Understanding Earth

Chapter 8:The Rock Record and the

Geologic Time Scale

Copyright © 2007 by W. H. Freeman & Company

John Grotzinger, Thomas H. Jordan Frank Press and Raymond Siever

Fifth Edition

Concepts you will need to know for the examsCHAPTER 8

Geologic Time ScaleRelative ageAbsolute age

Principle of original horizontalityPrinciple of superposition

Principle of cross-cutting relationahipsUnconformity

Angular unconformityDisconformity

Half-lifeGeologic Time Scale

Geologists differ from other professional scientists in their view of time.

Geologists deal with periods of time that go as far back as the origin of the universe.

Before we had the ability to read the “atomic clocks” of nature, geologists used the concept of relative ages.

We can see changes in the landscape over time.

We can see one type of organism replace another type of organism through the fossilized remains of these changes over time.

With the advent of atomic/radioactive decay techniques we began to obtain absolute dates.

Absolute dates or ages have specific numbers

Relative dating has been done in the past through the study of layers of sedimentary rock

(aka “the field of stratigraphy”)

Principle of Original Horizontality

sediments deposited innearly flat layers. Cross-bedded layers are contained within overall flat beds.

If layers are faulted (broken) or folded it means that they were deformed AFTER sediments were deposited.

Principle of Superposition

Lowest layers are older

If this is not so, then the layers have been

overturned by tectonic activity:

There are limitations for using stratigraphy

to keep time because,

(1) sedimentation rates are variable

e.g. Mississippi can deposit 1m of sediment in 1000 y

The deep ocean may deposit 1mm / 1000 years

We need to estimate time in another way and be able to recognize when the representation of time via sediments is incomplete.

Paleontology is the study of ancient life from fossilized remains.

William Smith in the 19th century was an engineer who noted that similar rocks in different areas had similar fossils and that different rock types had different fossils.

He made a composite (ideal, pieced-together) stratigraphic succession as in the following figure

Most of geologic time goes unrecorded

Sometimes NO SEDIMENT is DEPOSITED over an interval of time (hiatus)

Other times sediment is eroded away.

This is happening here: Mississippi Valley is filling whereas the higher ground is being eroded away.

An unconformity in which the layers above an erosional surface are parallel to the layers below is known as a DISCONFORMITY

One way of producing a disconformity is to change global sea-level by glaciation and glacial melting.

Cross-cutting relationships in sedimentary rocks allow is to date the relative timing of events such as faulting and igneous intrusion

The Geologic Time Scale…..is still under cosntructionLife began only 2500 Ma

Radioactive Dating Methods

Mining Uranium OxideU3O8

U-238 99.3%, U-235 0.7%

Millingcrushedsoaked in sulfuric acidYellow Cake U3O8

CONVERSION:yellowcake is heated to about 147 F, converted into uranium hexafluoride gas (UF6).

ENRICHMENT:The UF6 gas centrifuged heavier U-238 moves outside U-235 concentrates inwardConvert to UO2 powder

Nuclear Reactors: enriched 3-5% U-235Nuclear Weapons: enriched 90% U-235

All lead, except 204Pb (1.4%), is the end product of a complex radioactive decay. The most common isotope of Lead is: 208Pb 52.4% (stable)

The END ….. For now