Geologic TimeWhen time rocks….

Get It?!!

Geologic Time

• Geologic Time:• Is the study and interpretation of Earth’s past. • By looking at a cross-section of land, or an

outcrop, we can gain evidence about how it formed.

• It is like solving a puzzle

If It’s a Puzzle, We Need….

• The puzzle pieces.• Two types of ages:• Relative Dating: when we compare the age of

a rock or event to another rock or event in a geologic sequence (no numerical age, just a round about age)

• Absolute Dating: the actual, numerical age, of a rock or event.

• We are going to begin with relative aging.

1.) The Principle of Uniformiatrianism

• WHAT!!!!!!• Uniform: • The same• -trianism:• Processes

• “the present is the key to the past”• All of the geologic processes that shape the Earth

today are the same ones that shaped the Earth in the past.

• This rule always starts us out.

Geologic Processes• Early Earth:• Volcanism• Pangea:• Tectonic Plate Movement• Formation of Landscapes:• Weathering and Erosion

• All of these were found in rock records from the past and the present.

• So we can assume it will occur in the future as well.

2.) The Principle of Original Horizontality

• Sediments are deposited in horizontal layers that are parallel to the surface on which they were deposited.

• If layers are not completely horizontal, an unconformity (erosion)must have come after the layer was created.

• This is usually our second piece of evidence in most landscapes.

3.) The Principle of Superposition

• In a series of undisturbed layers, the oldest layer is on the bottom and each layer on top of that gets progressively younger.

• This does not apply in cases where layers have been overturned or where older rocks have been forced over younger layers along a fault.

Folding and Faulting

• These mess up the principle of superposition.

Index Fossils

• Index Fossils:• Fossils that can help us determine the relative

age of a layer.• They are distributed over a large geographic

area but for a short amount of time.• (horizontally not vertically)

Wide Spread Geographic Area

Time

Looking At Events

• 4.) Igneous Intrusion and Extrusions:• Igneous INtrusion:• when molten magma forces its way through

other layers is called an intrusion. • Contact Metamorphism shows this.• Igneous EXTrusion:• When lava solidifies at Earth’s surface, it forms

a mass of igneous rock called an extrusion.

Rules

1. PU2. POOH3. PSP4. U/E5. F/F6. Ig (int/ext)7. CCR

EVENTS

Radioactive Decay• Radioactive Decay:• Occurs when unstable atoms break down

(decay) into radioactive elements (decay product).

• Atoms with the same number of protons but a different number of neutrons are called isotopes.

C6 protons

6 neutrons

12

8 neutrons

6 protons

C14

Radioactive isotopeStable

atom

Half-LifeHalf-life is the amount of time it takes for the original product to disintegrate (break down) by half into the decay product

CARBON-14 is used for fossils b/c of its short half-life

Number of Half-Life

Years Orig. % Decay % Orig. GramsLeft

Pie Chart