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The goal of today is to get a better “feel” for geologic time. The Earth’s age is about 4.5 billion years.
That’s an immense amount of time! In many respects, geologic time is marked by unique,
never-to-be-repeated events, such as the origin of the Earth itself. However, in other respects, Earth
events are cyclic: the same things happen over and over again. Former Harvard paleontologist, Stephen J.
Gould (1941-2002), has described these two aspects of geologic time as time’s arrow and time’s cycle.
Time’s arrow refers to the non-repeatable events that have occurred through Earth’s history, whereas
time’s cycle refers to repeatable events that continue to shape the Earth, over and over again.
Here’s an analogy to better help you appreciate the distinction between time’s arrow and time’s cycle.
Imagine a bicycle (Figure 1). If we focus on the progress of the rider from Pt. A to Pt. B, she moves
between these two points in a non-repeatable way (she doesn’t retrace her path). That’s time’s arrow.
This represents the idea that specific geologic events, like Earth’s origin, are non-repeatable and
unique–they only happen once. However, the same kinds of geologic events (like mass extinctions and
plate boundary motions) keep happening over and over again. If we focus on a point on one of the bicycle
tires, notice that it traces out a circular motion compared to the rider. That’s time’s cycle.
Putting the two ideas together, let’s consider the motion of the point on the bicycle tire (red dot) in
comparison with Earth’s surface (bottom diagrams, Figure 1). On the one hand, the red dot keeps
returning to the ground as it traces out a sort of hopping motion. On the other hand, it progresses from
Pt. A to Pt. B as it “hops” along. The take-home message is that to really understand the character of
geologic time, we need to consider both its non-repeatable aspects (time’s arrow) as well as it’s
repeatable aspects (time’s cycle).
Part 1: Time’s Arrow—unique events in Earth history
Please review some of the key events in Earth history, summarized on pages 4-15. Then, answer the
1. Review the possible causes of the “big five” mass extinction events (Mass Extinctions #1 through #5)
below, during which much of Earth’s existing life was eradicated in geologically short spans of time. In
general, what processes are thought to cause mass extinctions?
2. Based on your review of pages 4-15, in what ways do later events in Earth history depend on earlier
events? Discuss three examples.
3. Approximate dates for the big five mass extinctions are given below. Based on these data, calculate
the average time between mass extinctions, as follows:
_________my _________my _________my
*Earlier age – later age
mya. = million years ago
my = million years
Average time between mass extinctions = Sum of Differences =
4. Do you think your calculated result for the average time between mass extinctions could be used to
predict when the next mass extinction might occur? Why or why not? Justify your answer.
5. Life on Earth has only become abundant within the last 540 million years or so (to 2 significant digits).
What percentage of geologic time does this span of time represent? Do this calculation as follows:
divide 540 m.y. (when life became abundant) by 4,540. m.y. (age of the Earth), then multiply by 100
(show your work).
***After answering Questions 1-5, proceed to page 16***
Important Events in Earth History (age dates are approximate)
4.54 Billion Years Ago: Birth of a planet
Earth grew from a cloud of dust and rocks surrounding the young Sun. Earth formed when some of these
rocks collided—a process called accretion. Eventually they were massive enough to attract other rocks
with the force of gravity, and vacuumed up all the nearby debris, becoming the Earth. The Moon probably
formed soon after, when a planet-sized chunk of rock smashed into the Earth and threw up a huge cloud
of debris, which eventually condensed into the Moon.
4.1-3.5 Billion Years Ago: First organisms
Nobody knows exactly when life began. The oldest confirmed fossils, of single-celled microorganisms,
are 3.5 billion years old, and are found in bulbous mounds called stromatolites. Other evidence
suggests that life may have begun a bit earlier than that (3.8-4.1 billion years ago) in warm-water vents
on the seabed, in open water, or on land.
3 Billion Years Ago: Continents form; plate tectonics begins (?)
Today, Earth’s surface is divided into a few dozen mobile slabs of rock, called tectonic plates, that slide
around atop a weak, plastic interior. Plate tectonic processes may have begun around 3.3 billion years
ago, based on the observation that Australian rocks as old as ~ 3.3 billion years show magnetic
evidence of continental drift.
2.7-2.2 Billion Years Ago: First complex cells
The first organisms were simple cells like modern bacteria, but some of them became much more
internally complex. These more complex life forms, called eukaryotes, developed lots of specialized
equipment within their cells. They also had a new source of energy: sausage-shaped objects called
mitochondria that were once free-living bacteria, but which were absorbed in a process called
endosymbiosis. Every animal and plant you’ve ever seen is a eukaryote.
