View
3
Download
0
Category
Preview:
Citation preview
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.
Recommended