Microsoft PowerPoint

GEOLOGY 1--Physical Geology

Lecture #2, 2/9/2006

Topics:

•Lithospheric plates and their motions

•Types of plate boundaries or margins

•The present is the key to the past

•Relative Time

•Numerical Age

•Age of the Earth

Three mechanical layers of the crust and mantle:

•

Lithosphere

(crust and uppermost mantle) is strong,

brittle, rigid.

•

Asthenosphere

(mantle) is plastic and deformable –it

contains a small amount of molten rock or magma.

•

Mesosphere

is strong, but not brittle

Lithosphere:

Uppermost

mantle and

crust

Theory of Plate Tectonics

Plate tectonics regards the lithosphere

as broken into plates that are in motion.

The plates move relative to each other,

sliding on the underlying asthenosphere.

The plates are much like the segments of

the cracked shell on a boiled egg.

Three Kinds of plate boundaries

1. Divergent boundary – plates move away from each other

2. Convergent boundary – plates move toward one another, and

one plate sinks or is subducted beneath the other

3. Transform boundary – plates move past one another

The

lithosphere

is broken into plates called

lithospheric

or tectonic plates

• Plates contain both oceanic and continental lithosphere

• Plates move over the asthenosphere

• Boundaries of plates are sites of convergence, divergence, shearing

This shows the simplest possible convection pattern, with

rising beneath ridges and sinking in subduction zones.

Why do plates move?

The earth cools by

mantle convection

The motions of lithospheric plates are driven by

convection – the Earth’s interior (once molten) is

cooling

Chapter 8 Time and Geology

The present is the key to the past

James Hutton, father of geology, realized that

geologic features in the past could be explained

through present-day processes.

He realized that our mountains are not permanent

but have been carved into their present shapes and

will be worn down by the slow agents of erosion now

working on them. The great thickness of

sedimentary rocks on the continents are products

of sediments removed from land and deposited in

oceans. “We found no sign of beginning and no

prospect for an end”. He wrote in 1788. The time

required for these processes to take place had to

be incredibly long.

The present is the key to the past

Hutton’s concept of geological processes requiring

vast amount of time also influenced Charles Darwin

and led the development of theory of evolution

that revolutionized biology.

Charles Lyell,

Principles of Geology

, referred to

Hutton’s concept that geological processes

operating at present are the same processes that

operated in the past as the principle of

uniformitarianism.

Actualism: the same processes and natural laws

that operated in the past are those we can actually

observe or infer from observation as operating at

present. Physical laws are independent of time and

location. Actualism=~ uniformitarianism

Relative time

1. Principles used to determine relative age

1) Original Horizontality

2) Superposition

3) Lateral Continuity

4) Cross-cutting Relationship

5) Inclusion

2. Unconformities (contact that represent a GAP in

geological records)

1) Disconformity

2) Angular unconformity

3) Nonconformity

3. Correlation (time equivalency of rock units)

1) Physical continuity

2) Similarity of rock types

3) Correlation by fossils

The principle of

superposition

states that

within a sequence of undisturbed

sedimentary or volcanic rocks, the layers

get younger from bottom to top.

The principle of

original horizontality

states that beds of sediments deposited

in water formed as horizontal or nearly

horizontal layers.

Principles used to determine relative age

The principle of lateral continuity states that

an original sedimentary layer extends laterally

until it tapers or thins at its edges.

The principle of cross-cutting relationships

states that a disrupted pattern is older than

the cause of disruption. A layer cake (the

pattern) has to be baked (established)

before it can be sliced (the disruption)

The principle of inclusion states that

fragments included in a host rock are older

than the host rock

Principles used to determine relative age

Unconformities

(contact that represent a GAP in geological records)

1) Disconformity

2) Angular unconformity

3) Nonconformity

1) Disconformity

Contact representing missing rock strata

separates beds that are parallel to each

other.

Implication:

• The older rocks were eroded away parallel

to the bedding plane

• Renewed deposition later buried the

erosion surface.

Unconformities

(contact that represent a GAP in geological records)

1) Disconformity

2) Angular unconformity

3) Nonconformity

Younger strata overlie an erosion surface on

tilted of folded layered rock.

Implications:

• Deposition and lithification of sedimentary

rocks

• Uplift accompanied by folding or tilting of the

layers

•Erosion

• Subsidence

• Renewed deposition

2) Angular unconformity

A nonconformity is a contact in which an erosion

surface on plutonic or metamorphic rock has

been covered by younger sedimentary or

volcanic rock.

