time introduction to earth science

Lecture 10 Stratigraphy and Geologic Time  Stratigraphy  Basic principles of relativ e age dating  Unconformities: Markers of missing time  Correlation of rock units  A bsolute

Lecture 10 Stratigraphy and Geologic

Time

 Stratigraphy

 Basic principles of relativ e age dating

 Unconformities: Markers of missing time

 Correlation of rock units

 A bsolute dating

 Geologic Time

How old is the Earth? W hen did v arious geologic ev ents

occur? Interpreting Earth history is a prime goal of geology Some know ledge of Earth history and geologic time is also required for engineers in order to understand relationships betw een geologic units and their impact on engineering

construction

 Stratigraphy :

 Stratigraphy is the study of rock lay ers (strata)

and their relationship w ith each other

 Stratigraphy prov ides simple principles used to

interpret geologic ev ents

Two rock units at a cliff in Missouri (US Geological Survey)

 Basic principles of relativ e age dating

Relativ e dating means that rocks are placed in

their proper sequence of formation A formation is a basic unit of rocks Below are some basic principles for establishing relativ e age betw een formations

 Principle of original horiz ontality

 Principle of superposition

 Principle of faunal succession

 Principle of cross-cutting relationships

 Principle of original horiz ontality :

Lay ers of sediment are generally

deposited in a horiz ontal position

Thus if w e observ ed rock lay ers that are folded or inclined, they must, w ith

exceptions, hav e been mov ed into that

position by crustal disturbances sometime after their deposition.

 M ost lay ers of sedim ent are dep osited in a nearly horiz ontal p osition Thus, w hen w e see incline d rock lay ers as show n, w e can assum e that they must hav e b een mov ed into that position after dep osition

Hartland Quay , Dev on, England by Tom Bean/DRK Photo.

 Principle of superposition:

In an undeformed sequence of

sedimentary rocks, each bed is older

than the one abov e and y ounger than the one below

The rule also applies to other

surface-deposited materials such as lav a flow s and v olcanic ashes

Principle of superposition (W.W Norton)

 A pply ing the law of superposition to the lay ers at the upper

portion of the Grand Cany on, the Supai Group is the oldest and the K aibab Limestone is the y oungest (photo by Tarbuck).

 Principle of cross-cutting relationships:

W hen a fault cuts through rocks, or w hen magma intrudes and cry stalliz es, w e can assume that the fault or intrusion is

y ounger than the rocks affected.

 Cross-cutting relationships: A n intrusiv e rock body is

y ounger than the rocks it intrudes A fault is y ounger than the rock lay ers it cuts (Tarbuck and Lutgens)

 Unconformities: Markers of missing time

W hen lay ers of rock form ed w ithout interruption, w e cal l them conform able

A n unconform ity represents a long period during w hich depos ition ceas ed and eros ion rem ov ed prev iousl y form ed rocks before deposition res um ed

 A ngular unconform ities

 Di s conform ity

 Nonconform ity

 A ngular unconformities:

A n angular unconformity consists of tilted

or folded sedimentary rocks that are

ov erlain by y ounger, more flat-ly ing

strata.

It indicates a long period of rock

deformation and erosion

Formation of an angular unconformity An angular unconformity

represents an extended period during which deformation and erosion occurred (Tarbuck and Lutgents)

Angular unconformity at Siccar

Point, southern Scotland, that

was first described by James

Hutton more than 200 years ago

(Hamblin and Christiansen and

W.W Norton)

 Disconformity :

A disconformity is a minor irregular

surface separating parallel strata on

opposite sides of the surface

It indicates a history of uplifting abov e sea (w ater) lev el, undergoing erosion, and low ering below the sea lev el again

Formation of disconformity (W.W Norton)

 Disconformities do not show angular disc ordance, but an erosi on

surface separate s the tw o rock bodies The channel in the central part of this outc rop rev eals that the low er shale units w ere deposi ted and then eroded bef ore the upper units w ere deposited (Hambli n and

