Question

What is the next smaller division of geologic time after the era? A) period C) epoch B) stage D) eon

Short answer

Period is the next smaller division after an era.

Step-by-step solution

Understanding Geologic Time Scales

To solve this exercise, we need to understand the hierarchy of geologic time scales. Geologic time is divided into several levels of subdivisions, organized from largest to smallest: eon, era, period, epoch, and stage.

Identifying the Order of Divisions

From the hierarchy, we can see the order: eon > era > period > epoch > stage. Each division represents a significant period characterized by major geological or paleontological events.

Determining the Next Smaller Division After an Era

Given that the order is eon > era > period > epoch > stage, the next subdivision smaller than an era is a period.

Key concepts

Era

An era is a large division in geologic time that marks significant changes on our planet. These changes can be the result of large-scale geological activities or major shifts in the types of life forms that inhabit Earth. For instance, the Paleozoic, Mesozoic, and Cenozoic are renowned eras that have been defined by substantial evolutionary milestones and continental shifts.
Erally, eras cover hundreds of millions of years and are characterized by the dominance of particular life forms.

• The Paleozoic Era saw the rise of complex life forms in the oceans before moving onto land.
• The Mesozoic Era is famously known for the age of reptiles, including dinosaurs.
• The Cenozoic Era is often referred to as the age of mammals.

These eras are part of a larger time division within geological history known as eons.

Period

A period is a subdivision of an era and tends to last tens of millions of years. Periods are typically defined by geology and paleontology, marking distinct patterns in the rock layers and fossil records that differ from those of other periods in the same era.
For instance, the Mesozoic Era is divided into the Triassic, Jurassic, and Cretaceous periods. Each of these periods is characterized by unique geological and biological attributes:

• The Triassic Period witnessed the emergence of the first dinosaurs and the breakup of the supercontinent Pangaea.
• In the Jurassic Period, dinosaurs flourished and the first birds appeared.
• The Cretaceous Period ended with a mass extinction event that led to the demise of the dinosaurs.

Periods allow scientists to communicate about earth's past with more precision, recognizing specific geological events and shifts.

Epoch

Epochs are yet smaller divisions of geological time within periods. They help to refine and specify changes within periods based on finer geological and climatic details. Epochs can last millions of years and often mark crucial environmental changes.
For example, during the Cenozoic Era, the Tertiary Period, which was divided into the Paleogene and Neogene, comprises several epochs:

• The Paleocene Epoch saw mammals begin to diversify rapidly after the dinosaur extinction.
• The Eocene Epoch experienced further mammalian evolution and a rise in temperature leading to lush, plant-filled landscapes.
• During the Oligocene Epoch, climates began to cool, paving the way for new mammalian adaptations.

These further divisions allow geologists to gain insights into the environmental conditions and life forms that defined narrower timeframes in Earth's vast history.

Hierarchy of Geological Divisions

The hierarchy of geological divisions is a structured framework used to organize Earth's history into different time scales. This hierarchy helps scientists and researchers to examine the changes our planet has undergone over billions of years and communicate these changes in an organized manner.
The order of these divisions from largest to smallest is eon, era, period, epoch, and stage. By using this structured order:

• Eons represent the largest chunks of time, covering over half a billion years.
• Eras follow eons and usually span several hundred million years.
• Periods fall within eras, lasting tens of millions of years.
• Epochs come after periods, marking significant evolutionary or climatic changes within shorter spans.
• Stages are the finest divisions, reflecting more detailed changes usually over a few million years.

This systematic approach allows geologists to evaluate the Earth's geological and biological history in a clear and well-organized manner.