VIDEO LESSON

LESSSON OBJECTIVES

By the end of this lesson, you will be able to:

• Explain the basic principles that geologists employ during the study of the relative age of rocks
• Discuss the major geological, climatological and biological events of each Era
• Compare the relative and absolute age of rocks

BRAINSTORMING QUESTIONS

Have you ever heard about the geological history of the Earth? How was the age of the Earth established? How do scientists determine the age of the Earth and its products?

Dear Online Learner! Geological timescale is the time showing the possible age of the Earth and its associated life forms. The scale divides the age of the Earth into Eons, Eras, Periods, and Epochs. It provides a review of Earth’s history and the major changes that occurred over time. Scientists determine the Relative and Absolute ages of rocks.

  Relative and Absolute Age of Rocks

Relative Age

Dear Online Learner! Relative age mainly depends on the analysis of the sequence of geological occurrences without giving due regard to the exact time of origin. It principally depends on the study of sedimentary rocks and often applies to local conditions interpretations. Geologists employ three basic principles (rules) during the study of the relative age of rocks:

• The principle of original horizontality
• The principle of superposition
• The principle of cross-cutting relationships

Dear Online Learner! The principle of original horizontality indicates that layers of sediments are originally placed horizontally under the action of gravity. This means that except for the disturbed sequences, sedimentary rocks are always deposited in nearly horizontal beds.

The principle of superposition asserts that in an undisturbed sequence of sedimentary rock layers (beds) or lava flows, the overlying bed is younger than the underlying rock.

Figure 1.4: Sample sedimentary rock-layers

The principle of cross-cutting relationships indicates that a rock-layer that cross-cuts another rock-layer is said to be younger than the rocks it cross-cuts. This is a condition where older rocks are cut by younger geologic features or igneous intrusions.

Absolute age

Dear Online Learner! Absolute age refers to the actual age of rocks given in numerical values through the analysis of the spontaneous decay of radioactive isotopes. The term isotope refers to the presence of an element in different forms.

Radioactive decay stands for the conversion of unstable (Parent) elements into daughter (Stable) elements through the gaining or losing of particles in their nucleus. For instance, Potassium-40 (40K) decays into Argon-40 (40Ar).

Table 1.1 Parent and Daughter isotopes and time-taken (half-life) for conversion

The time taken to convert from parent element to Daughter element is commonly measured in half-lives. The half-life of an isotope is the time taken for half of the parent isotope to change to its product atoms. The relative proportions of the Parent and Daughter isotopes are used to determine the number of half-lives.

Table 1.2 Proportion of Parent and Daughter isotopes during radioactive decay

Radioactive decay occurs when elements recombine to form new minerals during the processes of metamorphism or when magma cools. Radioactive elements found in igneous and metamorphic rocks are commonly used in rock-dating studies.

Based on the geological timescale, the history of the formation of the Earth is classified into four longer geological periods named Eons. Hadean, Archean, Proterozoic, and Phanerozoic are four major sub-divisions of the known Eons. The Hadean, Archean and Proterozoic Eons are often called Precambrian by scientists to refer to the geological time before the emergence of life on Earth. The Phanerozoic is the most recent Eon. It is further sub-broken into three Geological Eras named Paleozoic, Mesozoic, and Cenozoic.

Table 1.3 Geological timescale

Geologic Eras

Dear Online Learner! Four known Geological eras are identified in the history of the Earth. They are the Precambrian, Paleozoic, Mesozoic, and Cenozoic.

Figure 1.7 Distribution of Precambrian rocks (shields)

Figure1.8: Life forms evolved during the different geological eras (Crawford, 1998)

Precambrian Era (4.5 billion – 600 million years before the present)

• The Precambrian is the oldest of all the geological eras.
• Covers the time from 600 million to 4.5 billion years.  
• The time of solidification of the Earth and the formation of the oldest rocks.
• Rocks created during called crystalline basement complex rocks.
• The Hadean Eon of the Precambrian covers the time 4600 – 3900 Ma and not much is known about it.
• Archean is the other Eon covering some 1400 (3900 – 2500 Ma).
• The latest of the three Eons is the Proterozoic lasting from 2500–570 Ma.

Paleozoic Era (600 – 250 million years before the present)

• Covered the time from 600 to 250 Ma from the present.
• It is believed that it marked the beginning of life and is commonly referred to as the age of ancient life.
• Trilobites and shelled animals were the common species of the time.
• The Devonian, the fourth period of the Paleozoic, was rich in fish species and referred to as the age of fish.
• By the end of the Paleozoic, all continents of the Earth had joined together and created the Supercontinent named Pangaea

Figure 1.9 The Supercontinent (Pangaea) (Gabler et al., 2007)

Mesozoic Era (250-70 million years before the present)

• Marked the time from 250-70 Ma.
• Referred to as the era of middle life and the age of Dinosaurs.
• Turtles, snakes, crocodiles, and lizards were among the life forms of the time.
• Transgressions, regressions and depositions were common
• The mid-Mesozoic era was experiencing the splitting of Pangaea into Laurasia and Gondwanaland.
• The end of the Mesozoic era saw the emergence of land mammals but marked the mass extinction of Dinosaurs.

Cenozoic Era (70 million – present)

• Covering the time since 70 Ma.
• Birds, mammals, and flowering plants dominante succeeded on Earth.
• Named an era of recent life and the age of mammals.
• The rifting of Pangaea has been fully achieved and the continents retained their present form.
• Great volcanism and orogenic folding and fault- block mountains formed.
•  Glaciations were experienced and extinction of some mammals happened.