How do scientists sequence Earth’s past events to create the geological time scale? Using evidence from rocks and fossils, scientists can determine the order of events that occurred in the past. Relative and Absolute age are used to determine WHEN the events occurred.

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Relative Age The age of an object compared to the ages of other objects. (without knowing the exact ages)

General Rules for Determining Relative Age Law of Horizontality Law of Superposition Law of Cross-cutting Relationships: igneous intrusions & faults Unconformities Index Fossils

Law of Horizontality Sediment layers are horizontal when originally deposited. Layers that are not horizontal have been deformed by movement of Earth’s plates.

Because sediments are deposited under water, they will form flat, horizontal layers. If a sedimentary rock is found tilted, the layer was tilted after it was formed.

Law of Superposition In an UNDISTURBED horizontal layer of rock, the oldest rocks are on the bottom and the rocks become younger toward the top.

DISTURBED horizontal layers: Law of Superposition as a taco shell. Movement of tectonic plates can disturb rock layers. A whole set of layers can get turned on its side. Rock layers can get bent, or even folded over, like a taco shell. When horizontal layers have been disturbed, the youngest may no longer be on top. Scientists determine the original order by comparing the disturbed rock layers with a similar undisturbed stack of layers.

Folded rock layers

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Law of Crosscutting Relationships: Igneous intrusions Igneous rock forms when magma or lava hardens. Beneath the surface, magma may push into bodies of rock. The magma cools and hardens into a mass of igneous rock called intrusions. An intrusion (magma) is always younger than the rock layers it cuts through.

Igneous intrusions

Intrusion

Extrusion

Law of Crosscutting Relationships: Faults A fault is a break in the Earth’s crust. Forces inside Earth cause movement of the rock on opposite sides of a fault. A fault is always younger then the rock it cuts through. To determine the relative age of a fault, geologists find the relative age of the youngest layer cut by the fault.

Law of Crosscutting Relationships: Faults

Unconformities Gaps in a sequence of rock layers, resulting from erosion. Even though rock layers are lost, this can help scientists understand how geologic forces acted on Earth.

Index Fossils These fossils are used to estimate the age of rock layers in which they are found. Index fossils can provide a relative age.

THREE REQUIREMENTS FOR A FOSSIL TO BE A GOOD INDEX FOSSIL Very common Very common Existed for a short period of time. Existed for a short period of time. Found in many areas on the Earth’s surface. Found in many areas on the Earth’s surface.

Example of Index Fossil The mollusk Inoceramus labiatus, for example, is a kind of sea animal that appeared 144 million years ago and went extinct 65 million years ago. So, if you find a rock that contains a fossil of this mollusk, the rock must be between 144 million and 65 million years old because this mollusk lived during that time span.

Law of Faunal Succession The law states that fossils are found in rocks in a very definite order. This law led others to use fossils to define increments of time within a relative time scale.

Fossil Record: A Timeline of Earth’s History

Review of Relative Age Dating 1. What can you tell from undisturbed rock layers? 2. Are only sedimentary rocks used for relative age determinations? 3. Explain the relative age relationship of cross- cutting formations to the rocks they cut? 4. How can index fossils help scientists determine the ages of rock layers?

Absolute Age The exact age of a fossil or rock formation

Can you ask a rock its age?

Scientist use radioactive dating to determine the absolute age (exact age) of fossils and rock formations. Radioactive dating relies on radioactive decay. How do scientists determine the exact age of rocks & fossils?

What is Radioactive Decay? Many elements are stable and do not change, but some elements are unstable and change over time. Unstable elements are used to find the exact age of fossils. These unstable elements are said to be radioactive.

What are some examples of radioactive elements? Carbon-14 – changes into Nitrogen-14 Potassium-40 – changes into Argon-40 Urianium-235 – eventually changes into Lead-207

What is radioactive decay? RADIOACTIVE DECAY is when an unstable element changes in some way to become stable. The atoms of an unstable element (parent material) break down to become atoms of another element (daughter material).

What is a half-life? Half-life is the time it takes for half of the radioactive atoms (unstable atoms) to change into stable element. Each different radioactive element has its own personal half-life. Think of it as a geologic CLOCK

Half-life Example The half-life of C-14 is about 5700 years. (Every 5700 years, HALF of the C-14 changes!) You have 60 grams of Carbon-14. After 5700 years pass, how much C-14 would you have? N-14? Now you have 30g of C-14 & 30g of N-14 Another 5700 years go by. How much C-14 would you have? N-14? 15g of C-14 & 45g of N-14 This would continue until all the unstable element becomes stable.

When does radioactive decay begin? Radioactive decay starts as soon as the molten magma or lava begin to harden.

How is absolute age determined from radioactive dating?

In radioactive dating, scientists first find out how much radioactive element is in the sample. Then, they compare the radioactive amount to the stable amount. The ratio between stable and unstable lets scientists know how many half- lives the fossil or rock has gone through.

Radioactive dating can only be used on what kind of rocks? Absolute age can only be determined from igneous rocks Igneous rocks are those rocks made from magma or lava.

Why can’t we find the absolute age of sedimentary rocks? Sedimentary rocks are formed from millions of pieces of particles that come from rocks of all different ages. Radioactive dating would find the ages of the particles not the age of the whole rock. We’d never know how old the rock really is!

How do scientists date sedimentary rocks? To date sedimentary rocks, scientists find the age of any intrusions or extrusions that are near the sedimentary rock. So, any layer of sedimentary rock above an intrusion or extrusion, is YOUNGER! (RELATIVE AGE!!!)

How old is the Earth? The oldest rocks ever found on the Earth have been dated at about 4.0 billion years old, but scientists believe the Earth is really… 4.6 billion years old.

How do we know how old the Earth is? Scientist believe the Earth and the moon formed at the same time. Scientist have dated moon rock brought back to Earth by astronauts. Therefore the Earth is believed to be at least as old as the moon rocks.