Geologic Time

badlands viewpoint
Badlands National Park, South Dakota.

NPS photo by M. Reed.


Geologists start counting “geologic time” from Earth’s surface downward; that is, starting with younger surficial deposits and descending into older rocks and deeper time. Geologists count back more than 4 billion years to the oldest Earth materials. Astronomers help geologists count even farther back to the time of Earth’s formation, which may seem somewhat arbitrary because Earth did not appear instantaneously as a planet in space. Nevertheless, this “moment” may be defined as when Earth achieved its present mass. Scientific calculations presently place this event at ~4.6 billion years ago (International Commission on Stratigraphy).


Understanding the Depth of Geologic Time

Earth’s formation occurred ~4.6 billion years ago, that’s 4,600,000,000 or 4,600 million. You probably hear people use the number “one million” all the time, but a million is really big. Have you ever tried to count to a million? Counting once per second (easy at the start, but tough when you reach the hundred-thousand mark), 24 hours per day, seven days per week (no weekends off), it would take you 11 days, 14 hours to count to one million! There are a thousand millions in a billion, so counting to a billion would take you approximately 32 years. Taking this one step further, it is not humanly possible to count to 4.6 billion; that would take about 147 years of non-stop counting!

To help comprehend the length of geologic time, some analogies are provided below. Select an analogy:

If a piece of string an inch long (about 2.5 cm) represents one year, then 6 feet (about 183 cm) is equivalent to the average lifetime of a person living in the United States. A string representing all of recorded human history would be 1.6 miles (1.0 km) long. And a piece of string representing the age of Earth would be 72,600 miles (116,838 km) long. That length of string could wrap around Earth three times (Dalrymple 1991).
Say a quarter represents each year of Earth’s history. A stack of 4,600,000,000 quarters would be more than 5,000 miles (8,047 km) high. Such a stack could reach from where you are through the center of Earth and halfway to the other side (Dalrymple 1991).


Relative Age Dating

Relative age dating involves placing geologic events such as an ocean’s existence, a volcanic eruption, or the duration of a dune field in a sequential order. Rock formations can record these events: an ocean will result in marine limestone, a volcanic eruption in basaltic lava or a layer of ash, and a sand dune in sandstone. For a layer of rock to be considered a formation, it must spread across a relatively large area that can be depicted on a geologic map. Geologists determine the sequence of events from their position in the rock record with older events/rocks usually occurring in the lowest layers and later events higher in the rock sequence. Relative dating does not tell when a particular event occurred or how long it lasted—relative dating simply puts events in order of occurrence with respect to one another.

Geologists deduced the various principles used to determine relative dating hundreds of years ago. This set of Fundamental Geologic Principles, still in use today, is the basis for the construction of the relative geologic time scale.



Absolute Age Dating

Absolute age dating results in specific ages for rock units. Radiometric dating is the most common method for obtaining absolute-age dates. After the discovery of radioactivity and its application to age dating, geologists were able to make realistic determinations of Earth’s numeric age. They were also able to truly appreciate the antiquity and duration of the relative subdivisions of the geologic time scale. These dating tools have resulted largely from increasingly precise laboratory methods that enable geochemists to analyze very small quantities of particular elements with remarkable accuracy. Radiometric dating also has made possible the determination of rates of physical and biological processes, which has shed light on past developments of our planet.



Geologic Time Scale

The geologic time scale began to take shape in the 1700s. Geologists first used relative age dating principles to chart the chronological order of rocks around the world. It wasn't until the advent of radiometric age dating techniques in the middle 1900s that reliable numerical dates could be assigned to the previously named geologic time divisions.

To help comprehend the divisions of geologic time, some analogies are provided below. Select an analogy:

Geologic time began ticking when Earth formed ~4.6 billion years ago. Scaling this large amount of time to our calendar year, each of the 12 months of the geologic calendar year represents 383 million years (4.6 billion / 12). Generally speaking, each year has 365 days, so each day represents 12.6 million years (4.6 billion / 365) on our geologic calendar. Each day has 24 hours, so one hour represents 525,114 “geologic years” (4.6 billion / [365 × 24]). Each hour has 60 minutes, so one minute represents 8,752 “geologic years” (4.6 billion / [365 × 24 × 60]). Finally, each minute has 60 seconds, so each “geologic second” represents 146 years (4.6 billion / [365 × 24 × 60 × 60]).

Scaled to our geologic calendar, here are some geologic “holidays”:
January 1 Formation of Earth
February 13 Formation of oldest known rocks
March 27 First recorded forms of life
November 19 Cambrian “explosion” of hard-shelled life-forms
November 23 Life moves onto land (Ordovician)
November 26 First mass extinction (end of Ordovician time)
December 3 Second mass extinction (end of Devonian time)
December 12 Third and greatest mass extinction of all time (end of Permian time)
December 15 Fourth mass extinction (Triassic)
December 15 Dinosaurs become dominant
December 19 Fifth and most famous mass extinction; dinosaurs become extinct
December 19 Flowering plants begin to cover the landscape
December 31 Pleistocene ice ages (last 3 hours and 26 minutes)
December 31, 11:38 pm Homo sapiens (modern humans) appear
December 31, 11:59 pm Beginning of the geologic time in which we live (Holocene Epoch)
This analogy highlights the relative length of each geologic time period. Spread your arms wide. With the span of your arms representing all geologic time, look at one hand; your fingertips represent the formation of Earth and the beginning of geologic time. Now look at your other hand; the Cambrian Period begins in the wrist area of this hand, and the Permian extinction is at the other end of the palm. The Cenozoic Era is in a fingerprint, and with a single stroke of a nail file, you eradicate human history (McPhee 1998).

The Earth is about 4.5 billion years old, a number too large for people to conceptualize. If we were to shrink the Earth down to the size of a basketball and compress those 4.5 billion years into a few hours we would be able to observe radical changes. Continents would race around the globe, sink beneath the sea, rise up again, smash into other continents, build mountains, and erode back into the sea. Volcanoes would continually erupt and then quickly be weathered away. An astounding array of life would evolve and most of it would pass into extinction seconds later. Asteroids would occasionally slam into Earth. Indeed, the Earth would look like an extraordinarily dynamic little sphere before us.

From our reference point, change of this magnitude is hard to appreciate. Yet if we begin to grasp the immensity of geologic time, we can begin to recognize the changing nature of Earth.





Learn More

photo illustration oldest rocks!

Oldest Rocks in the Parks

Learn about the oldest rocks found in the parks that range in age from 3 billion to 600 million years old.

Earth from Space

Video: Big Ideas in Geoscience

From the American Geosciences Institute comes Big Idea 2: Earth is 4.6 Billion Years Old. Watch Earth form, and learn about Earth's history and the events of deep time. See what processes shaped the Earth we know today.

Educational Resources


Learning Activity: It's About Time

Have you ever wondered how geologic time works? This interactive classroom learning activity helps build the basic understanding of geologic time for grades 4-9.

Geologic Time Poster

Geologic Time Classroom Poster

Every park contains a slice of geologic time. In this classroom resource we highlight a few parks associated with each geologic time period.

Ranger Led Program

Geology, Relatives, and Time

Using a simple three or four generation family tree, students will construct a relatives time tree that mimics the major divisions of the geologic time scale (Precambrian, Paleozoic, Mesozoic, and Cenozoic). For Grades 9-12.


Last updated: February 12, 2024


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