Transcript Absolute and Relative Dating of Rocks

Ways to tell the age of
a rock
 Relative Dating:
 Places events in geologic history in
the proper order.
 The basis for the geologic time scale
 Mainly Sedimentary Rocks
 Does not provide a true “age”
Ways to tell the age of
a rock
 Absolute Dating:
 All you need is a tiny sample of material
(mineral, bone) no larger than a grain of
rice.
 Gives us the true “age” of a fossil or rock
 Mainly organic tissue or igneous crystals
 Measure the amount of unstable isotopes
that have “decayed” to figure out age
Rock Age Determination
• Relative Dating
– Superposition - The youngest rocks are on
the top, oldest at the bottom.
Cross-cutting relationships
– Geologic features that cut through and
across rocks are younger than those rocks.
• Mostly Faults (cracks) and Igneous intrusions
Law of Inclusions – Rocks embedded in other rocks are older
than those rocks they are embedded in.
http://www.earth.ox.ac.uk/~oesis/field/medium/xenolith-1365.jpg
Review Relative Dating
– #1 Superposition
– #2 Cross-cutting relationships
– #3 Law of Inclusions
Hikingtripsreport.com
What are the relative
age relationships shown
here?
Can you explain this rock strata?
STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION
•How do we correlate events and the passage of time from
one outcrop of rock to another and even around the world?
Fossils!
the main tool for correlating strata (and
intervals of time represented by strata) from
one rock outcrop to another outcrop
.
STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION
•
Different kinds of organisms have lived during different periods in
Earth's history and then died off (or went extinct). This is called
faunal succession.
•
If a strata in different outcrops contain the same fossil assemblages,
then the outcrops represent the same interval of time. These strata
correlate.
M&W4 Fig. 17.6; M&W5 Fig. 17.6
STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION
•
From correlation of
formations from different
locations, the history of the
entire region can be
deciphered.
•
Sequences of layers (from
differente places) overlap,
like when you create a
panoramic photo from
individual shots.
M&W4 Fig. 17.14; M&W5 Fig. 17.14
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
•
Radioactivity was first
discovered by Henri Becquerel
in 1896 and Polish-French
chemist Marie Curie discovered
that radioactivity produced
new elements (radioactive
decay).
•
Ernest Rutherford first
formulated the law of
radioactive decay and was the
first person to determine the
age of a rock using radioactive
decay methods.
Marie Curie
Ernest Rutherford
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
• The number of protons (the atomic number) is fixed for
any element and is unique for each element but the
number of neutrons in atoms of different elements can
vary.
Atoms of an element having different numbers of neutrons
are referred to as the isotopes (of that element).
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
Radioactive decay occurs when an isotope of one element is transformed
into a different element by changes in the nucleus. There are three
different decay mechanisms:
“Parent”
“Daughter”
M&W4 Fig. 17.18; M&W5 Fig. 17.18
How can we tell age based on
the number of parent
isotopes?
Radioactive isotopes “decay” at a particular rate.
We express this rate as the “HALF-LIFE”, which is the
time it takes for HALF of the parent isotopes to decay.
GEOLOGIC DATING: ABSOLUTE OR RELATIVE DATING?
•
To the oldest materials ever dated are found in the Jack Hills of
western Australia and are tiny zircon grains contained in sandstones
and conglomerates. The zircons are 4.4 billion years old. Why?
Scanning
electron
microscope
image of a
Jack Hills
zircon. Scale
bar is 0.1 mm
The very remote
“outback” of
western Australia-the Jack Hills
C14 is an isotope of
carbon that forms from
Nitrogen in the
atmosphere. Living
things consume this
radioactive carbon.
Once dead, no new
carbon is absorbed, and
C14 turns back into
Nitrogen.
The Half-Life of C14 is
5,730 years.