1. EARTH’S HISTORY

2. In this unit we learn to:
THINK logically about the sequence of geologic events. . . Kind of like putting together pieces of a puzzle!
Explore how rocks and fossils tell us about past environments and extinct life!

3. There are a few basic guidelines for solving the mysteries of ancient earth:
SEQUENCE (the order of events)
CORRELATION (matching rocks, events and locations)
FOSSILS AND ROCKS AS CLUES (about earth’s past)

4. SEQUENCE IS IMPORTANT: What happened first … and what’s most recent?
Determining sequence involves looking at the position and characteristics of rock layers

5. Just think about it. When you know how sedimentary rocks form, it’s very logical!
The bottom layer of sedimentary rocks is the oldest, unless it has been disturbed.
This is the “Law of Superposition”

6. One basic guideline is the “Principle of Uniformitarianism”
It’s NOT a religion!
Basic idea is that geologic forces affecting earth today were also shaping earth millions of years ago

7. And you know, age is relative!
Young and old. . .

8. In geologic terms, Relative Age is quite basic:
Which layer is older?
Which geologic event is younger?
Sequence involves looking at rock layers, igneous intrusions, faults and folds as well as layers of volcanic ash

9. Igneous intrusions are younger than the rocks they cut through, as are faults and folds
The sedimentary rocks have to form before they can be folded or intruded upon! ! !

10. In a drawing an intrusion is indicated by “little hairy lines”
When the magma flows into existing rock, it forms contact metamorphism…
altering what it comes in contact with!

11. Volcanic ash is an excellent time marker
Eruptions can deposit ash rapidly over large areas, sometimes in multiple layers during different time periods

12. This physical evidence can be observed, analyzed and mapped

13. SOMETIMES GEOLOGISTS FIND AN UNCONFORMITY
A gap in the rock record
Physical evidence (like layers of rock) that once existed but are now missing
Caused by erosion

14. So, how can rocks and events from different locations be CORRELATED?
By “walking the outcrop”
Looking for similarities in thickness, color and fossils from one place to another

15. OH, SPEAKING OF FOSSILS, THEY...
Give us clues about where and when plants and animals lived in the past
Show evolutionary changes in species
Form if an organism is buried in sediment soon after death
Are usually preserved remnants of shells, bones, teeth or other hard body parts

16. INDEX FOSSILS HELP US KNOW THE AGE OF ROCKS,SO THEY ARE VERY SIGNIFICANT !
They are fossils of organisms (species) that lived for a short time period but were widespread geographically
So, if you find an index fossil, you know that the rock it’s in formed during that particular time period!

17. And speaking of time, earth’s history is divided into GEOLOGIC TIME PERIODS
Geologic time is based on the fossil record
Time is divided by periods when life forms are present in fossils . . .
Jurassic Period!

18. THE GEOLOGIC TIME SCALE IS ENORMOUS… SPANNING 4.5 BILLION YEARS!

19. It’s based on analysis of the rocks and the record of earlier life!

20. And then there is Absolute Age
An actual age of rocks or fossils determined by radioactive dating using
naturally occurring radioactive elements in rocks
Analyzes the amount of radioactive decay over time

21. Radioactive Decay Is predictable regardless of pressure or heat
WOW… THINK about it!
That’s deep!

22. Radioactive Decay:
The unstable radioactive “parent material” breaks down into the more stable “daughter product” at a predictable rate called
half-life

23. Half-life is predictable
Carbon-14 has a half-live of 5700 years, perfect for absolute dating of more “recent” geologic events or fossil remains