1. Time and History of Earth
Why study Earth history and time?
Relative dating
Absolute dating
The Geologic Time Scale
Ages and rates of earth objects and events.

2. Why Study Earth History?
1. Exploration for natural resources
- ie, locations of tectonic/hydrothermal activity
2. Natural evolution of earth systems
- ie, climate change, past to future
3. Assessing geological hazards
- ie, risk = frequency x magnitude
4. Understanding our surroundings
- ie, how did the Grand Canyon form?

3. Geologic Time
Concept of geologic time is based upon the Principle of Uniformitarianism or Actualism. This concept assumes that the same processes and natural laws that have shaped Earth’s history in the past are operative today.
“The present is the key to the past”

4. Timing of Events Relative time Absolute time
chronological sequence of events without regard to age.
Absolute time
dates and rates of events in years.

5. Relative Dating Stratigraphic relationships Fossil sequence Key beds
Original horizontality
Superposition
Cross-cutting and inclusions
Unconformities
Fossil sequence
Faunal secession
Index fossils
Key beds

6. Stratigraphic Principles
Original horizontality
Sediments are deposited in horizontal layers parallel to Earth’s surface.
Superposition
Younger layers are deposited on top of older layers.
Cross-cutting and inclusions
Feature that cuts is younger than feature being cut, and inclusions are older than host rock.

7. Unconformities Represent interruptions in deposition.
Disconformity - sedimentary layers are parallel above and below.
Angular unconformity - older beds are tilted relative to younger beds.
Nonconformity - sedimentary rocks lie on igneous or metamorphic rocks.

8. Fossil Sequence Principle of Faunal Succession Index Fossils
Fossil organisms succeeded one another through time in a definite and recognizable order.
Index Fossils
Geologically widespread organism (fossil) that existed for a relatively short time interval, was abundantly preserved, and is easily identified in the field.

9. Key Beds Thin, widespread and easily recognizable rock/sediment layer.
Often layers of volcanic ash from large volcanic eruptions.
Useful for both relative and absolute dating.

10. Correlating Rock Units
Physically separated rock units can be correlated using:
Principle of lateral continuity
Physical similarities
Similar stratigraphic sequence/position
Index fossils and fossil assemblages

11. Absolute Dating
Used to provide an age for an event or object in years.
Historical records
few thousand years
Tree rings/glacial ice core records
10,000 years
Radioactive decay
age of solar nebula

12. Radioactive decay
Decay of unstable isotopes provides an internal clock for minerals and rocks.
Unstable parent isotope spontaneously decays to daughter isotope at a constant rate.
The decay rate is typically reported as the half-life of the unstable isotope, where the half-life represents the amount of time (in years) required for one half of an unstable isotope to decay.

13. Isotope Systems Commonly Used for Dating Rocks
Isotope Half-life Dating range Materials Used
(parent/daughter) (years) (years)
40K/40Ar 1.25 billion 50, billion mica,K-feldspar
238U/206Pb 4.5 billion 10 million-4.6 billion zircon, galena
235U/207Pb 713 million 10 million-4.6 billion zircon, galena
87Rb/87Sr 49 billion 10 million-4.6 billion mica, hornblende
14C/14N 5, ,000 organic material,
trapped CO2

14. Geologic Time Scale
Based upon fossil assemblages (absolute ages were determined after the scale was established).
Consists of Eons, Eras, Periods, and Epochs
Cenozoic Era - “age of mammals”
Mesozoic Era - “age of reptiles”
Paleozoic Era - “age of fishes and amphibians”

15. Important Events in Earth History
Age of Earth 4.6 billion years
Oldest Terrestrial Rock 4.26 billion years
First Marine Organisms 570 million years ago
First Land Plants 408 million years ago
Dinosaurs million years ago
First Mammals 200 million years ago
First Flowering Plants 140 million years ago
Earliest Human Ancestors 4-5 million years ago

16. Rates of Earth Processes
Magma Ascent (Mt. St. Helens) 15->66 m/hr Basaltic lava flow m/hr Sea-floor Spreading mm/year Uplift of Mountains 1-8 mm/year Subsidence mm/year Erosion of River Valleys mm/year Accumulation of Deep Sea Sediments mm/year

17. Review - Earth History What is relative time?
How do we determine relative time?
What is absolute time?
How do we determine absolute time?
What is the Geological Time Scale, what is it based on?