2.4-2.0 Billion Years Ago: Great oxidation event
For the first half of Earth’s history, there was hardly any oxygen in the air. But then some bacteria
began harnessing sunlight to make sugar from carbon dioxide and water, just like green plants
today—a process called photosynthesis. These microbes pumped out oxygen as a waste product,
creating the oxygen-rich atmosphere we have today. But the first oxygen may have caused the entire
planet to freeze over into a ‘Snowball Earth’, by destroying the greenhouse gas, methane.
2.2 Billion-640 million Years Ago: Snowball Earths
Earth may have frozen completely over at least three times throughout its history—at 2.2 billion, 710
million, and 640 million years ago. Evidence of these worldwide events includes glacial deposits
thought to have formed near the equator.
1.2 Billion Years Ago: First sex
Between 1.8 billion and 800 million years ago, the fossil record looks fairly dull – so much so that the
period is sometimes called the ‘Boring Billion’. But behind the scenes plenty was happening. For one
thing sex may have evolved for the first time. It’s not clear why, or when, some organisms stopped
simply dividing in two and started the messy business of sex. But it was definitely going on 1.2 billion
years ago: there are fossils of red algae from that time that were clearly forming specialized sex cells
such as spores.
570-530 Million Years Ago: Cambrian explosion
Soon after animals evolved, evolution went through a major growth spurt. During the so-called
“Cambrian Explosion,” the ancestors of almost every group of modern animals appeared within a few
tens of millions of years. This apparent ‘explosion’ may be partly due to better fossilization, as many
animals now had hard shells. Rising atmospheric oxygen levels may have also played a role. And
perhaps the eroded remnants of ancient mountains fertilized the oceans with much-need nutrients.
465 Million Years Ago: Plants colonize the land
Plants were the first to take up permanent residence on land. The first land plants were relatives of
green algae, but they rapidly diversified.
445 Million Years Ago: Late Ordovician—Mass Extinction #1
The Ordovician period was a time when life flourished. But towards its end, the world cooled
dramatically and ice sheets spread from the poles. The deep freeze led to the second-worst mass
extinction on record. Most life was still confined to the sea, and 85% of marine species were wiped
out. In the aftermath, fish became much more common.
375 Million Years Ago: From fins to legs
With plants well-established on land, the next step was for animals to move out of the water. Insects
were among the first, around 400 million years ago. But they were followed soon after by big,
backboned animals such as Tiktaalik, a fish that looked a bit like a salamander. Fish like Tiktaalik would
eventually evolve four limbs, and give rise to amphibians, reptiles, and mammals.
374 and 359 Million Years Ago: Late Devonian—Mass Extinction #2
At the end of the Devonian period, two closely-spaced extinction events pummeled existing life with a
one-two punch. Thought to have been caused by global cooling and widespread glaciation, the late
Devonian extinction events may have been related to the development of the first trees and forests,
which absorbed atmospheric carbon dioxide, a greenhouse gas.
320 Million Years Ago: Dawn of the Reptiles
When the first reptiles appeared, Earth was in the middle of a long cold snap called the Late Paleozoic
Ice Age. Reptiles evolved from newt-like amphibians. Unlike their ancestors they had tough, scaly skin
and laid eggs with hard shells that did not have to be left in water. Thanks to these advantages, they
quickly became the dominant land animals. The reptile-like Dimetrodon reached 4.5m long – but it
was not a dinosaur.
330 Million Years Ago: Pangea
Thanks to the plate tectonic processes, all Earth’s continents came together to form one giant
supercontinent. Known as Pangea, this so-called supercontinent was surrounded by a world-spanning
ocean called Panthalassa. It lasted until around 200 million years ago, when it began to tear itself apart
over tens of millions of years to become the familiar modern continents of today.
252 Million Years Ago: The Great Dying—Mass Extinction #3
Just as the reptiles were flourishing, life on Earth faced perhaps its greatest challenge. The Permian
extinction was the worst mass extinction in the planet’s history, obliterating up to 96% of marine
species and similar numbers of land animals. Climate change associated with massive volcanic
eruptions in what is now Siberia were probably to blame. In the aftermath, the first dinosaurs evolved.
240 Million Years Ago: First Dinosaurs
Around this time in Earth history, a new reptile arrived on the scene: the dinosaur. Unlike their reptile
ancestors, dinosaurs had legs situated underneath their bodies, allowing them to run faster and with
greater endurance than other reptiles. Modern birds are thought by most paleontologists to be one
kind of dinosaur because they share features common to other dinosaurs, such as the three-toed foot
and an s-shaped neck.
220 Million Years Ago: First Mammals
At the same time that the dinosaurs began to flourish, the first mammals evolved from reptilian
ancestors. Early mammals were small, hairy, and probably only active at night. This may have spurred
them to evolve warm-bloodedness: the ability to keep their body temperature constant.
