Time Detectives

Scientists have discovered several methods to determine a rock’s age.

(1)Relative Age

(2)Absolute Age

(3)Fossils in sedimentary rocks and the worldwide geological time scale which is defined by fossils. (http://www.mnh.si.edu/earth/text/3_1_2_3.html)

When scientists talk about

age, they talk about relative

versus absolute age.

(1)Geologists can work out the sequence of

events recorded in a rock and its age relative to other rocks around it.

What is relative age?

• Geologists can

work out the

sequence of

events recorded in

a rock and its age

relative (or

compared to) to

other rocks

around it.

What is the Law of Superposition?

• In horizontal sedimentary rock layers, the

oldest layer is at the bottom. Each higher layer

is younger than the layers below it

Oldest

Youngest

What is Uniformitarianism?This principle states that the

same processes that operate today operated in the past.

"The Present is the Key to the Past"

Proposed by Scottish geologist James Hutton, which was refined by John Playfair and popularized by Charles Lyell's Principles of Geology in 1830

Theory of the Earth

What other clues help to find relative age?

• A. Clues from igneous rock: extrusions (hardens on surface) and intrusions (hardens beneath surface) are always younger.

• B. Clues from faults: a fault is always younger than the rock it cuts through.

• C. Clues from an

unconformity: where an

old, eroded rock

surface is in contact

with a newer rock layer.

• An unconformity is a

gap in the geologic

record where some

rock layers have been

lost because of erosion.

Unconformity

The Great UnconformityGrand Canyon, AZ

• D. Clues from fossils: index fossils

(widely distributed around the world;

existed only for a short time period)

(2)Geochemists can use dating techniques based on radioactive decay to

determine a rock’s absolute age.

What is absolute age?

• Geochemists can use

dating techniques

based on radioactive

decay to determine a

rock’s absolute age.

• Absolute = actual

What is radioactive dating?• Rocks are a form of matter.

• All matter is made out of atoms.

• During radioactive decay, the atoms of one

element break down to form atoms of

another element.

Think about it…• Radioactive

dating works for

igneous rocks,

not for

sedimentary

rocks.

• Why not?

•

http://orchard.sbschools.net/users/dgurwicz/fieldtrips/buttonsandboats/sedimentaryrock.htm

How can a scientist find the absolute age of a sedimentary layer?

• To date a

sedimentary

layer, a scientist

must date the

igneous

intrusions and

extrusions.

What is the difference between relative and absolute dating??

• Generally (relative)

versus specifically

(absolute)…

• EX: “Generally” 7th

graders are 11 – 13

years of age. An

individual 7th

graders “absolute”

age is determined

by their actual

birthday

(3)Paleontologists can look for fossils in

sedimentary rocks to figure out the rocks’ ages

based on the worldwide geological time scale

defined by fossils.

What is the Geological Time Scale?

• A record of

the life

forms and

geologic

events in

Earth’s

history.

http://coolwallz.com/sitebuildercontent/sitebuilderpictures/earthCLOCK.jpg

0 hour

Earth Forms

7 hrs.

One Celled Organisms

21 hrs.

Complex Organisms

14 hrs.

Simple Organisms

22 hrs.

Reptiles, Insects

23 hrs.

Dinosaurs

Humans

• Scientists noticed

major changes in

life forms and in

the Earth and

used these units

of time.

Let’s take a look at Arizona…

• Using the law of

superposition, clues

from intrusions,

extrusions, faults,

unconformities and

fossils, the geologic

time scale, and

radioactive dating,

scientists have

made a geologic

map showing the

ages of rocks and

landforms.