Unit 1- Earth Systems and Resources

Geological Time Scale

AIM: How have stratigraphy and radiometric dating been used to construct the geological time scale?

The History of Earth

Earth’s history has been studied through 2 methods:1) Examining sequences of rock layers2) Radioactive dating

Stratigraphy

• Stratigraphy- branch of geology that studies rock layers (strata)

Rules of Stratification

1) Rock layers (strata) are laid down in succession with each strata representing a “slice” of time

2) The principle of superposition- younger layers are on top, older layers are on bottom

Geology

• The fossil record of life on earth is captured in the rock layers of earth

• The fossil record is our greatest source of information about the evolution of life on earth

The History of Earth

Using fossil evidence and the concepts of stratification, Earth’s history has been organized into various units according to major events that took place in each period.

This is called the Geologic Time Scale

• Earth’s history has been divided into 4 major Eras, which are furthers categorized into Periods.

Earth’s Eras

• Cenozoic Era• Mesozoic Era• Paleozoic Era• Precambrian Era

TODAY

• Today, we are in the Cenozoic Era, in the Quaternary Period

• Cenozoic Era (65 mya)- begins after extinction of dinosaurs; characterized by the evolution and diversification of mammals

• Quaternary Period (1.8 mya) - Ice ages, evolution of human

The History of Earth

Scientists estimate that Earth is 4.5 billion years old

Radiometric Dating- decay of radioactive elements in rocks are quantified to determine age in years

• Relative Age- rules of stratification to identify a chronological series of evolutionary and geological events– Fossil evidence

• Strata are laid down in succession• For any given strata, the rock layer above is younger,

the rock layer below is older.

• Absolute Age- radioactive decay of elements in rock to determine age in years– “radiometric dating”

The Structure of Earth

Crust• Uppermost layer of earth• Continental Crust- 20-30 miles deep• Oceanic Crust- 7 miles deep• Rocky and brittle, and can fracture in earth

quakes

Mantle

• Composed of iron, magnesium, aluminum, and silicates

• Mostly solid, but the uppermost portion is more “plastic-like”

Core

• Center of the earth• Mosly iron– Outer core = solid– Inner core = liquid

HOMEWORK

15 Questions