Geologic Principles and Relative Time Determination

Which Came First? Relative Time Puzzles

Introduction

In this lesson, students will be engaged in the process of ordering rock layers to establish their relative age. They will utilize geologic principles to determine the relative age of rock formations and provide supporting evidence concerning the geologic history of Earth.

Vocabulary

  1. Absolute age – The numeric age of a rock layer or fossil.

  2. Erosion – The mechanical abrasion of material on the Earth’s surface caused by glaciers, wind, and water.

  3. Fault – A fracture in the Earth’s crust where one side moves relative to the other.

  4. Fold – Bent or curved strata that have formed due to compressional forces.

  5. Intrusion – Igneous rock that is formed by the crystallization of magma that has pushed into preexisting strata.

  6. Relative age – The chronologic sequence of a rock, fossil, or geologic feature compared to other rocks, fossils, or features without indicating a specific numeric age.

  7. Strata – Layers of rock.

  8. Unconformity – A surface of erosion or non-deposition that separates rock layers of significantly different ages.

The Study of Strata

Geologists study rock layers, known as strata, to gain insights into the various climates, environments, and organisms that have existed over Earth’s 4.6-billion-year timeline. The examination of strata and their relative positioning commenced in the early 19th century; geologists leveraged direct observations of rock layers to correlate strata from different locations. Unlike methods that provide a specific numeric age, these observations enable scientists to determine the relative ages of rock layers based on their chronological arrangement.

Principles of Relative Age Determination

Various principles guide the determination of relative age:

  1. The Principle of Uniformitarianism
    This principle posits that changes in the Earth’s crust arise from continuous and uniform processes (e.g., storms, earthquakes, volcanism, weathering). The processes observed today are analogous to those that have occurred throughout geologic time, and their effects can be traced within the rock record.

  2. The Principle of Superposition
    According to this principle, newer layers of rock are situated atop older layers. Each layer is older than the one that lies above it.

  3. The Principle of Original Horizontality
    This principle states that sediments are typically deposited in flat, horizontal layers aligned with the Earth’s surface. Any layers of sedimentary rock that are not horizontally aligned suggest they have undergone alterations due to tectonic activities like folding or faulting.

  4. The Principle of Lateral Continuity
    This principle asserts that rock layers extend horizontally in all directions until they thin out or encounter a physical barrier. Breaks in rock layers occur due to barriers during deposition or through subsequent erosion.

  5. Cross-Cutting Relationships
    Involves features, such as intrusions or faults, that cut across older rock layers, which indicates that the cross-cutting features must be of a younger age compared to the layers they intersect.

  6. Inclusions
    Inclusions refer to fragments of older rocks found encased within younger rocks. Consequently, when a rock layer contains inclusions, it is established that this layer must be younger than the rock from which the inclusions originated.

Unconformities

An unconformity is defined as a surface of erosion or lack of deposition that separates rock layers of considerably different ages. There are three primary types of unconformities:

  1. Angular Unconformity
    This occurs when horizontal sedimentary rock is deposited atop tilted and eroded rock layers. Initially, the lower layers, which are older, become inclined due to tectonic activities (e.g., folding or faulting), followed by erosion, upon which younger sediment layers are deposited.

  2. Disconformity
    A disconformity arises when horizontal sedimentary rock is layered on top of older, eroded sedimentary rock. In this case, the strata are parallel yet separated by an erosional surface.

  3. Nonconformity
    A nonconformity results when horizontal sedimentary rock is deposited over older eroded igneous or metamorphic rock. The disparate rock types are separated by an erosional surface.

    • Example: The Grand Canyon exemplifies a nonconformity, with Precambrian metamorphic rocks lying beneath horizontal Cambrian sedimentary rocks.