Guided Notes

Geologic History

Relative Age Sequence of Events

Correlation Techniques Volcanic Ash Markers

Index Fossils Geologic Time Scale

Evolution Radioactive Dating

1) How do we determine how long ago events on Earth occurred?

• Geologic history: study and interpretation of Earth’s past

Relative Age Absolute Age

Example: Example:

The age of rocks or events compared to the age of other rocks or events

The age of a rock, object, or event in years

I am a teenager. I am 16 years old.

2) How do scientists determine the relative ages of rock layers?

▪ To determine order, or chronology, of rock layers, geologists use a few major principles:

1. Uniformitarianism: the geologic processes that took place in the past are generally similar to those that take place now.

▪ The best way to understand the geologic events that shaped our planet is to look at the geologic processes at work today.

2) How do scientists determine the relative ages of rock layers?

2. Principle of Original Horizontality: sediments are deposited in horizontal layers that are parallel to the surface they were deposited on.

• Therefore, tilted or folded layers indicate that the crust has been deformed.

2) How do scientists determine the relative ages of rock layers?

3. Principle of Superposition: in undisturbed layers, the oldest layer is on the bottom and each overlying layer is younger

▪ Does not apply if layers have been overturned or faults have forced older rock over younger rock.

Exposed Rock Layers Photograph & Diagram

3) What events can disrupt horizontal bedrock?

1. Igneous Intrusions & Extrusions: As hot magma squeezes into cracks and zones of weakness, the cooling magma passes its heat energy to the nearby rock.

3) What events can disrupt horizontal bedrock?

▪ Examine the rocks on either side of the igneous mass…

Intrusion is younger Extrusion is older

Contact metamorphism on both sides of bedrock

Contact metamorphism is only on the rock layer beneath the igneous mass

Youngest Igneous IntrusionSandstoneShale

Oldest

Youngest SandstoneIgneous IntrusionShale

Oldest

3) What events can disrupt horizontal bedrock?

2. Erosion: carrying away of weathered rock materials

▪ Unconformity: A gap in the rock record cause by the burial of an eroded surface by younger sediments. Shown on rock outcrop by a wavy line.

Unconformity Picture & Diagram

3) What events can disrupt horizontal bedrock?

3. Folding: layers of sedimentary rock that have been bent or warped by crustal forces

3) What events can disrupt horizontal bedrock?

4. Tilting: formally horizontal rock layers that have been tilted at an angle by crustal activity

Bedrock Tilted at a 90˚ Angle

3) What events can disrupt horizontal bedrock?

5. Faulting: a crack of weakness in Earth’s crust along which movement occurs during an earthquake

4) How can geologists determine which disruption happened first?

▪ Law of Cross-Cutting: a disruption that cuts through rock or another geologic feature must be younger than the rock or the other geologic feature

✓ Checkpoint – Cross-Cutting: Determine the sequence of events for the diagrams below from youngest to oldest.

Youngest: _________ _________ _________ _________ _________ Oldest: _________