Telling Geologic Time

Putting it in perspective

Determining geological ages

Relative age dates placing rocks and

events in their proper sequence of formation

Numerical dates specifying the actual

number of years that have passed since an

event occurred (known as absolute age

dating)

Figure 1-15 (p. 16)

Geologic Time Scale.

Uniformitarianism The physical, biological and

chemical

principles acting

on the Earth in

the present also

acted on the

Earth in the past.

Getting Dates

Relative Dating

Law of Superposition

Developed by Nicolaus

Steno in 1669

In an undeformed

sequence of sedimentary

rocks (or layered igneous

rocks), the oldest rocks

are on the bottom

Superposition in the Grand Canyon

Principles of relative dating

Principle of original horizontality

Layers of sediment are generally deposited

in a horizontal position

Rock layers that are flat have not been

disturbed

Original Horizontality (d)Lateral Continuity (e)

Figure 1-6 (p. 5)Illustration of original lateral continuity.

Law of Cross-cutting Relationships

Inclusions

An inclusion is a piece of rock that is

enclosed within another rock

Which layer is younger?

Inclusions

Baked ContactsContact Metamorphism

Lava flow

A cross-section through the earth reveals the variety of geologic features. View 1 of this animation identifies a variety of geologic features; View 2 animates the sequence of events that produced these features, and demonstrates how geologists apply established principles to deduce geologic history. [by Stephen Marshak]

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Geologic History

Gaps in the Record: Unconformities

Angular unconformity, Siccar Point, Scotland

Erosional Surfaces

Unconformities represent gaps in the

geologic record because of surface erosion

Figure 1-8 (p. 8) Siccar Point, Eastern Scotland. It was here that James Hutton first realized the historical significance of an unconformity.

Formation of an angular unconformity

Development of a nonconformity

Development of a disconformity

Unconformities

Four types of

erosional

unconformities.

(A) Angular

unconformity. (B)

Nonconformity. (C)

Disconformity. (D)

Paraconformity.

Sub-bottom seismic profile

Gaps in Time in the Grand Canyon

Types of Unconformity

This animation shows the stages in the development of three main types of unconformity in cross-section, and explains how an incomplete succession of strata provides a record of Earth history. View 1 shows a disconformity, View 2 shows a nonconformity and View 3 shows an angular unconformity. [by Stephen Marshak]

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Top: Block diagram

Bottom: Geologic map

Figure 1-11 (p. 11)An example of how the sequence of geologic events can be determined from cross-cutting relationships and superposition.

From first to last, the sequence indicated in the cross-section is first deposition of D, then faulting to produce fault B, then intrusion of igneous rock mass C, and finally erosion followed by deposition of E. Strata labeled Dare oldest, and strata labeled E are youngest.

Interpreting the Sequence of Events

Principle of Faunal Succession

Correlation often relies upon fossils

Principle of fossil succession fossil

organisms succeed one another in a definite

and determinable order, and therefore any

time period can be recognized by its fossil

content

Determining the ages of

rocks using fossils

Getting Dates

By the numbers:

Absolute Dating

Using radioactivity in dating

Radioactivity

Spontaneous changes (decay) in the

structure of atomic nuclei

Radioactive Decay Isotopes

Elements with the same number of protons and electrons but with different numbers of neutrons in their nucleus

Radioactive (radiogenic)

Parent Original unstable radioactive isotope

Daughter The isotopes resulting from the decay of a parent

Half-life Time it takes for half of the parent isotope to

decay into the daughter isotope

Given as an average time

Figure 1-20 (p. 23)Radioactive decay series of uranium-238 (238U) to lead-206 (206Pb).

Figure 1-25 (p. 26)Rate of radioactive decay of uranium-238 to lead-206.

Figure 1-27 (p. 28)Decay curve for potassium-40.

Using radioactive C14 in dating

Dating with carbon14 (radiocarbon dating)

Half-life of only 5730 years

Used to date very recent events

Carbon-14 is produced in the upper

atmosphere

Useful tool for anthropologists,

archeologists, and geologists who study very

recent Earth history

Carbon-14 Dating

Figure 1-29 (p. 29)Carbon-14 is formed from nitrogen in the atmosphere. It combines with oxygen to form radioactive carbon dioxide and is then incorporated into all living things.

Difficulties in dating the

geologic time scale

Not all rocks can be dated by radiometric methods

Grains comprising detrital sedimentary rocks are

not the same age as the rock in which they

formed

The age of a particular mineral in a metamorphic

rock may not necessarily represent the time

when the rock formed

Datable materials (such as volcanic ash beds and

igneous intrusions) are often used to bracket

various episodes in Earth history and arrive at ages

Dating sedimentary strata

using radiometric dating

Dating sedimentary strata

using radiometric dating

Figure 1-23 (p. 25)Igneous rocks that have provided absolute radiogenic ages can often be used to date sedimentary layers. (A) The shale is bracketed by two lava flows. (B) The shale lies above the older flow and is intruded by a younger igneous body.