l1 l2 part 1 basic geology

7/31/2019 L1 L2 Part 1 Basic Geology

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CALIFORNIAGEOLOGY

Earth109

ProfessorBusby

INTRODUCTIONAND

CHAPTER1

SimplifiedgeologicmapofCalifornia,bythe

CaliforniaGeological

Survey-

Seeinsidebookcoverof

yourtextbook:California Geology,

Deborah R. Harden,

2nd Edition

REQUIRED

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I have been researching and teaching about California geology ever since I was

an undergraduate at Berkeley, a PhD student at Princeton, and for 28 years as

a professor here at UCSB.

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READ YOUR SYLLABUS CAREFULLY AND REFER TO IT OFTEN!

HUGE range in the educational backgrounds of class members.

To get everyone on the same page, the class will spend the first 6 weeks of the quarter

studying the textbook.

The last 4 weeks will go beyond the book, using pdfs of my powerpoints, to be posted

on Gaucho Space.

Foreach class meeting, you will prepare a neatly drawn 3-page maximum cheat

sheetin pencil , and be prepared to go to the board and answer any given question.

You will bring these cheat sheets on the field trip and to both exams.

GRADING:

First exam (including quality of cheat sheets): 45% of grade - Feb 15 IN

CLASS.Second exam (including quality of cheat sheets): 45% of grade - March 13

IN CLASS. NO EXAM DURING FINALS WEEK.

Participation (preparation for class answers, as well as field exercise

participation): 10% of grade.

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REQUIRED Field Exercise: 7:00 am Friday March 2 to 9:00 pm Monday

March 5.

You will be barced $150 lab fee for the trip.

This is an elective class; do not take the class if you have conflict for those dates

(be it personal or class conflict). The only excuse for missing the trip will be a

doctors letter saying you were too ill to attend.

Each stop on the field trip will have questions for you to answer, with sketches. You

will turn in your field notes at the last stop of the field trip, and I will return them toyou with comments at the next class meeting so you can revise them for the final.

There is no limit to the length of your field exercise cheat sheet.

WEEK 1: Review of Basic Geology. Reading: Chapters 1-4

GET STARTED TODAY!

ANSWER WEEK 1 QUESTIONS BY WEDNESDAY.

ANSWER WEEK 2 QUESTIONS BEFORE NEXT MONDAYS CLASS.

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WEEK 1: Review of Basic Geology

Can you label the major plates?

Do you know what these three symbols mean?

SimilartoFigure1-1inyourbook5


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Queson1a-TheLayeredEarth:rightsideshowslayeringby

COMPOSITION,getsdenserinward.

Iron-rich (metallic) core (9 - 16 X the density of water)

Stony mantle (~ 6 - 3X the density of water) - more Fe and Mg, less Si thancrust.

Stony crust,less dense than mantle (~2.7 - 3.2X the density of water)

Similar to

Figure 1-2 of

your book

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Inner core is solid,Outer core is liquid with convection currents,responsible for Earths magnetic field.

Mesosphere and asthenosphere are plastic.

Lithosph,ere is brittle, and makes the tectonic plates.

Question 1b - left side shows layering by STRENGTH

Similar to Figure

1-2 of your book

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Tectonic (or

lithospheric) plate -

May include:

continental crust

(lightest, thicker),

oceanic crust

(denser, thinner),and the uppermost

part of the mantle

(densest).

These float on theplastic

asthenosphere by a

process called

isostacy (see next

slide).

Question 1c - Similar to Figure 1-3 of your book:

Oceanic crust is made of basalt, formed by sea-

floor spreading.

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Isostasy(Question 1c)

Less dense materials float

higher than more dense

materials, like an iceberg

floating in the ocean.

Continental crust floats

highest, with a thick

hidden root - thecontinent is the tip of

the iceberg.

Oceanic crust is denser and

floats lower than the

continents - that is what

makes the ocean basins,

which hold most of the

water on earth.

A thicker continent sticks up

higher.

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Question 2a - Divergent plate boundary on the sea floor.

Figure 1-5 of your

book:Sea-floor spreading =

injection of mantle-

derived melts into the

crack created as two

plates are pulled apart.

The process repeats

over and over. Sea floor

is created by this

process.The asthenosphere is virtually at thesurface at a mid-ocean ridge.

Transform faults accommodate right

or left steps along the mid-ocean

ridge.

How do we know this?

MAGNETIC

STRIPING OF THE

SEA FLOOR.10


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What is MAGNETIC STRIPING OF THE SEA FLOOR?

Earthsmagnecfieldactslikegiantbarmagnet,

withnorthendneartheNorthPoleandsouth

endneartheSouthPole(causedbydynamoin

liquidoutercore).

