada (batholith-formed deep inside the earth)

Common Minerals found in Igneous Rocks:

Mineral Chemical Composition Appearance

Quartz Silicon Dioxide Glassy, clear, cloudy,

white, gray, pink

Feldspar Calcium or Sodium Blocky dark gray to white

Plagioclase aluminum silicate White

Orthoclase Potassium aluminum Blocky, pink silicate

Mica Complex iron silicates Thin, shiny black sheets

Biotite

Muscovite Complex potassium Thin, shiny clear sheets silicates

Ferromagnesian minerals complex iron, magnesium
Short, stubby crystals,

Pyroxenes silicates green to black

Amphiboles Complex iron, mag. silicates Grains or long crystals,

light green to black

Olivine Complex iron, mag. silicates Glassy to grainy, light

green

There are two types of igneous rocks: fine grained (volcanic) and coarse grained (plutonic). The volcanic rocks are Rhyolite (light),

Andesite (medium) and basalt (dark). The plutonic are granite (light), diorite (medium) and Gabbro (dark).

Of the volcanic rocks, basalt is the highest in ferromagnesian minerals. It's dark black and located in the eastern side of the Sierra's. It is

also the main component of the Hawaiian Islands.

Andesite, which is lighter than basalt, is primarily found on the borders of continents. The name comes from Andes.

Rhyolite has almost the same composition as granite but it has cooled quicker. It is found in the Mojave Desert. It is sticky when it

flows which causes stones to embed into it and also traps bubbles of gas.

Obsidian has the smoothest surface of extrusive rocks. It's non-crystalline. Lacks ordered atomic structure. An amorphous mixture of the

same elements fund in granite but due to rapid cooling, atoms did not have time to become arranged in an ordered structure. Black

obsidian cooled in the absence of oxygen. It's color is due to non-oxidized iron.

Pumice is a rock that is so light that the wind can pick it up. It is a light weight volcanic "froth."

Sedimentary Rock

5%of the earth's crust and 75% of the rocks found on the surface.

Formed by the consolidation of particles transported from another source. This formation is significant in the rock-cycle.

Rock-Cycle

When mechanical and chemical weathering causes rock material to break up into smaller pieces.

Erosion transports the particles and they begin to form sediment.

Over time the rested particles become cemented or consolidated into rock.

Sedimentary rock is clastic or formed of clasts and particles. They are named according to the size of the particles that makes up 50% of

the rock.

Fine particles = shale

Silt = siltstone

Sand = sandstone

Grave size = conglomerate>>>>>and rocks embedded in the conglomerate that are sharp and angular = breccia pg. 51

Mountains composed of sedimentary rock have a distinct band appearance. The bands can be of different colors. This is from the many

layers of sediment that laid down over millions of years.

Roof Pendants: the remnants of overlying sediment found along the crests of mountain ranges. Still present today on the Sierra Nevada.

The size of the particles in the sediment is dependent on the amt. of energy in the water flow at the time of deposition.

Fast water = conglomerate bottom such as river bottom or beach.

Slow moving water = sand bottom

Still water = mud bottom

Four common sedimentary rocks have a chemical origin

1.limestone

2.gypsum Solid form known as alabaster Plaster of Paris component

3.halite Common table salt

4.chert Quartz that has many color forms

Salt flats are produced by evaporation and the minerals are known as evaporates. These minerals are of economic value
ie: trona, potash, borate

Biological origin

1.limestone majority produced this way

2.apatite: a mineral formed by the accumulation of calcium phosphate. It is used as a source for fertilizers.

3.coal

Metamorphic rocks

Rocks that have been altered from their original form by heat and pressure. Many of the oldest rocks on the continent are metamorphic

Metamorphic Ex: slate, quartzite, schist, gneiss, marble, serpentinite

Unconformity: absence of rock layers. It represents a period of erosion in which the materials were carried away.

