Clocks in Rocks: Timing the Geologic Record
Chapter 8: Clocks in Rocks - Timing the Geologic Record
Introduction to Clocks in Rocks
Concept of "Clocks in Rocks":
Geologists utilize the concept of "clocks in rocks" to uncover deep geological time.
These clocks serve as a tool to measure the duration of various geological processes and cycles within the Earth system.
Geologists categorize ages into relative age and absolute age.
Lecture Outline
Reconstructing geologic history from the stratigraphic record
Geologic time scale: relative ages
Measuring absolute time with isotopic clocks
Geologic time scale: absolute ages
Recent advances in timing the Earth system
Reconstructing Geologic History from the Stratigraphic Record
Principles of Stratigraphy
Stratigraphy: The study of rock layers (strata) and layering (stratification).
Key Principles
Original Horizontality:
States that sediments are initially deposited in horizontal layers. This principle suggests that any rock layers that are not horizontal must have been influenced by tectonic activity after their formation.
Superposition:
As a general rule, in an undeformed sequence of sedimentary rocks, the oldest layers are at the bottom, and the younger layers are at the top.
Faunal Succession
Faunal Succession Principle:
Fossil organisms succeed one another in a definitive and recognizable order. Therefore, the presence of certain fossils in rock layers allows geologists to determine their relative ages.
Examples of Stratigraphic Principles
Sedimentation Example:
Sediments, like those in Marble Canyon, Arizona, are deposited in horizontal layers in a lake or ocean.
Thought Questions
William Smith's contributions in 1793 highlighted how fossils could be utilized to determine relative ages of sedimentary rocks.
This foundational work led to the principle of faunal succession. Consider why animal fossils were predominantly used over plant fossils by Smith in his mapping efforts.
Stratigraphic Principles and Unconformities
Unconformities
Definition: Gaps in the geologic record where layers have been eroded or were never deposited.
Types of Unconformities:
Disconformity: Layers of sediment become eroded, leading to a gap.
Angular Unconformity: Occurs when sedimentary layers are tilted and then eroded before new layers are deposited on top.
Case Study on Disconformity
**Process of Disconformity:"
Below the ocean, sedimentary beds accumulate in contiguous layers A-D.
Tectonic uplift exposes these layers to erosion.
Erosion removes layer D and part of layer C, leaving an irregular ground surface.
Subsequent subsidence allows new layers (layer E) to be deposited over the eroded surface (layer C), forming a disconformity.
Case Study on Angular Unconformity
Visualization and Process:
Initial Accumulation: Sediments accumulate in beds at the ocean floor.
Tectonic Forces: Compression leads to uplift and deformation.
Erosion: The tops of folds are eroded, exposing folded beds.
New Sediment Deposition: Subsidence below sea level permits the deposition of new sediments over the folded beds, creating an angular unconformity.
Cross-Cutting Relationships
Definition: In geology, when a geological feature cuts across another, the one that has been cut is older.
Example: Cross-Cutting Relationships Process
Sedimentary layers accumulate in beds below the ocean.
Uplift and deformation occur due to tectonic forces.
A dike intrudes into the folded beds, cutting across them, indicating digestion and folding proceeded the intrusion.
Faulting finally displaces both the sedimentary beds and the dike, thus indicating that the faulting occurred after the formation of these features.
Geologic Time Scale
Relative Ages
Divisions of geologic time include eras, periods, and epochs. The time scale is used to understand relationships between layers and determines the characteristics of strata based on geological evidence.
Example of a Stratigraphic Cross-Section
The geologic column includes various formations characterized by distinct fossils, aiding in the understanding of relative ages among geological materials.
Measuring Absolute Time with Isotopic Clocks
Isotopic Dating Theory
Basic Principles:
Utilizes radioactive decay of isotopes to determine numerical ages. The terms include parent isotopes and daughter isotopes.
Half-Life: The time required for half the quantity of a radioactive isotope to decay.
Methods of Isotopic Dating
Uranium-Lead Dating
Potassium-Argon Dating
Rubidium-Strontium Dating
Carbon-Nitrogen Dating
Major Radioactive Elements Used in Radiometric Dating
Table of Isotopes:
Parent Isotope
Daughter Isotope
Half-Life (years)
Effective Dating Range (years)
Uranium-238
Lead-206
4.4 billion
10 million-4.6 billion
Uranium-235
Lead-207
0.7 billion
10 million-4.6 billion
Potassium-40
Argon-40
1.3 billion
50,000-4.6 billion
Rubidium-87
Strontium-87
47 billion
10 million-4.6 billion
Carbon-14
Nitrogen-14
5730
100-70,000
Example of Rubidium-Strontium Decay
Rubidium-87 decays into Strontium-87 by ejecting an electron. This transformation can affect the isotopic composition of rock samples examined.
Geologic Time Scale: Absolute Ages
Four Eons of Geologic Time
Eons: Hadean, Archean, Proterozoic, Phanerozoic.
**Eras and Periods: ** The Phanerozoic is subdivided into Paleozoic, Mesozoic, and Cenozoic eras, each containing distinct periods.
Recent Advances in Timing the Earth System
Developments include:
Sequence Stratigraphy: The study of sedimentary deposit sequences over time.
Chemical Stratigraphy: Analyzing the chemical signatures of rock formations for dating purposes.
Paleomagnetic Stratigraphy: Utilizing the magnetic properties of rocks to aid in dating.
Clocking the Climate System: Understanding climatic changes through stratigraphic methods.