Implications:

• Crystallization of plutonic or metamorphic

rocks at depth

• Strong erosion of several >kms of overlying

rocks (the great amount of erosion further

implies considerable uplift of this portion of

the crust)

• Deposition of new sediment

3) Nonconformity

Unconformities

(a GAP in geological records)

Disconformity Angular unconformity Nonconformity

Erosion slight moderate strong

Lower strata sedimentary sedimentary igneous

Geometry parallel not parallel not parallel

Folding no yes yes

Correlation (determining time equivalency of rock units)

1. Physical continuity

2. Similarity of rock types

3. Correlation by fossils

Fossils are common in sedimentary rocks and their

presence is important for correlation.

Different sedimentary layers are characterized by

distinctive fossil species and that fossil species

succeed one another through the layers in a

predictable order. William Smith’s principle of

faunal succession allows rock layers in different

places to be correlated based on their fossils.

Index fossils: 1) very short-lived, 2) geographically

widespread, 3) known existed in a specific period

of time.

Correlation by fossils

Fossils are common in sedimentary rocks and

their presence is important for correlation.

Principle of Faunal Succession: different

sedimentary layers are characterized by

distinctive fossil species and that fossil

species succeed one another through the

layers in a predictable order. William Smith’s

principle of faunal succession allowed rock

layers in different places to be correlated

based on their fossils.

Index fossils: 1) very short-lived, 2)

geographically widespread, 3) known existed in

a specific period of time, e.g., Trilobite.

The Age of the Earth

OLD IDEAS:

1. In 1625, Archbishop James Usser: 4004 B.C. (Western

Countries), before the birth of Christ October 21,

9:00 in the morning

. His age determination was made

by counting back generations in the Bible.

2. Hindus regarded Earth as very old (2 billion years)

3. Earth scientists in early 1800s (Uniformitarianism): very

old, >hundreds of millions of years

Other Early Attempts

Sedimentation rates - 3 my – 500 my

Halley/Joly - Ocean Salinity – 100 my

Lord Kelvin (famous English Physicist) in

1866: 20-40 Myrs, calculated from the

rate of cooling.

The Age of the Earth

Isotopic Dating

Discovery of radioactivity in 1986 invalidated Lord

Kelvin’s claim because it provided a heat source that

had not known about. The decay of radioactive

elements generate heat and add to the heat already in

the earth.

The discovery of radioactivity also provided means to

determine how old Earth is. In 1905, the first crude

isotopic dates were indicate an age of about 2 billion

years.

The Age of the Earth

Isotopic Dating (continued)

In 1955, CalTech geochemist Clair Patterson

determined the age of the Earth at 4.55 byrs by

U-Pb isotope dating. And this age has sustained

enormous tests by other scientists using different

radioactive isotopes.

Radioactive Revolution around 1900

Radioactive decay - spontaneous

transformation of an element to

another isotope of the same

element or another element.

Alpha Decay – loss of a positively charged

Helium ion (two protons and two neutrons)

Beta Decay – neutron splits into proton

and electron

atoms

• Protons - positively charged

• Neutrons - no charge

• Electrons - negatively charged

Helium 3

Radioactive Decay (Beta)

Tritium

Helium 3

unstable stable

nuclear

decay

(daughter)

(parent)

Beta Decay – neutron changes into proton and electron

Theory of radioactive dating

N is the number of radioactive atoms

dN/dt=-λN

λ is the rate of decay in year

-1

N=N

-λt

The age of the rock is thus

t=(1/λ)ln(N

/N)

Half-life

The fixed period of time during

which half the parent atoms

present in a closed system decay to

form daughter atoms.

Half-life is related to the rate of decay

(decay constant) by

1/2

=(1/λ)ln(N

/N)= (1/λ)ln2=0.693/ λ

1/2

=0.693/ λ

Half-Life

1 2 3 54

6.25%

3.125%

Radiometric Dating Methods and their

half lives

Cosmogenic

• C-14: 5700 Yr.

• Be-10: 2.5 M.Y.

Primordial

• K-Ar (K-40): 1.25 B.Y.

• Rb-Sr (Rb-87): 48.8 by

• U-235: 704 M.Y.

• Th-232: 14 B.Y.

• U-238: 4.5 B.Y.

Primordial

• Nd-Sm (Sm-147-Nd-

143): 106 B.Y.

• Re-187 43 B.Y.

• Lu-Hf (Lu-176) 36 B.Y

Geologic

Time Scale

65 Ma

251 Ma

544 Ma

Era Period