201 Million Years Ago: End-Triassic—Mass Extinction #4
As dinosaurs were flourishing on land, giant reptiles called ichthyosaurs had become the top predators
in the sea. Then, another disaster struck: the end-Triassic extinction, which killed off around 80% of all
known species. In the aftermath, the dinosaurs became the dominant land animals and eventually
reached massive sizes. Although various hypotheses have been suggested for the cause of this
extinction event, massive volcanism in the central Atlantic region, which occurred as Pangea broke
apart, probably played a role.
160 Million Years Ago: First birds
The most famous early bird (or was it a birdlike dinosaur?), Archaeopteryx, lived 150 million years ago.
In recent years slightly older fossils of bird-like organisms have been found in China. Modern birds are
essentially velociraptors with beaks instead of snouts and wings instead of arms.
130 Million Years Ago: Flower power
Flowers are a relatively recent invention, appearing after the rise of both dinosaurs and mammals.
Insect evolution is closely connected to the rise of flowering plants because many insect species came
to rely on pollen and nectar for food. Many modern flowering plants are pollinated by insects.
66 Million Years Ago: End Cretaceous—Mass Extinction #5
Boom, you’re extinct! 66 million years ago, a huge chunk of rock from outer space smashed into what
is now Mexico. The explosion was devastating, but the longer-term effects were worse. Dust was
thrown into the upper atmosphere and blocked out sunlight, and in the ensuing cold and darkness
Earth suffered its fifth and most recent mass extinction. The dinosaurs were the most famous
casualties, but pterosaurs and giant marine reptiles were also wiped out. Intense volcanism in what is
now India may have also played a role in this mass extinction.
60-55 Million Years Ago: First primates
Almost immediately after the dinosaurs were wiped out, mammals evolved the ability to nourish their
young inside their wombs, just like modern humans. Soon, some of these early placental mammals
evolved into the first primates. They would ultimately give rise to monkeys, apes, and humans. But the
first ones were small creatures, which lived in the hot and humid rainforests of Asia.
23-6 Million Years Ago: Road to humanity
Based on fossil evidence, the first apes appeared in Africa around 23-25 million years ago. Then at
some point, the group split into the ancestors of modern humans and the ancestors of modern apes.
It’s hard to say exactly when, but thanks to modern genetics and a host of fossil discoveries, we have a
rough idea. The oldest known common ancestor of chimps and humans probably lived around
6 million years ago.
200,000 Years Ago: The thinking ape
Our species, Homo sapiens, is ridiculously young. We have only existed for less of a quarter of a million
years. In that time we have expanded from our African birthplace to reach every continent, and even
outer space. Our activities are causing what some consider to be a sixth mass extinction and have
unleashed rapid, planetary-scale climate change. Yet we are also the only species that has ever
managed to piece together the history of Earth. As Carl Sagan once said, “We are a way for the
universe to know itself.”
Part 2: Time’s Cycle—repeatable events in Earth history
how Earth’s tectonic plates interact at divergent, convergent, and transform boundaries. We’ll consider the cyclic
nature of plate boundary processes through time. Study Figure 2, which illustrates the plate tectonic cycle.
Figure 2: The plate tectonic cycle.
Occasionally on Earth, most or all of the world’s continents join together via continental collision (Steps 5 and 6) to
create a supercontinent. Within the last 2 billion years or so, as many as five supercontinents may have existed.
However, supercontinents don’t last forever. Supercontinents eventually break up into multiple continental
fragments again after assembling, only to be reassembled again as a new supercontinent forms. Although
supercontinent assembly and break-up are cyclic processes (they happen over and over again), each supercontinent
is unique because its continental fragments are arranged in different ways from the other supercontinents.
Here are brief descriptions of the five supercontinents, from oldest to youngest, thought to have existed within the
past 2 billion years. Click each link to see how continental fragments are arranged within each supercontinent.
Nuna (1820-1350 mya): Also called Columbia, this mostly northern hemisphere supercontinent contained
continental fragments that are now part of North America, China, Australia, Siberia, and parts of Africa and
Rodinia (1130-750 mya): This supercontinent was assembled from continental fragments produced by the
break-up of Nuna. It straddled the ancient equator.
Pannotia (633-573 mya): This short-lived supercontinent mostly existed in the southern hemisphere. Africa
was located at the center of the supercontinent. Africa, in turn, was surrounded by South America, Arabia,
Madagascar, India, Antarctica, and Australia.
Gondwana (550-336 mya): Like Pannotia, Gondwana was a mostly southern hemisphere supercontinent
created by the collision of several continental land masses, including South America, Antarctica, Australia,
India, and Africa.
