3.3.1: Deep Time

Unit 3.3.1 - Deep Time

• Please read and watch the following Mandatory Resources
• Reading the material for understanding, and taking notes during videos, will take approximately 1.5 hours.
• Optional Activities are embedded.
• Bolded terms are located at the end of the unit in the Glossary. There is also a Unit Summary at the end of the Unit. 
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Learning Objectives

• Identify the major time frames on the geologic timeline
• Explain the evidence scientists use to explain the age of the Earth and mass extinctions

Geologic Time  

The geological time scale in Figure \(\PageIndex{1}\) is one of the crowning achievements of science in general and geology in particular. It is a reference and communication system for comparing rocks and fossils from throughout the world and is geology's equivalent of the periodic table of the elements. It is also an important way to help organize when important milestones in the history of life on Earth occurred. For example, a paleontologist can call her colleague and say, "I just found an amazing new trilobite from the Devonian of British Columbia" and her colleague will immediately understand when in geological time that trilobite lived.

Most of the boundaries on the geological time scale correspond to the origination or extinction of particular kinds of fossils. Knowing when major groups of fossils first appeared or went extinct is therefore incredibly useful for determining the ages of rocks in the field. For example, if someone finds a rock with a trilobite fossil upon it, they would immediately know that the rock is Paleozoic in age (541 Ma to 252 Ma) and not older or younger; knowing the species of trilobite allows even greater precision.

The geological time scale provides a global summary of countless small-scale temporal correlations of rock layers made at local and regional scales. It is based almost entirely upon careful observations of the distributions of fossils in time and space.

Figure \(\PageIndex{1}\): The geological time scale. Ka = thousands of years, Ma = millions of years, and Ga = billions of years. (Jonathan R. Hendricks CC-BY-SA 4.0)

Eons

The eon is the broadest category of geological time. The Earth is 4.54 billion years old. Earth's history is characterized by four eons; in order from oldest to youngest, these are the Hadeon, Archean, Proterozoic, and Phanerozoic. Collectively, the Hadean, Archean, and Proterozoic are sometimes informally referred to as the "Precambrian". (The Cambrian period defines the beginning of the Phanerozoic eon; so, all rocks older than the Cambrian are Precambrian in age.)

Humans live during the Phanerozoic, which means "visible life". This is the interval of geological time characterized by abundant, complex fossilized remains. Being the youngest eon of time, it is also very well represented by rocks at Earth's surface. Because of these two factors, most paleontologists and geologists study fossils and rocks from the Phanerozoic eon. The Phanerozoic eon began 541 million years ago (or, 0.541 billion years ago). Thus, the Phanerozoic eon represents only 12% of Earth's history!

Most of Earth's history is represented by the three Precambrian eons. These older eons tell the story of Earth's beginning, life's origin, and the rise of complex life. The Hadean and Archean are difficult eons to study, however, because they are exposed in very limited places on Earth's surface. Since they are the oldest eons, rocks that are Hadean and Archean in age are often buried far below younger rocks at Earth's surface. Proterozoic rocks--which span nearly 2 billion years (42% of Earth's history)--are much more accessible, but, until recently, have received significantly less attention from paleontologists than rocks from the younger, fossil-rich Phanerozoic eon. That is slowly beginning to change, however, as more clues about the origins of complex life begin to be revealed from Proterozoic-aged rocks.

Eras

Eons of geological time are subdivided into eras, which are the second-longest units of geological time. The Phanerozoic eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic.

Most of our knowledge of the fossil record comes from the three eras of the Phanerozoic eon. The Paleozoic ("old life") era is characterized by trilobites, the first four-limbed vertebrates, and the origin of land plants. The Mesozoic ("middle life") era represents the "age of dinosaurs," though also is noteworthy for the first appearances of mammals and flowering plants. Finally, the Cenozoic ("new life") era is sometimes called the "age of mammals" and is the era during which we live today.

