Geologic Timeline — Reading Comprehension

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This educational passage for grade 4-5 students introduces the concept of geologic time on Earth, helping learners understand how our planet has changed over millions of years. The passage explains key scientific terms like fossils, eras, and extinction, and describes how scientists use clues from rocks and fossils to learn about Earth's history. Students will discover why geologic time is divided into different eras and periods, and how major events, such as the rise of dinosaurs or the extinction of certain species, have shaped life on our planet. The passage is aligned with NGSS science standards and supports vocabulary development with a glossary. Activities include a multiple-choice quiz and writing prompts that encourage critical thinking and connections to real-world science. Integrated audio and a Spanish translation make this resource accessible for diverse learners. The content is ideal for classroom or home learning and is designed to spark curiosity about geology and Earth's past.

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Geological time spiral - sharper — "Geological time spiral - by United States Geological Survey. Philcha at English Wikipedia.
Wikimedia Commons

Geologic time is the way scientists describe the long history of Earth. Instead of using days or years, they use millions and billions of years to show how our planet has changed over time. The geologic time scale helps scientists organize Earth’s history into different sections, called eras and periods.

Earth is about 4.6 billion years old. Over this huge amount of time, the planet has gone through many changes. Scientists use fossils as clues to learn about what life was like long ago. A fossil is the remains or imprint of a plant or animal that lived a long time ago and became trapped in rock. For example, dinosaur bones and ancient leaves are fossils. By studying them, scientists can tell when certain animals lived and when they disappeared.

The geologic time scale is divided into major eras: the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. Each era is marked by big changes in life on Earth. For example, the Mesozoic Era is known as the Age of Dinosaurs, because dinosaurs were the main animals alive then. The Cenozoic Era is called the Age of Mammals, because mammals became more common after the dinosaurs went extinct.

Extinction happens when a type of plant or animal dies out completely. Mass extinctions, like the one that ended the dinosaurs, changed the course of life on Earth. These events help scientists divide time into different eras and periods.

Geologic time helps us understand how Earth’s surface, climate, and life have changed. For example, rocks and fossils found in the Grand Canyon tell us about ancient oceans and creatures that lived there millions of years ago. Learning about geologic time teaches us how slow and amazing Earth’s changes can be.

Interesting Fact: The oldest rocks on Earth are over 4 billion years old, almost as old as the planet itself!

What are fossils?

Remains or imprints of ancient lifeTypes of rocks found in riversTools used by scientistsWeather patterns from the past

How old is Earth?

About 4.6 billion years oldAbout 500 million years oldAbout 2,000 years oldAbout 6 million years old

What is an era?

A major division of geologic timeA type of dinosaurA type of rockA kind of fossil

Why do scientists use geologic time?

To organize Earth's historyTo measure weather changesTo find goldTo grow plants

What caused the Age of Mammals?

Dinosaurs went extinctVolcanoes formedOceans dried upIce Age started

What do fossils help scientists do?