Magnecpolarityreverses

Everyseveralthousandtotensofmillionsof

years,theorientaOonofmagneOcfield

switchesfromnorth(normal)polarityto

south(reverse)polarity.

InclinaOonofmagneOclinescanalsobeusedto

inferwhatlatudetherockformedat

(determinepaleolaOtudinalposiOonsofthe

conOnents).

APrimeronEarthsMagnecField

Question 2a continued.11


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MagnezaonofVolcanicRocks:Whenlavacoolstobelow550oC(Curiepoint),atomsiniron-bearingminerals

lineupindirecOonofEarthsmagneOcfield,recordingbothpolarity(normal

vs.

reversed)andinclinaon(laOtudeatwhichthelavacooled).

Astackoflavaflowsmayrecordseveral

reversals,whichcanbedated

radiometrically.Bythismethoda

magneOcpolarityreversalOmescalecan

beconstructed.



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MagnezaonPaXernsontheSeafloors

ceanfloorisstripedbyparallelbandsofmagneOzedrockwith

alternaOngpolariOesStripesareparalleltomid-oceanridges,andpaXernofstripesis

symmetricalacrossmid-oceanridges(paXernononesideofridge

hasmirroroppositeonotherside)

VaryingwidthsofalternaOngpolaritystripesmatchlengthsofOme

betweensuccessivereversalsofEarthsmagneOcfield



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MagnezaonPaXernsontheSeafloors

Magmaisinjectedintotheoceanridgestocoolandformnew

rockimprintedwiththeEarthsmagneOcfieldSeaflooristhenpulledawayfromoceanridgeliketwolarge

conveyorbeltsgoinginoppositedirecOonsseafloor

spreadingCross sectional

view

Question 2a, divergent plate boundary, continued.

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Question 2a, divergent plate boundary on the sea floor, continued:

ceanfloordepthsincreasesystemaOcallywithseafloorage,moving

awayfrommid-oceanridges.WHY?

Theplategetsthicker,byslowcoolingofoceaniccrustand

uppermostmantle,andbecomesdenser,thereforesinkingaliXle

lowerintomantleasthenosphere.

SedimentslowlypilesupwithOme(thickeronolderseafloor)andits

weightonplatealsocauseittosinkaliXleintomantle

Well talk about divergent plate boundaries on continents later.15


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Question 2b - CONVERGENT PLATE BOUNDARY with oceanic

plate subducting under a continent.Figure 1-6 of your book:

UPPERPLATE (over-

riding plate)LOWER

PLATE (going

back down intothe mantle)

The Benioff zone is

defined by earthquakes

where the lower

(subducting) plate grinds

under the upper plate.

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Question 2b - CONVERGENT PLATE BOUNDARY - what

happens where light crust gets carried into the subduction zone?

Ocean-continentconvergence (top): oceanic

plate is subducted,

but when a piece of light

continental crust getsbrought in on the subducting

slab,

it cant subduct (toobouyant), and

continental upheaval results.

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Question 2b - CONVERGENT PLATE BOUNDARY

India moved 2,000 km north toward Asia

by subduction of Indian Ocean plates.

Pre-collision, Indian and Asian crusts were

35 km thick.

Crust under Tibetan plateau is now 70 km

thick, highest continental area on Earth.

India is still

shoving

under Asia.

Some muchsmaller parts

of California

were

accreted in a

similar way.19


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Question 2c - TRANSFORM PLATE BOUNDARY

Figure 1-7 of your book,

illustrating right-lateral slip

along the San Andreas fault and

the Hayward fault. Below: map

view.

This fault shows left-lateral slip.

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Strike-slip faults: horizontal movement

When straddling a fault, if

right-hand side moved

towards you, it is a right-lateral fault

When straddling a fault, if

the left-hand side has moved

towards you, it is a left-lateral fault

Convention works in either

direction

Question 2c - TRANSFORM PLATE BOUNDARY continued.

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Question 2 summary: Tectoniccycle(takesabout250millionyears): Meltedasthenosphereflowsupwardasmagma

Coolstoformnewoceanfloor(lithosphere)

Newoceaniclithosphere(slab)divergesfromzoneofformaOonatopasthenosphere(seafloorspreading)andcoolsasitisraedaway.

Whenslabofoceaniclithospherecollideswithanotheryounger,warmerslab,theolder,

colder,denserslabsubductsandisreabsorbedintothemantle

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Question 4 - Fixed hot spot rising up through the mantle under the Pacificplate, which is moving toward the NW - so Hawaiian Islands are older toward

north, and sink under their own weight.