As temporal points of reference, it is worth memorizing the ages of the boundaries that separate the three eras of the Phanerozoic eon. Long before geologists knew these absolute age dates, they realized that the boundaries represent important events in the history of life: mass extinctions. For example, many fossils that are commonly found in the youngest Paleozoic rocks are not found in overlying Mesozoic rocks. Similarly, dinosaur fossils found in the youngest Mesozoic rocks are never again found in the overlying Cenozoic rocks. Mass extinctions are when large numbers of species go extinct all within a very short period of geologic time. Paleontologists and geologists used these mass extinction events to define these (and other) boundaries within the Phanerozoic portion of the geological time scale. It is therefore no coincidence that some of the major boundaries coincide with mass extinction events!

The older Archean and Proterozoic eons are similarly divided into several eras. For example, the youngest era of the Proterozoic eon is called the Neoproterozoic. For the sake of simplicity, these older eras are not included on the time scale shown at the top of this page; they do, however, exist.

Periods

Just as eons are subdivided into eras, eras are subdivided into units of time called periods. The most well-known of all geological periods is the Jurassic period of the Mesozoic era. The Paleozoic era is divided into six periods. From oldest to youngest, these are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Note that in the United States, the Carboniferous is divided into two separate periods: the Mississippian and the Pennsylvanian. The Mesozoic era is divided into the Triassic, Jurassic, and Cretaceous periods. Finally, the Cenozoic era is divided into three periods: the Paleogene, Neogene, and Quaternary.

Epochs and Ages

Periods of geological time are subdivided into epochs. In turn, epochs are divided into even narrower units of time called ages. For the sake of simplicity, only the epochs of the Paleogene, Neogene, and Quaternary periods are shown on the time scale at the top of this page. It is important to note, however, that all of the periods of the Phanerozoic era are subdivided into epochs and ages.

The Paleogene period is divided into--from oldest to youngest--the Paleocene, Eocene, and Oligocene epochs. The Neogene is divided into the Miocene and Pliocene epochs. Finally, the Quaternary is divided into the Pleistocene and Holocene epochs. Some geologists now think that--since humans are having such a notable impact on the Earth and its life--a new, youngest epoch should be added to the Quaternary: the Anthropocene. There is still considerable discussion in the geological community about whether this epoch should be added, as well as debate about what characteristics should define its beginning.

Video

This 12-minute video details the geologic record and important events through Earth's history that have shaped life.

Video

The first era of the current eon, the Paleozoic Era, is probably the most deceptively fascinating time in Earth’s history. This 11-minute video describes it and the conditions of the Permian Extinction.
Question after watching:
Why do scientists say that mass extinctions provide opportunities?

The Geologic and Evolutionary Record

A remarkable feature Earth's history is how often evolutionary changes coincided with geologic and climatological changes. In addition, consider that changes in geology (e.g., mountain formation or lowering of the sea level) can cause changes in climate and climate changes (e.g. loss or gain of global ice) can cause changes in geology. These two together alter the habitats available for life. Past and present climate change will be covered in depth in Unit 5. 

Here, we look at two types of geologic change that seem to have had especially dramatic effects on life: continental drift and the impact of meteors.

Continental Drift  

A body of evidence, both geological and biological, supports the conclusion that 200 million years ago, at the start of the Mesozoic era, all the continents were attached to one another in a single landmass, which has been named Pangaea, seen in Figure \(\PageIndex{2}\). This drawing of Pangaea is based on a computer-generated fit of the continents as they would look if the sea level were lowered by 6000 feet (~1800 meters).

Figure \(\PageIndex{2}\): The supercontinent Pangaea in the early Mesozoic (at 200 Ma). The modern continents are highlighted for reference. (Fama Clamosa; CC-BY-SA 4.0)

During the Triassic, Pangaea began to break up, first into two major landmasses:

• Laurasia in the Northern Hemisphere
• Gondwana in the Southern Hemisphere.

The present continents separated at intervals throughout the remainder of the Mesozoic and through the Cenozoic, eventually reaching the positions they have today. Evidence for this theory includes the shape of the continents, the geologic record, and fossils.