Christiansen)

 A nonconformity is a break surface that dev eloped w hen igneous or metamorphic rocks

w ere exposed to erosion, and

y ounger sedimentary rocks w ere subsequently deposited abov e the erosion surface (Tarbuck and Lutgens)

 A nonconformit y at the Grand Cany on The

metam orphic rocks and the igneous dikes of the inner

go rg e w ere formed at great depths and subsequently

uplifted and eroded Y ounger sedimentary lay ers w ere then deposited on the eroded surface of the igneous and

Types of Unconformity

 This animation show s the stages in the

dev elopment of three main ty pes of unconformity

in cross-section, and explains how an incomplete succession of strata prov ides a record of Earth

history V iew 1 show s a disconformity , V iew 2 show s a nonconformity and V iew 3 show s an

angular unconformity [by Stephen Marshak]

 Play A nimat ion W indow s v ersion >>

 Play A nimat ion Macintosh v ersion >>

 Distinguishing nonconformity and intrusiv e contact

 Nonconformity :

The s edim entary rock i s y ounger The eros ion surface is

generally smooth Di kes m ay cut through the igneous body but s top at the nonconform ity

 Intrusiv e contact:

Intrus ion is y ounger than the s urrounding sedi mentary

rocks The contact s urface m ay be quite irregular A z one of contact m etam orphism m ay form s urrounding the igneous body Cros s-cutting dikes m ay penetrate both the igneous body and the sedim entary rocks

 Contrasting field cond itions for (a) a nonconformity

and (b) an igneous intrusion (W est, Fig 9.4)

 The three basic ty pes of unconformities illustrated by this cross-section of the Grand Cany on (Tarbuck and

Geologic History

 A cross-section through the earth rev eals the

v ariety of geologic features V iew 1 of this

animation identifies a v ariety of geologic features;

V iew 2 animates the sequence of ev ents that

produced these features, and demonstrates how

geologists apply established principles to d educe geologic history [by Stephen Marshak]

 Play A nimation W indow s v ersion >>

 Play A nimation Macintosh v ersion >>

 Principle of faunal succession:

Groups of fossil animals and plants occur the geologic history in a definite and determinable order and a period of geologic time can be

recogniz ed by its characteristic fossils.

 Fossils are the remains

(Bottom) Dinosaur footprint in fine-grained limestone near Tuba,

A z

The principle of fossil succession Note that each species has only a limited range in a

succession of strata (W.W Norton)

 C orrelation of rock units

The method of relating rock units from one

locality to another is called correlation

 O ne w ay of correlation is to recogniz e the rock

ty pe or rock sequence at tw o locations

 A nother w ay of correlation is to use fossils A

basic understanding of fossils is that fossil

organisms succeeded one another in a definite and determinable order, and therefore a time period can be recogniz ed by its fossil content

The principle of correlation of rock units The rock columns can

be correlated by matching rock types (W.W Norton)

 W ill iam Sm ith, a ci v il engineer and surv ey or, could piece

together the sequence of l ay ers of di fferent ages containing different fos sils by correlati ng outcrops found in southern England about 200 y ears ago In this exam ple, Form ation II

w as exposed at both outcrops A and B, thus Form ation I

and II w ere y ounger than Form ation III (Press and Siev er).

 Corre lation of strata at three locations on the Colorado Plateau rev eals the total extent of sedimentary rocks in the region.

The geologic column was constructed by determining the relative ages of rock units from around the world (Next) By correlation, these columns were stacked one on top of the other to give relative ages of rock units (W.W Norton)

 A bsolute dating

 The geologic time based on stratigraphy and

fossils is a relativ e one: w e can only say w hether one formation is older than the other one.

