# Determining Geologic Ages Lab # 8 pg 91

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Determining Geologic Ages Lab # 8 pg 91

Relative Dating Is a series of Principles and laws The Doctrine of Uniformitarianism Principle of Original Horizontality Law of Superposition Law of Cross-cutting relationships Law of Inclusions

Doctrine of Uniformitarianism
Proposed by James Hutton in 1700’s. The present is the key to the past. States that the physical, chemical and biological process that occur today have occurred throughout the Earth’s history. The process have not changed, but the rates and intensities at which they occur can change.

Principle of Original Horizontality
1st proposed by the Danish geologist Nicholas Steno ( ) This applies to sedimentary rocks. Sediments when deposited form horizontal layers. If we see sedimentary rocks that are folded or inclined, we can assume that something has occurred after the sediments were deposited.

Principle of Original Horizontality

Law of Superposition 1st proposed in the 11th century by the Persian geologist, Avicenna (Ibn Sina), and later formulated more clearly Nicolas Steno This applies to Sedimentary rocks. Each layer is a bed. The oldest is at the bottom, and the youngest is at the top. Each layer of rock represent a specific period in time.

Law of Cross-Cutting Relationship.
If a layers cut diagonally through another then the diagonal layer is younger. The layer that is cutting through another has to be younger because the original layer had to be there before the newer one could cut through it.

Law of Superposition & Cross-Cutting Relationship

Law of Inclusion Inclusions are pieces of one rock that is contained within another. Consequently, the one the pieces are from is older than the one the pieces are in.

This is a gap in the geologic record.
Unconformity This is a gap in the geologic record. Occurs when sedimentation stops because the area is above sea level, thus no sediments can be deposited. i.e. - this location was once the surface of the Earth. They can be Disconformity (Sedimentary Rock) or Nonconformity (Igneous or Metamorphic Rocks)

Sedimentary layers are parallel above and below the erosional contact.
1 2 4 3 5 6

Nonconformity Older Igneous or Metamorphic rocks are separated from younger Sedimentary layers. Uplift and erosion of overlying rocks had to occur. Once these rocks are on the surface, they are weathered and erosion take place. After erosion sedimentation occurred.

Nonconformity K Q J P IM Deposition of rocks Uplift, & erosion
J & K. Intrusion or Metamorphism of Rock IM Uplift, & erosion Of rocks J,K,& IM Subsidence & Deposition of rocks P & Q Sea Level Nonconformity K J IM P Q

Development of Angular Unconformity
When the angle of the rock below is different then the angle of the rocks above. During the pause in sedimentation, folding and or tilting has occurred.

Faults A movement of rock .
The movement has to be younger then the rock it cuts through.

Absolute Dating Quantify the date in years.
We use fossils and Radiometric Dating (half lives) to determine the age of a layer.

Fossils and Principle of Fossil Succession
Fossils - evidence of past life. For fossils to occur you must have a rapid burial; if not the remains will be destroyed. Bones, shells make good fossils. We know that different organisms lived during the same time; if one is found then the other should be found in the same layer Used for relating age of rocks regardless of depth or distance apart.

Principle of Fossil Succession

Principle of Fossil Succession

Discovered in 1896 by Henri Becquerel. Radioactive Decay- radioactive materials break down into other materials at known rates. Radioactive parent elements decay to stable daughter elements. (Carbon14 → Carbon12) Once this rate is known, geologists can estimate the length of time over which decay has been occurring.

Decay occurs at a constant rate.

Radiocarbon dating - Measures the decay of carbon-14 in organic material and can be best applied to samples younger than about 60,000 years. Uranium-lead dating - Measures the ratio of two lead isotopes (lead-206 and lead-207) to the amount of uranium in a mineral or rock. This method is one of the two most commonly used (along with argon-argon dating) for geologic dating, applied to samples older than about 1 million years. Uranium-thorium dating - Used to date corals, carbonates, and fossil bones. Its range is from a few years to about 700,000 years. Potassium-argon and argon-argon dating - These techniques date metamorphic, igneous and volcanic rocks, for samples older then a few thousand years.

Geologic Scale The largest time unit Eon Era Period
Epoch Era Period The largest time unit The smallest time unit

Geologic Timeline Precambrian Eon 88% 3 Eras Phanerozoic Eon 12%
Cenozoic Era Mesozoic Era Paleozoic Era 2 Periods 3 Periods 6 Periods 7 Epochs

Geologic Timeline Precambrian Eon Three Eras
4.5 Billion-to 540 million years ago No life forms to multi cell organisms Phanerozoic Eon Paleozoic Era (Ancient Life) 540 million years ago to 248 million years ago Marine invertebrates dominant then undergo mass extinction On land, insects evolve Mesozoic Era (Middle Life) The age of Reptiles 248 million years ago - ~65 million years ago Pangea & early dinosaurs Mass extinction of dinosaurs and early trees bloom.

Cenozoic Era (Recent Life)
2 periods Tertiary & Quaternary Tertiary Period: (Age of Mammals) ~65 to ~2.6 million years ago Mammalian life diversifies to all modern mammals present Quaternary period (Humans Develop) ~2.6 million years ago to the present The oldest species of Homohabilis evolves to Human civilization develop.

Geologic Clock

Chapter Summary pg Old Book Questions 1-2a-d, 3 & 6 Ignore question 2 parts e-g & 4-5 New Book Questions 1-7 & 10