Shape of the Continents

The east coast of South America and the west coast of Africa and are strikingly complementary (Figure \(\PageIndex{2}\)). This is even more dramatic when one tries to fit the continents together using the boundaries of the continental slopes, e.g., 6000 feet (~1800 meters) down, rather than the shorelines.

Geology

• In both mineral content and age, the rocks in a region on the east coast of Brazil match precisely those found in Ghana on the west coast of Africa.
• The low mountain ranges and rock types in New England and eastern Canada appear to be continued in parts of Great Britain, France, and Scandinavia.
• India and the southern part of Africa both show evidence of periodic glaciation during Paleozoic times (even though both are now close to the equator). The pattern of glacial deposits in the two regions not only match each other but also glacial deposits found in South America, Australia, and Antarctica.

Fossils

• Fossil reptiles found in South Africa are also found in Brazil and Argentina.
• Fossil amphibians and reptiles found in Antarctica are also found in South Africa, India, and China.
• Most of the marsupials alive today are confined to South America and Australia. But if these two continents were connected by Antarctica in the Mesozoic, one might expect to find fossil marsupials there. In March 1982, this prediction was fulfilled with the discovery in Antarctica of the remains of Polydolops, a 9-ft (2.7 m) marsupial.

Note

This 9-minute video discusses supercontinents and how their formation and breakup shaped life.

Meteors: The Impact Hypotheses

The Cretaceous period, the last period of the Mesozoic, marked the end of the Age of Reptiles. It was followed by the Cenozoic era, the Age of Mammals. Although extinctions have occurred throughout the history of life, an extraordinary number of them occurred in a relatively brief period at the end of the Cretaceous. Why?

The Alvarez Theory

Louis Alvarez, his son Walter, and their colleagues proposed that a giant asteroid or comet striking the earth some 66 million years ago caused the massive die-off at the end of the Cretaceous. Presumably, the impact generated so much dust and gases that skies were darkened all over the earth, photosynthesis declined, and worldwide temperatures dropped. The outcome was that as many as 75% of all species — including all dinosaurs — became extinct.

The key piece of evidence for the Alvarez Hypothesis was the finding of thin deposits of clay containing the element iridium at the interface between the rocks of the Cretaceous and those of the Paleogene period (called the K-Pg boundary after the German word for Cretaceous). Iridium is a rare element on earth (although often discharged from volcanoes), but occurs in certain meteorites at concentrations thousands of times greater than in the earth's crust.

After languishing for many years, the Alvarez theory gained strong support from the discovery in the 1990s of the remains of a huge (180 km in diameter) crater in the Yucatan Peninsula that dated to 65 million years ago.

The abundance of sulfate-containing rock in the region suggests that the impact generated enormous amounts of sulfur dioxide (SO₂), which later returned to earth as a bath of acid rain. A smaller crater in Iowa, formed at the same time, may have contributed to the devastation. Perhaps during this period, the earth passed through a swarm of asteroids or a comet and the repeated impacts made the earth uninhabitable for so many creatures of the Mesozoic.

Video

This entertaining video provides an overview of the evidence that lead researchers to conclude that an asteroid impact led to the demise of the dinosaurs.
Question after watching: 
One researcher, in tongue -n-cheek fashion, claims that we still live in the age of the dinosaurs. How do you think that the ages are named (i.e. why is this called the age of the mammals)? Dinosaurs are often thought to be ectotherms ("cold-blooded") like reptiles. But birds are particularly endothermic ("warm-blooded"). Based on this observation, do you think that dinosaurs were ecothermic or endothermic? Explain your reasoning.

Other Impacts

Mass extinctions have occurred at other times in Earth's history. There was another large extinction of non-dinosaur reptiles that occurred earlier, at the end of the Triassic. It was followed by a great expansion in the diversity of dinosaurs. The recent discovery of a layer enriched in iridium in rocks formed at the boundary between the Triassic and Jurassic suggests that impact from an asteroid or comet may have been responsible then just as it was at the K-Pg boundary.

Contributors and Attributions

• Hendricks, J.R. Geologic Time Scale. https://www.digitalatlasofancientlife.org/learn/ CC-BY-SA 4.0