Unraveling Earth's Age: Geological History and Dating Methods

Historical Ideas on Earth's Age

• 1650: Archbishop James Ussher

  Anglican Archbishop James Ussher published a study that fixed the creation of the Earth in 4004 BC, precisely at 9 pm on October 23.

• Until the 18th Century

  The accepted age of the Earth was less than 6,000 years. This is known as short chronology or biblical chronology because it was based on a literal interpretation of the Bible.

The Answer Lies in the Rocks

Everything began to change in the 19th century when it was discovered that the past was preserved in the rocks. From this point, scientists attempted to "decipher" the geological code.

Reconstructing the Geological Past

Geologists seek answers to three fundamental questions:

1. What happened?

   Identifying the major events that have affected the Earth.

2. When did this happen?

   Determining the temporal order of events (what happened before or after).

3. How did it happen?

   This can be detected through analyzing:

   • Originating Materials: For example, a volcanic eruption leaves pyroclastics; a glacier leaves moraines; and a flood leaves sludge.
   • Generated Forms: For example, abrasion leaves a cliff platform; a glacier excavates a U-shaped valley.
   • Resulting Structures: Compression efforts result in folds or reverse faults, whereas periods of détente (extension) result in normal faults.

For this reconstruction, geologists use the Principle of Uniformitarianism, evaluating current processes to interpret the past.

Determining Geological Chronology

Dating Methods

• Relative Dating: Establishes the sequence of events (what happened before or after).
• Absolute Dating: Provides specific numerical figures indicating how many years ago an event occurred.

Fundamentals of Relative Dating

1. Original Horizontality

   Sediments originally form in horizontal layers. If inclined strata are found, it is deduced that they have suffered tectonic stress.

2. Superposition of Layers

   Sediments are deposited over one another. Therefore, upper layers will be more modern (younger) than the lower, older layers.

Criteria for Dating Layers

• Fossils

  The existence of fossils of known age may be sufficient to sort layers from oldest to newest (Principle of Faunal Succession).

• Graded Bedding (Granulometric Sorting)

  Sedimentation of materials ordered by size. This allows geologists to know which is the basis (the wall) and which is the top (the roof) of a stratum.

• Desiccation Cracks

  These are formed when clay sediments dry out. They appear as cracks on the surface that close with depth, indicating the original top surface.

Reconstruction of Geological History

The Principle of Cross-Cutting Relations

This principle states that any geological process (like faulting, folding, or intrusion) is younger than the materials it affects. This is often called the Principle of Succession of Events.