1. The Geologic Time Scale
Ch. 2: How do scientists study earth’s past?
The Geologic Time Scale
Lesson 4:
What is the Geologic Time Scale?

2. Scientists were able to put rock layers in order by relative age
The geologic time scale is a record of the geologic events and evolution of life forms as shown in the fossil record.
Geologists have to use the geologic time scale to show Earth’s history. This is because the time span of Earth’s past is so great.
The geologic time scale was developed by
Studying rock layers world wide
Studying index fossils world wide
Next, radioactive dating was used to find the absolute ages of where one part of the geologic time scale ends and a new one begins. (Igneous rocks have radioactive elements which decay over time and change into another element. Scientists can use this process to determine the absolute age of rocks.)
Scientists were able to put rock layers in order by relative age

3. Dividing Geologic Time
By studying the fossil record, major changes in life forms were found to occur at certain times. This is where one unit of geologic time ends and a new one begins.
Precambrian Time: -Covers 80% of Earth’s history
-started 4,600 million years ago ended 542 million years ago
-few fossils survived
Paleozoic Era: million years ago to 251 million years ago
-The Periods in this era are:
Cambrian, Ordovician, Devonian, Carboniferous, Permian
Mesozoic Era: million years ago to 66 million years ago
Triassic, Jurassic, Cretaceous
Cenozoic Era: - began 66 million years ago
-The Periods in the era are:
Paleogene, Neogene, Quarternary

4. Lesson 5: How did Earth Form?
Ch. 2: How do scientists study earth’s past?
Early Earth
Lesson 5:
How did Earth Form?

5. The oldest rocks ever found on Earth are about 4 billion years old!
4,000,000,000
But scientists think Earth formed even earlier than that—about 4.6 billion years old.

6. The Age of Earth Earth Takes Shape
Scientists have also used radioactive dating on moon rocks brought back to Earth by astronauts. They hypothesize that the moon is about the same age and that it was formed soon after Earth collided with another object.
Earth Takes Shape  
Scientists think that Earth began as a ball of dust, rock, and ice in space. Gravity pulled this mass together. Gravity increased as Earth grew larger.  
The collision of the dust, ice and rock created energy that raised Earth’s temperature, making it hot enough to melt, sinking the densest materials to Earth’s core and leaving the less dense material to harden as crust and mantle on Earth’s surface.

7. Gases from comets & volcanos:
1st: The Atmosphere  
Earth’s first atmosphere consisted of light gases such as hydrogen and helium. The gases were blown away by solar wind.
Earth’s second—and present—atmosphere was formed after collisions with comets added carbon dioxide, water vapor, nitrogen, and oxygen to the atmosphere. A comet is a ball of dust, gas, and ice that orbits the sun.
Gases from comets & volcanos:
-Carbon dioxide
-Water vapor
-Nitrogen
-Oxygen
Hydrogen
Helium

8. 2nd: The Oceans 3rd: The Continents
As the Earth’s surface cooled, scientists believe that oceans were formed when water vapor condensed to form rain, which gradually accumulated.  
Oceans formed about 4 billion years ago.
3rd: The Continents  
Scientists have found that Earth’s large landmasses, or continents, move over the Earth’s surface. For millions of years, they have repeatedly formed, broken apart, and crashed together again.
The first signs of continents were formed less than 500 million years after Earth formed.

9. Early Organisms 
Scientists have found fossils of the earliest known form of life on Earth. These single-celled organisms formed about 3.5 billion years ago.  They do not know when or where life began on Earth.
Fast forward 1 billion years to 2.5 billion years ago. Many organisms started to use the sun’s energy to make food. Photosynthesis played a crucial role in the spreading of life on Earth. During photosynthesis, organisms created oxygen.
The amount of oxygen in the atmosphere grew and allowed a layer of ozone to protect Earth from the sun’s ultraviolet rays. The ozone layer allowed organisms to live and grow on land.