ways to tell the age of a rock relative dating: places events in geologic history in the proper...
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Ways to tell the age of a rock
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”
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
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
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
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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.
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http://www.earth.ox.ac.uk/~oesis/field/medium/xenolith-1365.jpg
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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. 17.4M&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 Do three meters of strata at place A record the same amount of time as three meters at place B?same amount of time as three meters at place B?
• How do we correlate events and the passage of How do we correlate events and the passage of time from one outcrop of rock to another and time from one outcrop of rock to another and even around the world? even around the world?
Fossils!Fossils!the main tool for correlating strata the main tool for correlating strata
(and intervals of time represented by (and intervals of time represented by strata) from one rock outcrop to strata) from one rock outcrop to
another outcropanother 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
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GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
Marie Curie
Ernest Rutherford
• 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.
M&W4 Fig. 3.3; M&W5 Fig. 3.4
• 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:
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
M&W4 Fig. 17.18; M&W5 Fig. 17.18
“Parent” “Daughter”
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
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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
M&W4 Fig. 17.24; M&W5 Fig. 17.24
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?
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