Geologic Time
Cenozoic ("Recent Life") Era
This is the "Age of Mammals" in which whales took over the oceans, saber-tooth cats shared
the land with elephants and giant sloths, and humans finally appeared. The Cenozoic period
began about 65 million years ago with the extinction of the dinosaurs and continues through
the present.
During the last 65 million years, Pangea has broken up into the continents, and they have
moved into the positions, which we see them in now. The Atlantic Ocean has opened from a
narrow valley to a vast ocean. India has moved across the Indian Ocean and collided with
Asia to make the Himalaya Mountains. North and South America have moved westward over
part of the Pacific Ocean. The pressure has crumpled the western coasts of both continents
to form the Rocky and Andes Mountains. Part of the Pacific sea floor has been forced into
the warm interior under the American continents, causing melting and the formation of the
Cascade and Andes volcanoes on the surface.
The global climate has turned somewhat colder, and the last few million years have seen the
return of giant glaciers and ice caps to North America, Eurasia, and Antarctica.
After the disappearance of the dinosaurs, there were suddenly many empty places on Earth
where animals could live. Mammals, which were small, mouse-like animals at the beginning
of the Cenozoic, quickly spread out, diversified in kind, and grew in size. Soon the plains and
forests of Earth were occupied by giant rhinos and elephants, lions and saber-tooth cats,
horses and deer. The skies filled with bats and birds, and the seas filled with whales and
porpoises, as well as with fish and octopi. There have been mass extinctions during the
Cenozoic as there were during the Mesozoic and Paleozoic, but not as many animals and
plants have disappeared.
Finally, humanity appeared during the last two million years. In the last 10,000 years, a blink
of the eye in geologic time, humanity has spread across the lands and seas of Earth, altering
the face of Earth with cities and farms, destroying some plants and animals and
domesticating others. Humans have become the dominant terrestrial life form: more
numerous than any other large animal, and more fearsome than the most terrible of
dinosaurs.
Geologic Time
Mesozoic ("Middle Life") Era
This is the era we all think of when we imagine the Ancient Earth!
Rampaging dinosaurs! Dive-bombing pterodactyls! Endless forests of
giant ferns! Erupting Volcanoes! (Sorry, no cave men! They didn't show
up until the end of the Cenozoic.) The Mesozoic Era lasted about 180
million years, from about 245 million years ago to about 65 million years
ago.
The supercontinents Gondwanaland and Laurasia collided some time
back to form a single super-super continent called Pangea ("All-Earth").
But plate tectonics continues its irresistible motions, and even as we
look, Pangea is beginning to break up into the continents we know now.
At upper left, North America is just breaking away from the northwest
coast of Africa, and the Atlantic Ocean and Gulf of Mexico are beginning
to form. The Appalachian Mountains of the eastern United States are a
high, rugged mountain range, something like the Rocky Mountains of
today. Over the next fifty million years or so, South America, India, and
Antarctica will all break away from Africa and move toward their present
positions.
The dominant animals on both land and sea are reptiles, the most
famous of which are the dinosaurs. Dinosaurs began in the Triassic,
spread during the Jurassic, and dominated Earth in the Cretaceous. They
are so prominent that the Mesozoic is also called "The Age of Reptiles."
Birds and mammals also appear during the Mesozoic, as well as
deciduous trees and flowering plants.
The climate is so warm that there are no ice caps at all, even at the
poles! Plants grow like crazy in the warmth and moisture, so there is
food everywhere for your average hungry 50-ton Ultrasaurus! So what
happened to this Dino Paradise? A mass extinction like those in the
Paleozoic ended the idyllic Mesozoic Era. More than half of all existing life
forms disappeared, including virtually all of the dinosaurs. Why? There
are many hypotheses, including disease, volcanic eruptions, and giant
impacts.
Geologic Time
Paleozoic ("Ancient Life") Era
The Paleozoic period lasted about 325 million years, from about 570
million years ago to about 245 million years ago.
Here in the Paleozoic, Earth's interior has cooled down to something like
modern levels, so that volcanic activity is usually about as humanity
experiences it: a few minor eruptions like Mount St. Helens each year,
and major ones like Krakatoa every century or so. However, gigantic
"hot-spot" type eruptions still occur every hundred million years or so.
