CHAPTER 32

It is hard to measure history of earth since no one (human) was actually alive to record it.

Relative Time: places events in a sequence but does not actually assign them a certain time.

• Middle schoolers are 11-14 years old
• You are in middle school
• You are probably between 11-14 years old

Absolute Time: identifies the actual date of an event.

• Based on chemical tests, that rock is 2 million years old
• Physical tests show that you are 14 years old
• You were born on November 12, 2001

Absolute time can allow us to use relative time more accurately. If John is 14, then Tim is about 14 too…because they are in the same class.

Most work in telling geologic time however, is done using relative time.

Telling time (relative) involves a few rules:

• Law of Superposition: in a sequence of rocks, the oldest rock should appear on the bottom and the newer rocks "piled" on top.
• Law of Cross-cutting Relationships: the rock layer that intrudes or cuts across other rock layers is younger than the layers it intrudes.
• Law of Included Fragments: the a mass of rock layers, the fragments of other rock types found within the layers will be older than the layers of rock they are found in. (They would have had to have been first, prior to being found in the rock layers!)
• Unconformity: Like pages missing in a book…when you discover missing years in the rock record.

The Geologic Timetable: (Page 600-601!!!)

• Describes and charts out earth’s history!
• Made up of 5 major distinctions:
• Archean Era: 4-5 billion years ago
• Proterozoic Era: 2.5 billion years ago; simple plant life is found in this rock record
• Paleozoic Era: 570 million years ago; land and ocean plants and animals are found
• Mesozoic Era: 250 million years ago; Dinosaurs thrived during this period
• Cenozoic Era: 65 million years ago; still going today! Age of the mammals…that’s us!
• The eras are divided into periods and further subdivided into epochs.

The Periods:

• Cambrian – 570-510 million years ago
• Ordovician – 510-440 million years ago
• Silurian – 440-410 million years ago
• Devonian – 410-355 million years ago
• Mississippian – 355-320 million years ago
• Pennsylvanian – 320-290 million years ago
• Permian – 290-250 million years ago
• Triassic – 250-205 million years ago
• Jurassic – 205-135 million years ago
• Cretaceous – 135-65 million years ago
• Tertiary – 65-2 million years ago
• Quaternary – 2 million years ago to present

 

The Fossil Record

Fossil – any evidence of earlier life preserved in rock

• May be examples of shells, bones, petrified wood, impressions, footprints, and borrows

Fossils can be preserved several ways:

• Original remains: like bones found in rock
• Replaced remains: like trees preserved in the petrification process
• Molds & Casts: the process of making an imprint of a preserved item
• Trace fossils: trails, footprints, borings, or even waste

Evolution of the Fossil Record

Evolution – the process of change that produces new life forms over geologic time

Charles Darwin explained that through "natural selection", we can account for the changes that produce new life forms over time. He meant that those organisms who survive to produce offspring are those who inherited the most beneficial traits for surviving in a particular environment! Hence, he found that all organisms evolve gradually over many generations.

Evolution is may not follow a gradual path of change, but may follow a path interrupted by short period of dramatic change.

Additional methods of telling time:

Index fossils – known fossils in a rock layer that allow identification of other fossils found in the same rock layers.

Key bed – much like an index fossil; known layer used for correlating ages of nearby rock layers.

Correlation – the matching of rock layers from one layer to another; usually in different regions of the continent, etc. IE – layer of rock in New York matches layers found in Wisconsin… therefore fossils found in each can be the same.