Pangea (336-175 mya): Earth’s most recent supercontinent, Pangea, was a “C”-shaped land mass that
stretched from the north pole to the south pole. Pangea was created during the collision of Gondwana with
several northern hemisphere continents.
6. Study Table 1 (below), which lists estimated assembly and break-up times for the previous five supercontinents.
Based on the data in Table 1, how long, on average, do supercontinents persist on Earth*?
*To calculate average supercontinent persistence time, subtract the break-up age from the assembly age for each
supercontinent to calculate persistence time (Table 1). Then, add up all five persistence times and divide by 5.
Table 1: Supercontinent Assembly and break-up ages
Persistence Time (my)
mya = millions of years ago
my = million years
**Gondwana didn’t break up; instead, it joined several southern hemisphere continents to become Pangea.
7. Given the data in Table 1, is it reasonable to hypothesize that a sixth supercontinent may form in the future?
8. Can you use your answer to Question 6 to make an accurate prediction for when the next supercontinent will
form? Why or why not? Hint: do the supercontinent persistence times you calculated in Table 1 constitute a
high-precision (all values close to each other) or a low-precision (all values very different from each another)
9. How long will it take for this next supercontinent, called Pangea Proxima, to form (ages in millions of years)?
As a second example of time’s cycle, consider the rock cycle (Figure 3). Given the right set of processes, any one
rock type (igneous, sedimentary, or metamorphic) can be converted into any other rock type; however, when this
occurs, information about the previous rock type is typically lost as it’s converted into a new rock type.
Figure 3: The rock cycle.
Igneous Rock: Formed by melting and cooling of molten (liquid) rock (magma) beneath Earth’s surface.
● Sedimentary Rock: Formed by compaction and cementation of pre-existing rock fragments and chemical
residues derived from pre-existing rock.
● Metamorphic Rock: Igneous, sedimentary, or pre-existing metamorphic rock that has undergone solid-state
transformation (without melting) by heat, pressure, and migrating fluids.
10. A popular advertising campaign proclaims that, “Diamonds are forever.” Given your understanding of the rock
cycle, is this statement scientifically accurate? Explain.
11. The Earth’s current age is estimated to be 4.54 billion years old. Given what you’ve learned about the rock cycle
(Figure 2), is it likely that we’ll ever find extensive terrestrial rock left over from Earth’s initial formation? Why
or why not?
As a way of summarizing what you learned today, please address the following questions (3 paragraphs, minimum):
What aspects of today did you find most interesting?
Briefly explain how mass extinctions can reflect both time’s arrow and also time’s cycle.
Briefly explain how plate tectonic processes can reflect both time’s arrow and time’s cycle.
Review the possible causes of the
“big five” mass extinction events
(Mass Extinctions #1-#5) below,
during which much of Earth’s
existing life was eradicated in
geologically short spans of time.
In general, what processes are
thought to cause mass
Based on your review of pages
4-15, in what ways do later events
in Earth history depend on earlier
events? Discuss three examples.
Average time between mass
extinctions (show your work)
Do you think your calculated
result for the average time
between mass extinctions could
be used to predict when the next
mass extinction might occur?
Why or why not? Justify your
Life on Earth has only become
abundant within the last 540
million years or so (to 2
significant digits). What
percentage of geologic time does
this span of time represent?
Do this calculation as follows:
divide 540 my (when life became
abundant) by 4,540. my (age of
the Earth), then multiply by 100
(show your work).
my = million years
Study Table 1 (p. 17), which lists
estimated assembly and break-up
times for the previous five
supercontinents. Based on the
data in Table 1, how long, on
average, do supercontinents
persist on Earth?
Given the data in Table 1, is it
reasonable to hypothesize that a
sixth supercontinent may form in
the future? Explain.
Can you use your answer to
Question 6 to make an accurate
prediction for when the next
supercontinent will form? Why or
why not? Hint: do the
supercontinent persistence times
you calculated in Table 1
constitute a high-precision (all
values close to each other) or a
low-precision (all values very
different from each another) data
How long will it take for this next
supercontinent, called Pangea
Proxima, to form ( ages in
millions of years)?
A popular advertising campaign
proclaims that, “Diamonds are
forever.” Given your
understanding of the rock cycle,
is this statement scientifically
The Earth’s current age is
estimated to be 4.54 billion years
old. Given what you’ve learned
about the rock cycle (Figure 2), is
it likely that we’ll ever find
extensive terrestrial rock left over
from Earth’s initial formation?
Why or why not?
As a way of summarizing what
you learned today, address the
following questions (3
What aspects of today did
you find most interesting?
Briefly explain how mass
extinctions can reflect
both time’s arrow and also
Briefly explain how plate
tectonic processes can
reflect both time’s arrow
and time’s cycle.
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The Great Dying
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