Figure 1-7 of your book

Hot spot track records plate

motion directions for the

last 80 million years.

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Question 5 - FAULTS - not fully discussed in book but you need to

know this.

How is a fault different from a joint in a rock?

Its a fracture in the Earth in which two sides move relative to each

other.

Jointing: brittle rocks fracture and crack but a there is no offset across a joint.

Examples of bedding offset by faults.

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Strike = intersection of the fault plane with the earths surface.

Above: the strike of the Alpine

fault in New Zealand is about10 degrees east of north.

Below: the San Andreas fault

here strikes about 30 degrees

west of north (see north arrow);

offset of a creek is right lateral.

Question 5 continued - Three-dimensional view of a fault plane:

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Question 5 - Three-dimensional view of a fault plane, continued:

Dip = angle between the earths surface and the fault plane. Example

shown on block diagram is about 45 degrees.

Above: these small faults

dip steeply (about 80

degrees). Below: faultplanes can be curved. This

fault flattens with depth:

from a dip of about 45

degrees, to a dip of about 20

degrees.

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Question 5 continued - What is the difference between a

hypocenter and an epicenter?

Stress builds up until rupture starts at the weakest point along the fault

plane -- this is called the hypocenter or focus

The spot on the earth directly above the place of initial rupture is the

epicenter

Rupture then spreads out along

the fault surface.

The surface rupture will occur

directly above the hypocenter

ONLY if the fault plane is

vertical.

Sometimes faults dont rupture

all the way to the surface, and

then there is no ground

breakage.27


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Question 5 continued -

FAULTS THAT ARE NOT VERTICAL are referred to as dip slip

faults, because they are dominated by vertical movement.

Miners refer to the

block beneath them

as the

footwall (block

beneath the fault)

because they stand

on it, and the block

above them as the

hanging wall (blockabove the fault)

because they hang

their lantern on it

(the roof).

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Question 5, continued.

Figure 1-13 from your book,

illustrating compression,

extension, and transform

motion.

Its a little confusing

because it shows the faultplane as being vertical in all

three cases.

What is much more common

is that the fault plane is onlyvertical for transform

motion,and is NOT vertical

for compression and

extension (see next slide).

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Figure 1-15 from your

book:(a) reverse fault is a

dip slip fault that

forms by

compression,

(b) normal fault is a

dip, slip fault that

forms by extension,

and

(c) strike slip fault is a

vertical fault that

forms by transform

motionPrettier drawings on following

slides..

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Normal Fault : a dip slip fault

formed due to PULLING

APART = extensional.

Hanging wall moves downrelative to footwall.

Zone of omission - if you wanted to

drill down into the stippled bed to get

water or oil from it, you wouldnt find

it there.

Reverse Fault-a dip slip fault

formed due to PUSHING

TOGETHER = compressional.

Hanging wall moves up relative

to footwall.

Zone of omission - if you wanted to

drill down into the stippled bed to get

water or oil from it, you would hit it

twice!


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Figure 1-14 from your

book illustrates how

compression not only

produces reverse faultsbut also may cause

folding, especially in

layered rocks.

Example of folding of rock

layers: these sedimentary

layers were deposited flat and

then they got crumpled up

along the San Andreas fault.

Photo along Highway 14 just

south of Palmdale (San

Andreas fault zone).


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Figure 1-15 from yourbook illustrates how

extension not only

produces normal faults,

but can produce a graben

(left) or downsag (right).

A graben will be seen at

the surface as a basin or

valley.


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Left bend in a right-lateral

fault

(or right step in left-lateral

fault, not shown):

THE TWO SIDES PUSH

TOGETHER

Compression, uplift,hills and mountains

Steps or bends in Strike-Slip Faults: Earth does not rupturealong planes that are perfectly straight and continue forever. Instead, the

faults bend or make steps to the left or right.


TRANSPRESSION - see Figure 1-17

of your book. 34


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Right bend in a right-lateral fault

(or left bend in left-lateral

fault, not shown):

THE TWO SIDES PULL

APART

Extension, down-

dropping,

basins and valleys

Figure 3.14

Steps or bends in Strike-Slip Faults, continued:

TRANSTENSION - see Figure 1-17

of your book.


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Strike-Slip Faults, continued:

A TRANSFORM FAULT is a strike-slip fault that forms a plateboundary.

Example shown here connects two spreading centers on the ocean

floor.


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TRANSFORM FAULTScontinued:

Example shown here

(San Andreas fault)

connects spreading

centers in the Gulf ofCalifornia with the

subducting Juan de

Fuca plate.

It forms the plate

boundary between the

Pacific and North

American plates.


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l1 l2 part 1 basic geology

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