Ch 10 Ages of Rocks 10.2/10.3

10.2 Relative Ages of Rocks Principle of Superposition: states that in an undisturbed layer of rock, the oldest rocks are on the bottom and the rocks become younger as you go to the top. Why? Because sedimentary rocks are deposited in horizontal layers that build one on top of the other. This means that the first layer deposited must be on the bottom, 2nd on top of that, etc.

Is that always so? No, because forces within the earth are constantly changing the earth’s surface especially at the fault lines where rock layers can be overturned. If geologists find rocks that were overturned, they have to use a different method to date those rocks.

Types of Dating Relative Dating Radioactive Decay

Relative Dating: Allows scientists to determine the order of events and the relative age of the rocks by examining their position in a sequence. For example, an area of rocks has been overturned by a fault line, this means that these rocks must have been there before, the fault has moved them. It gives us a general timeline for ages, not a specific one.

CLUES FROM IGNEOUS ROCK EXTRUSION-when lava flows onto the surface of the Earth and hardens making it younger than the rock below it. INTRUSION-when magma or lava cools under the surface and the rock surrounding it is older.

CLUES FROM FAULTS A FAULT IS A BREAK IN THE EARTH’S CRUST. A FAULT IS ALWAYS YOUNGER THAN THE ROCKS IT CUTS THROUGH.

Unconformities: When you find a rock record or sequence that’s incomplete or missing a layer. These can occur by erosion, or lack of deposition. Types of unconformities-  1) Angular unconformities- when tilted layers become eroded and new horizontal layers are deposited on top of those.

Unconformities 2. Disconformity- when layers of rock are deposited, then uplifted, exposed and eroded away, and finally new layers are deposited again. 3. Nonconformity: occurs when sedimentary layers form on top of igneous of metamorphic layers. The layers are uplifted and eroded and sedimentary rocks are deposited once again.

Angular Unconformity

Nonconformity

Nonconformity

Disconformity

Label from youngest to oldest

Answers to Superposition example Youngest to Oldest F B K N A J D M H C L G E

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

Ammonites

10.3 Absolute Ages of Rocks Method used by scientists to determine the age of rocks or other objects in years. Radioactive decay- the process of an atom changing its structure over time Several ways an atom can decay- One of the atoms’ neutrons breaks down into a proton and an electron.-The electron leaves the atom as a beta particle and the proton is kept by the atom.

Absolute Ages of Rocks Atoms can give off two protons and 2 neutrons in the form of a alpha particle. When you change the number of protons, the identity of the atom changes. Example: Uranium 238 decays into Lead-206. Uranium is the parent, Lead the daughter. Example: Carbon-14 decays to Nitrogen-14.

Rate at which decay occurs The rate that it takes for the isotope to decay is called its half-life. The half-life is the amount of time it takes for half of the atoms inside the isotope to decay. Half-life of C-14 is 5730 years. This means that after 5730 years have passed, only half of those C-14 atoms will remain. If another 5730 years pass, that ½ will be cut to 25% of the original atoms. See p. 324 http://vcourseware5.calstatela.edu/VirtualDating/files/2.0_HalfLife.html

Dating Aging Rocks(tars) 4.6 Billion Years?

                                                

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