Ch 10 Ages of Rocks Ch 10 Ages of Rocks 10.2/10.310.2/10.3

10.2 Relative Ages of 10.2 Relative Ages of RocksRocks

Principle of Superposition: states that in Principle of Superposition: states that in an undisturbed layer of rock, the oldest an undisturbed layer of rock, the oldest rocks are on the bottom and the rocks rocks are on the bottom and the rocks become younger as you go to the top.become younger as you go to the top.

Why? Because sedimentary rocks are Why? Because sedimentary rocks are deposited in horizontal layers that build deposited in horizontal layers that build one on top of the other. This means that one on top of the other. This means that the first layer deposited must be on the the first layer deposited must be on the bottom, 2bottom, 2ndnd on top of that, etc. on top of that, etc.

Is that always so?Is that always so?

No, because forces within the earth No, because forces within the earth are constantly changing the earth’s are constantly changing the earth’s surface especially at the fault lines surface especially at the fault lines where rock layers can be where rock layers can be overturned. overturned.

If geologists find rocks that were If geologists find rocks that were overturned, they have to use a overturned, they have to use a different method to date those different method to date those rocks.rocks.

Types of DatingTypes of Dating

Relative DatingRelative Dating Radioactive DecayRadioactive Decay

Relative Dating:Relative Dating:

Allows scientists to determine the order Allows scientists to determine the order of events and the relative age of the of events and the relative age of the rocks by examining their position in a rocks by examining their position in a sequence.sequence.

For example, an area of rocks has been For example, an area of rocks has been overturned by a fault line, this means overturned by a fault line, this means that these rocks must have been there that these rocks must have been there before, the fault has moved them. It before, the fault has moved them. It gives us a general timeline for ages, not gives us a general timeline for ages, not a specific one.a specific one.

CLUES FROM IGNEOUS CLUES FROM IGNEOUS ROCKROCK

EXTRUSION-when lava flows onto EXTRUSION-when lava flows onto the surface of the Earth and the surface of the Earth and hardens making it younger than the hardens making it younger than the rock below it.rock below it.

INTRUSION-when magma or lava INTRUSION-when magma or lava cools under the surface and the cools under the surface and the rock surrounding it is older.rock surrounding it is older.

CLUES FROM FAULTSCLUES FROM FAULTS

A FAULT IS A BREAK IN THE A FAULT IS A BREAK IN THE EARTH’S CRUST.EARTH’S CRUST.

A FAULT IS ALWAYS YOUNGER A FAULT IS ALWAYS YOUNGER THAN THE ROCKS IT CUTS THAN THE ROCKS IT CUTS THROUGH.THROUGH.

Unconformities:Unconformities:

When you find a rock record or sequence When you find a rock record or sequence that’s incomplete or missing a layer. that’s incomplete or missing a layer. These can occur by erosion, or lack of These can occur by erosion, or lack of deposition.deposition.

Types of unconformities-Types of unconformities-

  1) Angular unconformities- when tilted 1) Angular unconformities- when tilted layers become eroded and new layers become eroded and new horizontal layers are deposited on top of horizontal layers are deposited on top of those.those.

2. Disconformity- when layers of rock 2. Disconformity- when layers of rock are deposited, then uplifted, are deposited, then uplifted, exposed and eroded away, and exposed and eroded away, and finally new layers are deposited finally new layers are deposited again.again.

3. Nonconformity: occurs when 3. Nonconformity: occurs when sedimentary layers form on top of sedimentary layers form on top of igneous of metamorphic layers. The igneous of metamorphic layers. The layers are uplifted and eroded and layers are uplifted and eroded and sedimentary rocks are deposited sedimentary rocks are deposited once again. once again.

UnconformitiesUnconformities

Angular Unconformity

NonconformityNonconformity

NonconformityNonconformity

DisconformityDisconformity

Label from youngest to Label from youngest to oldestoldest

Answers to Superposition Answers to Superposition exampleexample

MM HH CC LL GG EE

Youngest to Youngest to OldestOldest

FF BB KK NN AA JJ DD

USING FOSSILS TO DATE USING FOSSILS TO DATE ROCKSROCKS

INDEX FOSSILS ALLOW SCIENTISTS INDEX FOSSILS ALLOW SCIENTISTS TO MATCH UP ROCK LAYERS. TO MATCH UP ROCK LAYERS. CERTAIN FOSSILS WERE ALIVE CERTAIN FOSSILS WERE ALIVE DURING A PARTICULAR TIME FRAME DURING A PARTICULAR TIME FRAME AND FINDING THEM HELP TELL THE AND FINDING THEM HELP TELL THE AGE OF THE ROCK IT IS EMBEDDED AGE OF THE ROCK IT IS EMBEDDED IN.IN.

EX: AMMONITESEX: AMMONITES

AmmonitesAmmonites

10.3 Absolute Ages of 10.3 Absolute Ages of RocksRocks

Method used by scientists to determine Method used by scientists to determine the age of rocks or other objects in the age of rocks or other objects in years.years.

Radioactive decay- the process of an Radioactive decay- the process of an atom changing its structure over time atom changing its structure over time

Several ways an atom can decay-Several ways an atom can decay- One of the atoms’ neutrons breaks down into One of the atoms’ neutrons breaks down into

a proton and an electron.-The electron leaves a proton and an electron.-The electron leaves the atom as a beta particle and the proton is the atom as a beta particle and the proton is kept by the atom.kept by the atom.

Absolute Ages of RocksAbsolute Ages of Rocks

Atoms can give off two protons and 2 Atoms can give off two protons and 2 neutrons in the form of a alpha neutrons in the form of a alpha particle. particle. When you change the number of protons, When you change the number of protons,

the identity of the atom changes.the identity of the atom changes. Example: Uranium 238 decays into Lead-Example: Uranium 238 decays into Lead-

206. Uranium is the parent, Lead the 206. Uranium is the parent, Lead the daughter.daughter.

Example: Carbon-14 decays to Nitrogen-Example: Carbon-14 decays to Nitrogen-14.14.

Rate at which decay occursRate at which decay occurs The rate that it takes for the isotope to The rate that it takes for the isotope to

decay is called its half-life.decay is called its half-life. The half-life is the amount of time it takes The half-life is the amount of time it takes

for half of the atoms inside the isotope to for half of the atoms inside the isotope to decay.decay.

Half-life of C-14 is 5730 years. This means Half-life of C-14 is 5730 years. This means that after 5730 years have passed, only that after 5730 years have passed, only half of those C-14 atoms will remain. If half of those C-14 atoms will remain. If another 5730 years pass, that ½ will be another 5730 years pass, that ½ will be cut to 25% of the original atoms.cut to 25% of the original atoms.

See p. 324See p. 324 http://vcourseware5.calstatela.edu/VirtualDating/files/2.0_HalfLife.htmlhttp://vcourseware5.calstatela.edu/VirtualDating/files/2.0_HalfLife.html

Dating Aging Rocks(tars)Dating Aging Rocks(tars)

4.6 Billion Years?

                                                

What does it mean?What does it mean?

A common "parent-daughter" A common "parent-daughter" combination that geologists use is combination that geologists use is radioactive uranium and non-radioactive radioactive uranium and non-radioactive lead. As shown in the diagram above, lead. As shown in the diagram above, uranium is trapped in a newly formed uranium is trapped in a newly formed rock. As the rock ages, more and more rock. As the rock ages, more and more of the uranium changes into lead.of the uranium changes into lead.