Discovering Earth’s History

What is time?

Us

What is “history” ? Prehistoric Historic

Geology is based on the following statement: Most all geologic events that happened on Earth occurred during prehistoric times. By carefully observing what has been left behind - the rocks, the fossils, etc - we are able to read and interpret Earth’s history.

…and follows the fundamental principle of Uniformitarianism which states… the forces and processes that we observe today have been at work for a very long time.

“THE PRESENT IS THE KEY TO THE PAST.” We must understand present-day processes and events in order to understand the geologic past. “THE PRESENT IS THE KEY TO THE PAST.” Determining time on Earth can be done by two different methods: 1. Relative dating 2. Absolute dating

I. Relative Dating – Determines the sequence of events, not the exact time - Qualitative. Animations and Movies

A few rules to follow: A. Law of Superposition – For rock layers that are undisturbed, the bottom layers are the oldest, and the top layers are the youngest. *An exception to this MAY occur if there is a “floppy fold” or a thrust fault Fig. 15.21 - Types of Faults Fig. 15.17 - Styles of Folding

Where does the Law of Superposition not apply?

B. Principle of Original Horizontality – Layers of sediment are initially deposited horizontal.

C. Principle of Cross-Cutting - Any rock layer cut by an intrusion or fault is older than the intrusion or fault.

D. Unconformity - A gap in geologic time created when part of the rock record has been eroded and then later buried. Observe an animation showing the formation of an unconformity. http://www.wwnorton.com/college/geo/earth/flash/12_1.swf Stratigraphy is the study of rock layers.

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II. Fossils – The remains, imprints, or traces of something that was once living. Observe how fossils can form. An organism that is made of hard parts and is buried rapidly after death in a low oxygen environment has the greatest chance of becoming a fossil

Actual Remains – Ice and Amber A fossil may form as: Actual Remains – Ice and Amber Swedish Bogman – 700 y.o. Otzi – 5,300 y.o.

Fossil may form as: 2. Replacement – Petrified wood

Fossil may form as: Molds - Hollow depression left when an organism decays

Fossil may form as: 4. Casts – When material fills in the mold

What story do fossils tell? What happened here?

Fossils, together with the rock layers in which they are found, enable geologist to read the past. As an example, widespread marine fossils found across NYS lead to the logical inference that NYS was once under water.

Rochester North America about ~400 mya

What is the oldest fossil? As a rule, the age of a fossil is the same age as the rock layer in which it is found

A. Index Fossils – Fossils that are useful in identifying and dating specific rock layers because they: are easy to recognize 2. once covered a large geographic area 3. when alive, existed for a short period of time Volcanic ash layers also serve as good index markers because of their wide distribution and rapid placement.

These are all index fossils

Which fossil is the index fossil?

Which fossil is the index fossil?

Which fossil is the index fossil?

B. Correlation of Rock Outcrops – Matching rock strata from different locations

1. Index fossils – Match known index fossils 2. Rock Similarities – Match strata that the same distinctive characteristics

III. The Geologic Time Scale 1. The Earth is estimated to be about 4.6 billion years old. Humans have existed on Earth for only 0.033% of Earth’s total history First Humans

III. The Geologic Time Scale 2. The fossil record clearly shows that life has continued to evolve into more complex and diverse forms.

III. The Geologic Time Scale 3. The fossil record shows that there have been several mass extinctions on Earth. It is estimated that over 90% of life that is known to have once lived on Earth is now extinct.

III. The Geologic Time Scale 4. The Geologic Time Scale (Page 8-9 - ESRT’s) has been created based on fossil and rock evidence that was then correlated between many places on Earth.

Do now… What is the geologic timeline based on?

IV. Absolute Dating - The dating of past events that happened on Earth by accurate and precise means – Quantitative. A couple of methods - radiometric dating and counting tree rings

These are absolute values

Understanding Radiometric Dating starts with this: Certain materials on Earth, including some rocks, are made with elements that are radioactive - these are called radioisotopes.

What does "Radioactive" mean? Something that is radioactive contains unstable elements that are naturally decaying, or breaking down into a more stable form - This is called Radioactive Decay Harmful radiation is given off during radioactive decay.

Terms to know: Parent material - The original radioactive material Daughter product - The stable material that forms from the decay of the parent material. Half-Live - The time it takes for half of the parent material to decay into daughter product

Because radioactive materials: Decay at random decay at a constant rate that can be measured …an absolute age can be determined.

An absolute age for a rock can be determined by measuring the ratio of parent material to daughter product. Less parent material/more daughter product = old More parent material/less daughter product = young

Page 1 of ESRT Uses: Carbon-14 – organic, less ~ 50,000 Y.O. Potassium-40, Uranium-238, and Rubidium-87 – dating old rocks or inorganic material.

Parent ((Radioisotope) Half-Life Parent ((Radioisotope) % Fraction Daughter Age 1 2 3 4 5 6

Trials Heads Tails 20 1 2 3 4 5 6 7 8 How long will take to flip 20 pennies from heads to tails? Like radioactive decay, flipping pennies is random, but occurs at constant rate. How long would it take to reach the half way point (half-life) if this were done again?

Trials Heads Tails 20 1 2 3 4 5 6 7 8 tails heads

You find a

What’s wrong with this picture?

Observe an animation of convection in the mantle. Understanding Earth 4E - Content http://bcs.whfreeman.com/understandingearth/content/ann/ue13an01.rm http://www.wwnorton.com/college/geo/earth/flash/1_3.swf