Law of Superposition: a layering process of rock that states that unless the land is deformed by mountain building processes, the

youngest rocks are found on top. Pg. 56

Geologic Time Scale: A geologic time scale combines absolute age determinations, in years, with relative age, based on sequences of rock

strata. The decay rate of radioactive minerals is the most reliable estimation of absolute age, this combined with known rates of

sedimentation are used. Fossil organisms are used for relative age estimation in that certain fossils are associated with certain time periods.

Some time subdivisions are named for life-forms, such as, Proterozoic (first animals), Paleozoic (ancient animals), Mesozoic (middle

animals), and Cenozoic (recent or new animals). Others were named for localities where certain rocks are found, such as, Cambria is the

Roman name for Wales, Mississippian refers to the Mississippi Valley.

In California:

Rocks from Found
Proteozoic
San Gabriel Mountains

and desert ranges

Paleozoic Mojave and Basin-Range complex

Mesozoic Sierra Nevada

Transverse Ranges

Peninsular Ranges

Cenozoic Cascades

Modoc Plateau

Plate Tectonics:

The most important concept in understanding how California became so diverse. California is on the western margin of the North

American plate. First California was underwater as part of a diverging margin. Then 400 million years ago that motion reversed and Ca

became a converging margin. Then a subduction occurred when the Pacific plate slid under the North America plate creating a series of

offshore volcanic islands. California as we know it today occurred 25 million years ago. The relative motion of the two plates changed

form head-on convergence to a sideways motion know as a transform system. Faults formed and motion began to take place along the

faults. The two kinds of motion are called dip-slip, and up and down motion, and strike-slip, a sideways motion. Everything west of the

San Andreas fault was as much as 300 miles south 25 million years ago.

Geology is the study of the rocks of the earth. 97% of all rocks are composed of a combination of eight

elements: silicon (Si), oxygen (O), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K) and

magnesium (Mg).

A mineral is composed of a single element or compound. A mineral is a naturally occurring inorganic

substance with a definite chemical composition and ordered atomic structure.

Rocks are usually composed of several minerals.

Rocks may be divided into three primary categories: igneous, sedimentary, and metamorphic.

Igneous rocks are formed from cooling and solidification of molten rock.

Granite and diorite make up the Sierra Nevada. A batholith is a large intrusive block that originally was

beneath the surface.

Gabbro is a dark colored plutonic (intrusive) rock.

Basalt is highest in ferromagnesian minerals. It is a dark rock. (Devil's Postpile).

Andesite is the primary volcanic rock found on the borders of continents.

Rhyolite has many stones imbedded in it and holes from bubbles of gas.

Obsidian is smooth similar to glass. It can be black or brown.

Pumice is a lightweight volcanic "froth".

Sedimentary Rocks are formed by the rock cycle. Broken rock materials are transported by forces of

erosion when they come to rest they in time become cemented or consolidated into rock. If the sediment

is composed of fine particles the rock is called shale. If the sediment is composed of gravel-sized particles

it is known as a conglomerate. If the rocks imbedded in the conglomerate are sharp or angular the rock is

known as breccia.

Some sedimentary rocks have a chemical or biological origin. Some are from evaporation such as salt flats.

Others of biological origin are from organisms that extract calcium carbonate from water and use it to

form shells. Coal is also a sedimentary rock of biological origin. It is composed of carbon material produced

by vegetation.

Metamorphic Rocks have been altered from their original form by heat and pressure.

Types of metamorphic rocks: slate, quartzite, schist, gneiss, marble, and serpentinite.

Law of superposition states unless the land is deformed by mountin building processes, the youngest

rocks are on top.

Geologic Time Scale is used to combine age determinations in years, with relative age based on sequences

of rock strata. The eras are divided in Cenozoic, Mesozoic, and Paleozoic.

Plate Tectonics refers to the plates sliding on the surface of the earth. The plates can slide under another

plates, slide past each other, converge, or diverge from each other.

California originated about 25 million years ago, about the beginning of the Miocene. The relative motion

of the plates changed to a sideways motion known as a transform system.

The San Andreas is one of many faults in California. The San Andreas Transform System is the name used

for the total area. The fault moves at different rates. Some areas of the fault have many small

earthquakes during the year.

