Relative Dating

Relative Dating• Relative Age: the age of something compared to

something else.

• Absolute Age: the actual age of a rock or an event

The Principal of Superposition• In undisturbed sedimentary rock layers (strata),

the oldest rock is at the bottom.

YOUNGEST rock layer

OLDEST rock layer

Dating Intrusions, Extrusions, & Inclusions

• Intrusion: when magma squeezes into preexisting rocks and cools.– YOUNGER than any rock it cuts through

Dating Intrusions, Extrusions, & Inclusions

• Extrusion: when lava flows on the SURFACE and cools.– YOUNGER than rocks beneath it– OLDER than rocks above it.

Dating Intrusions, Extrusions, & Inclusions

• Inclusion: a piece of older rock within igneous rock.

Cross-Cutting Relationships• Types of Cross-cutting relationships: Intrusions,

faults, veins, etc. – These features are YOUNGER than the rocks they cut

across.

Cross-Cutting Relationships A fault is younger than

the rock layers it cuts across

An igneous intrusion is younger than the rockit penetrates

The folding and tilting of rock layers are events that are younger than the rock layers

they effect.

Dating Rock Features

Unconformity – A surface of erosion between rock layers that represents a missing gap in Earth’s history.

Deposition Uplifting and folding

Erosion SubsidenceUnconformity

Correlation• The process of showing that rock layers, or

geologic events, from different places are the same, or similar in age.

• Types of Correlation:1. Exposed bedrock2. Similarities in Rocks3. Use of Index Fossils4. Volcanic Ash or Meteorite Deposits

1. Correlation by Exposed Bedrock• Bedrock is usually

covered with soil, but can occasionally be exposed.– Correlation can be

accomplished by directly following the rock layers.

2. Correlation by Similarities in Rocks• Rocks can be correlated by having similar

appearance, color, mineral composition, and rock sequence.

3. Correlation by Index Fossils• Index Fossil: a fossil that is used to date the rock

layers in which it is found.1. An organism that lived during a relatively short,

well-defined time span.2. Have a wide distribution geographically.

Which of these is an INDEX FOSSIL?

4. Correlation by Volcanic Ash and Meteorite Deposits• Volcanic Ash: ash from each volcanic eruption has

unique composition– Ash deposits are widely distributed, and represent a

small period of time.

• Meteorite Deposits: rock particles and debris produced by an asteroid, comet, or meteorite impact can cover large areas of Earth.

“SEQUENCE OF EVENTS”PRACTICE WORKSHEETS

Sequence 1: Uplift & Erosion

1. Limestone deposited2. Sandstone deposited3. Shale Deposited4. Uplift5. Erosion

1. Limestone deposited

2. Sandstone deposited

3. Shale deposited4. Faulting

Sequence 2: Faulting

Sequence 3: Folding

1. Limestone deposited2. Sandstone deposited3. Shale deposited4. Folding

Oldest- Limestone

Youngest- Igneous Intrusion

Oldest- Shale

Youngest- Siltstone

Oldest- Sandstone

Youngest- Shale

Oldest- Sandstone

Youngest- Igneous Intrusion

Oldest- Shale

Youngest- Folding

Oldest- Shale

Youngest- Fault

12

3

456

Folding

12

34

6(erosion)7

1

23

4

1

2

34

5 1

2

34

5

6 12

3

4

5

6

5(tilting)

12

Oldest- Siltstone

Youngest- Igneous Intrusion

13link

1

2

3

5

64

7

unconformity formation