 A bsolute dating w as made possible only after the

discov ery of radioactiv ity

 Radioactiv ity

 A t the turn of the 20th century , nuclear

phy sicists discov ered that atoms of uranium, radium, and sev eral other elements are

unstable The nuclei of these atoms

spontaneously break apart into other

elements and emit radiation in the process

know n as radioactiv ity

 W e call the original atom the parent and its decay product the daughter For example, a radioactiv e 92U238 atom decay s into a stable nonradioactiv e 82Pb206 atom

 example ty pes of radioactiv e decay

 A lpha decay : an α particle (composed of

2 protons and 2 neutrons) is emitted from

a nucleus The atomic number of the

nucleus decreases by 2 and the mass

number decreases by 4.

 Beta decay : a β particle (electron) is

emitted from a nucleus The atomic

number of the nucleus increases by 1 but the mass number is unchanged.

Illustration of alp ha and b eta decay s ( adap ted from Tarb uck and Lutgens)

 The d ecay of U 2 38 A fter a series of rad ioactiv e d ecay s, the stabl e end prod uct Pb 2 06 is reached (Tarbuck and Lutgents)

 Decay constant

 The rate of decay of an unstable parent nuclide

is proportional to the number of atoms (N)

remaining at the time t

dN/dt=-λ*N

 The reason that radioactiv e decay offers a

reliable means of keeping time is that the decay constant λ of a particular element does not v ary

w ith temperature, pressure, or chemistry of a geologic env ironment.

 Half-life

 The half-life of an radioactiv e element is the

time required for one-half of the original number

of radioactiv e atoms to decay :

T1/2 =0 693/λ

 The half-liv es of geologically useful radioactiv e

elements range from thousands to billions of

y ears The age of the Earth (4.6 billion y ears)

w as first obtained using U/Th/Pb radiometric

dating The half-life of U 238 is 4.5 billion y ears

 The radioactiv e decay is exp onential Half of the radioactiv e parent remains after one half-life, and one-quarter of the parent remains after the second half-life (Tarbuck and Lutgens)

The concept of a half-life The ratio of parent-to-daughter changes with the passage of each successive half-life (W.W Norton)

 Geologic Time

The geol ogic tim e scale s ubdi v ides the 4.6-bi llion-y ear his tory of the Earth into many different uni ts, w hich are linked w ith the ev ents of the geologi c pas t

 The ti m e scale is div ided into eons : Precam brian and

Phaneroz oic and eras : Precam brian, Paleoz oic ("ancient life"), Mesoz oic ("m iddle life"), and Cenoz oic ("recent life") T he eras are bounded by profound w orl dw ide

changes i n li fe-forms

 The eras are di v ided into periods

 The peri ods are di v ided into epochs

The standard geologic

time scale was

developed using relative dating techniques Radiometric dating later provided absolute times for the standard geologic periods (W.W Norton)

 The aw esome span of geologic time

The geologic time represents ev ents of

aw esome spans of time If the

4.6-billion-y ear Earth histor4.6-billion-y is represented b4.6-billion-y a hour day w ith the beginning at 12

24-midnight, the first indication of life

w ould occur at 8:35am Dinosaurs w ould appear at 10:48pm and become extinct at 11:40pm The recorded history of

mankind w ould represent only 0.2 sec

before midnight.

 The K T extinction

 A t the b oundary b etw een Cretaceous (the last p eriod

of Mesoz oic) and Tertiary (the first p eriod Of

Cenoz oic) ab out 66 million y ears ago, know n as K T

b oundary , more than half of all plant and animal

s pecies died in a mas s extinction The b oundary

marks the end of the era in w hich dinosaurs and

other rep tiles dom inated and the b eginning of the

era w hen mam mals b ecame im portant

 The w idely held v iew of the extinction is the imp act

hy p othes is A large ob ject collided w ith the Earth,

p roducing a dus t cloud that b locked the sunlight

from m uch of the Earth’s surface W ithout sunlight for p hotos y nthesis , the food chains collap sed, w hich affected large animals most s ev erely