An area that has small earthquakes moves the earth slowly and is called creep zones.

Areas that have don't move often are called locked zones and have large earthquakes.

Richter scale is mathematic measure of magnitude based on a logarithmic scale that is used to measure

the size of a earthquake.

A seismograph measures vibrations in the earth. The use of many seismographs in different areas are

coordinated to locate the epicenter of a quake.

California is made up of rocks of different ages and origins. It is one of the most diverse areas in the world.

California has more endemic species of plants and animals than any region of equivalent size in the U.S.

oxygen, aluminum, iron, calcium, sodium, potassium and magnesium. A mineral is a

naturally occurring inorganic substance with a definite chemical composition either of a

single element or compound and an ordered atomic structure. Rocks are usually composed

of several minerals. Rocks are divided into 3 categories: igneous, sedimentary, and

metamorphic.

Igneous Rocks

The Earth's fiery core creates a convection of heat energy, which melts rocks into magma.

The cooling and solidification of molten rock creates igneous rocks. They vary in texture,

shape, size, color and composition. The length of time it takes to cool determines the size of

the rock's crystals (grain). These are typically called intrusive or plutonic. Those that slowly

cool are coarse-grained (granite). If is comes to the surface and cools rapidly is termed

extrusive or volcanic and is fine-grained and often no crystals may form (basalt). Color and

composition vary. Minerals are responsible for the color of the rock and the six main types

are shown in

In California there are several different types of rocks. A large intrusive rock block is called a

batholith. Granite and diorite are common of the Sierra Nevada. Gabbro is found in San

Diego. Of the extrusive rocks, basalt is most common. Rhyolite, chemically similar to granite

is found in the Mojave Desert. Obsidian, resembling glass, has the smoothest surface of the

extrusive rocks. It lacks minerals and atomic structure. Black obsidian is cooled in the

absence of oxygen and is black due to nonoxidized iron. There are large deposits of obsidian

around the Sierra

Sedimentary Rocks

Sedimentary Rocks are an important part of the rock cycle. Mechanical and chemical

weathering breaks down rock that is then transported to various locations by wind, water,

and glaciers. They then become cemented or consolidated into rock...sedimentary rock.

Most rocks are clastic (composed of particles). Sediment is classified by particle size (see

Table 3-3). If composed of small particles the rock is called shale, siltstone, and sandstone. If

composed of gravel-sized particles it is called a conglomerate. Breccia is a conglomerate with

sharp imbedded rocks.

Mountains composed of sedimentary rock have a banded appearance. The size of the

particles in the sediment depends on the amount of energy in the water at the time of

sedimentation. Larger particles are deposited as the flow is reduced while smaller particles

remain in suspension. Sandy bottoms indicate slow-moving water (shorelines) while mud

bottoms indicate still and/or deep water (ponds). Trilobite fossils as old as 600 million years

have been found in shale. They have been extinct for nearly 300 million years.

Summaries Chapter 3 (Naida pg.# 53-57)

SEDIMENTARY ROCKS

Sedimentary rocks make up 5% of the earths crust and 75% of the rocks of the surface. They

are formed by the consolidation of particles transported from another source. If sediment is

composed of fine particles it is called shale, at least 50% made of. Silt makes siltstone and

sand makes sandstone. Gravel sized particles are called a conglomerate and if it is sharp or

angular it is called a breccia.

Four common sedimentary rocks have a chemical origin; limestone (calcium carbonate),

gypsum (calcium sulphate), halite (codium chloride), and chert (silicon dioxide). Trona is a

town and a mineral mined from Searles Dry Lake. Potash is a potassium mineral mined from

Searles Dry Lake. This potassium is the primary source for fertilizer for farmers. Borates are

also found in Searles Lake. Borate is used in detergents and jet fuel. All these minerals are

caused by evaporation. Boron can be found in the Mojave Desert, Death Valley, and Boron.

Plaster of Paris is gypsum.

