Fossil Dating and Earth's Geological History

Understanding Fossils: Records of Past Life

Fossils are the only "documents" of the past, providing crucial information about how life existed historically. They serve to determine:

• Shape, size, and anatomy.
• Lifestyle and diet.
• Geographical distribution.

Fossils, Sediment, and Rock Formation

The sediment that buried the body became the rock containing the fossil today. Therefore, the environment in which the organism lived is usually the same in which the rock was formed.

Dating Rocks Using Fossils

If we know the time period an organism lived, we can determine the age of the rock containing the fossil, allowing us to date it.

Characteristics of Index Fossils

Index fossils are essential for correlating rock layers and must meet specific criteria:

• Lived for a short time period.
• Had a broad geographical distribution.
• Must be abundant in the rocks.

Absolute Dating Methods

Absolute dating provides a numerical age for geological materials, often relying on radioactive decay.

Radioactive Element Decay

Parent Element:

The initial radioactive element.

Daughter Element:

The final stable element resulting from decay.

Half-life:

The time required for half of the parent element atoms in a sample to decay into daughter elements.

By measuring the ratio of parent-to-daughter elements and knowing the transformation rate (half-life), we can calculate the time elapsed. This calculation determines the absolute age of the rock.

Defining Geological Time

Key Concepts in Geochronology

Geological time is the period since the Earth was formed until the present day (approximately 4,560 million years ago).

• Unit of Measure: A million years.

Divisions of Geological Time

Eons:

• Precambrian
• Phanerozoic

Eras:

• Archean
• Proterozoic
• Paleozoic
• Mesozoic
• Cenozoic

Relationship Between Eons and Eras

• Precambrian Eon: Includes the Archean and Proterozoic Eras.
• Phanerozoic Eon: Includes the Paleozoic, Mesozoic, and Cenozoic Eras.

The Formation of Earth and the Solar System

Five Stages of Planetary Origin

1. A rotating nebula, larger than the current solar system and consisting of huge amounts of dust and gas, began to concentrate.
2. Gravitational pull formed a central mass, the protosun, surrounded by a spinning disk of dust and gas. The size of the dust particles would have been only a few millimeters.
3. As the protosun concentrated and increased in temperature, the particles fused within the rotating disk, forming planetesimals, bodies several hundred meters in diameter.
4. Collisions and mergers of planetesimals originated larger bodies, protoplanets. The diameter of these protoplanets ranged from hundreds to thousands of miles. One of these protoplanets formed the Earth.
5. The protoplanets continued to accumulate more planetesimals, clearing their orbits until they were relatively clean of debris.

Materials that were not incorporated into planets and satellites during formation remain today as asteroids and comets, serving as the only witnesses left of the formation of our solar system.