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
Dating Rocks (determining
their age, that is)
• Relative Dating
– Superposition - The youngest rocks are on
the top, oldest at the bottom.
Dating Rocks (determining
their age, that is)
• Relative Dating
– Superposition
– Cross-cutting relationships - Geologic
features that cut through and across rocks
are younger than those rocks.
• Mostly Faults and Igneous intrusions
Dating Rocks (determining
their age, that is)
• Relative Dating
– Superposition
– Cross-cutting relationships
– 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
http://bloginitiative.typepad.com/photos/uncategorized/crowd.jpg
Dating Rocks (determining
their age, that is)
• Relative Dating
– Superposition
– Cross-cutting relationships
– Law of Inclusions
– Law of Original Horizontality (and Lateral
continuity)
Hikingtripsreport.com
What are the relative
age relationships shown
here?
How can you tell a sill
from a lava flow?
M&W4 Fig. 17.4; M&W5 Fig. 17.4
Dating Rocks (determining
their age, that is)
• Relative Dating
– Superposition
– Cross-cutting relationships
– Law of Inclusions
– Law of Original Horizontality (and Lateral
continuity)
– Law of Unconformities
A DISCONFORMITY is a
boundary between two
layers of non-continuous
ages. This boundary is
usually marked
by an erosional
surface and is
often irregular.
M&W4 Fig. 17.8; M&W5 Fig. 17.8
An ANGULAR
UNCONFORMITY is a
disconformity between
layers of different angles.
The underlying layers are
first tilted, then erosion
scours away a new,
horizontal surface.
New, horizontal layers form
on top
An NONCONFORMITY is a disconformity between
different rock types, one of them sedimentary.
M&W4 Fig. 17.11; M&W5 Fig. 17.11
STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION
• Do three meters of strata at place A record the same
amount of time as three meters at place B?
• 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
•
Formations: The fundamental stratigraphic units that are used to
correlate stratifed rocks are called formations. Formations have
between one (and preferably all) of the following characteristics:
• a distinctive set of physical properties (sedimentary rock type,
bedding, grain size)
• a distinctive fossil assemblage
• have a widespread (map scale) geographic distribution
•
Geologists can thus construct a regional stratigraphy that
represents much more geologic time than any single outcrop in any
single location.
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).
M&W4 Fig. 3.3; M&W5 Fig. 3.4
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 AGE DETERMINATION
•
For radioactivity dating we use igneous rocks and minerals. The clock
starts when radioactive atoms that are present in the magma get
incorporated in the crystalline structure of certain minerals in the rocks.
•
The crystals containing the parent atoms form and so we then have a
“container” with parents that can begin decaying to form daughters.
•
We can then use measure the parent-daughter ratio. This is our “atomic
clock” that records the time since the rock crystallized.
M&W4 Fig. 17.21; M&W5 Fig. 17.21
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
•
To the oldest materials ever dated by the radioactive method 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.
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.
This method works best for
fossils younger than 50,000
years. Why?
(end)
M&W4 Fig. 17.24; M&W5 Fig. 17.24