METAMORPHIC ROCKS

Metamorphic rocks have been altered from their original state by heat and pressure. Heat

and pressure occur from large plates moving around on the surface of the earth. San Gabriel

Mountains of California have metamorphic rocks over a billion years old. Metamorphic

rocks can be found in the Orocopia Mtns, the Newberry-Ord Mtns, the Old Woman Mtns, and

the Marble Mtns. There is and ancient shist found at the bottom of the Grand Canyon dating

at 1.1 billion years. Superposition means that the youngest of the rock is on the top.

Geological Time Scale

A geological time scale combines absolute age determinations, in years, with relative age

based on sequence of rock strata. Combining known rates of sedimentation and erosion with

decay rates of radioactive materials, the latter being more reliable, derives estimations of

absolute age.

PLATE TECHTONICS (Tammy Pg#58-62)

No other concept in geology is as important to understanding how California came to be so

diverse than that of plate techtonics, plates sliding about on the surface of the earth.

California is on the western margin of the North American plate. At first, California was

underwater as part of a diverging margin similar to that which is present today on the

Atlantic side of North America.

California as we know it today originated about 25 million years ago, about the beginning of

the Miocene. The relative motion of the two plates changed from head-on convergence to a

sideways motion known as a transform system. Deep cracks or faults formed in the earth's

crust. Motion began to take place along these faults as some blocks sank and others rose.

Some blocks were carried sideways. The western part of the United States began to be

stretched in a manner known as extension.

Earthquakes occur when these blocks move. If most of the motion is up or down along a

fault, it is called dip-slip. If most of the motion is sideways, it is called strike-slip.

The modern Sierra Nevada, Transverse Ranges, and Peninsular Ranges were uplifted along

dip-slip faults and are known as fault-block mountains.

Prior to 25 million years ago, everything west of the San Andreas fault was a good deal farther

south, some authorities contend- as much as 300 miles. An Andean type of continental

margin, associated with shallow, rapid subduction, characterized these periods.

Holocene - This epoch was characterized by continued motion along fault lines associated

with earthquakes and volcanism.

Pleistocene - This, the "Ice Age," was characterized by repeated glacial episodes in the

mountains and heavy rains elsewhere. Formation of the Cascade volcanoes occurred.

Coastal terraces formed by uplift and changing sea level.

Pliocene - Sedimentation occurred in the Great Central Valley, and marine sandstones

continued to be uplifted. A Californian type of continental margin characterized this epoch

as the San Andreas Fault began to move.

Miocene - Northern Coast Ranges began to be uplifted. A period of heavy erosion

transformed the ancestral Sierra Nevada to a gentle rising plain. The climate was warm and

moist.

Oligocene - These epochs were characterized by long periods of erosion.

Eocene - Freshwater sediments of the Peninsular Ranges were deposited at this time.

Paleocene - Seawater covered most of the present coastal plain.

The Salton Trough and the Gulf of California did not exist prior to 25 million years ago

because the entire Peninsular Ranges sequence including Baja California was actually the

west coast of what is now mainland Mexico. However, some authorities place the formation

of the Gulf of California as recently as 5 to 10 million years ago.

EARTHQUAKES AND FAULTS (Miriam pg.#63-68)

The San Andreas is but one of many faults in California. Motion on the San Andreas Fault is

right-lateral strike-slip. That means that the land is moving primarily sideways, and if a

person stood on either side of the fault during an earthquake and watched the other side, the

land would move to the right.

Parts of the San Andreas move at different rates, expressed by different degrees of

displacement at various points along the fault.

Areas where there are many quakes may move 2 inches a year, but there is seldom a serious

earthquake. Regions such as these are known as active areas or creep zones.

In contrast to creep zones are locked zones. Hard rock materials such as granite bound these

areas, so they do not move often. When they do move, a large earthquake is the result.

Apparently, tension or strain in a creep zone is released a little at a time, but in a locked zone

it builds up for many years. When the earth finally does move, a tremendous amount of

energy is released all at once, causing severe damage to buildings and reshaping the

surrounding terrain. In California, the most famous such earthquake took place in 1906 in

the locked zone north of San Francisco.

Go to Other Chapters!

Marilyn Cannon, Sept. 18, 2002