Plate tectonics continues to push land masses across Earth's surface. At
this particular time--the middle of the Silurian Period--most of the land is
still locked in two supercontinents called Gondwanaland, which happens
to be wandering over the South Pole in our view, and Laurasia, which is
on the other side of the globe. Huge glaciers cover the interior of
Gondwanaland, and Earth is experiencing one of its ice ages. Over the
next hundred million years, Gondwanaland will move north over the
equator and begin to break up, and the climate will warm up
substantially
The composition of the atmosphere has continued to slowly change,
mostly due to the increase of oxygen produced by photosynthetic algae
floating on the ocean. By the Paleozoic, the composition of the air has
reached something like what we breathe now: about 4/5 nitrogen, 1/5
oxygen, and small amounts of carbon dioxide, water vapor, and other
gases. At long last the air is capable of supporting large animals, and
almost in response, life explodes into the Paleozoic!
At the beginning of the Paleozoic, life existed only in or near the ocean.
Trilobites, shellfish, corals, and sponges appeared, followed by the first
fish. Land plants appeared near the end of the Ordovician and for the
first time we see the green of land plants in our global view. Huge
forests and swamplands formed during the warm climate of the
Mississippian and Pennsylvanian periods that later fossilized into the
giant coal beds of the eastern United States. Animal life also moved onto
the land, first the arthropods (spiders and insects to you), then the
amphibians, and later the reptiles. The most abundant animals on land
and sea during the Paleozoic were those like shellfish and insects that
lacked backbones, so the Paleozoic is often called "The Age of
Invertebrates."
The Paleozoic was also marked by several mass extinctions: geologically
Geologic Time
short periods of time during which large numbers of life forms died out.
Mass extinctions occurred at the end of the Ordovician, the Devonian,
and the worst one of all at the end of the Permian, when about 95% of
all life on Earth died! These mass dyings were probably caused by
climate changes and periods of giant volcanic eruptions.
We can follow the development of life in detail during the Paleozoic,
because at the beginning of that Era, life forms developed hard parts like
shells, teeth, bones, and woody parts that were easily preserved as
fossils. Earlier life forms were single-celled and soft-bodied, so older
rocks contain few fossils.
Geologic Time
The Precambrian Eon The name means: "before the Cambrian period." This old, but still
common term was originally used to refer to the whole period of Earth's
history before the formation of the oldest rocks with recognizable fossils
in them. In the last few decades, however, geologists have found that
there are some hard-to-discern fossils in some Precambrian rocks, so this
period also is now known as the Cryptozoic or "obscure life" Eon (from
the words "crypt" = "hidden," and "zoon" = "life").
This eon covers almost 90% of the entire history of Earth. It has been
divided into three eras: the Hadean, the Archean and the Proterozoic.
Each era is very different.
Hadean ("Hades-like") Era This era begins about 4.6 billion years
ago with the formation of Earth from dust and gas orbiting the Sun.
During this era the surface of Earth is like popular visions of Hades:
oceans of liquid rock, boiling sulfur, and impact craters everywhere!
Volcanoes blast off all over the place, and the rain of rocks and asteroids
from space never ends. It is hard to take a step without falling into a
pool of lava or getting hit by a meteor! The air is hot, thick, steamy, and
full of dust and crud. But you can't breathe it anyway: its made of
nothing but carbon dioxide and water vapor, with traces of nitrogen and
smelly sulfur compounds! Any rocks that do form from cooling lavas are
quickly buried under new lava flows or blasted to bits by yet another
impact. Some people think that an asteroid as large as the planet Mars
hit Earth near the beginning of the Hadean era, completely smashing
and melting Earth and forming the Moon as part of the "splash!" Wow!
No one has found any rocks on Earth from this era. Only meteorites from
space and moon rocks are this old. If any life formed on Earth during
this era, it was probably destroyed.
Archaean ("Ancient" or "Primitive") Era
This era begins about a billion years after the formation of Earth, and things have
changed a lot! Mostly everything has cooled down. Most water vapor in the air has
cooled and condensed to form a global ocean. Even most of the carbon dioxide is
gone, having been chemically changed into limestone and deposited at the bottom of
the ocean. The air is now mostly nitrogen, and the sky is filled with normal clouds and
rain. The lava also is mostly cooled to form the ocean floor. The interior of Earth is still
quite hot and active, as shown by the many erupting volcanoes. The volcanoes form
lots of small islands in long chains. The islands are the only land surface. The
Geologic Time
continents have not formed yet. The islands are carried over the surface of Earth by
the movement of rock deep in Earth's interior. Occasionally the small islands collide
with each other to form larger islands. Eventually these larger islands will collide to
form the cores of the continents we know today.
If you look closely, you will see evidence of blue-green algae (actually simple bacteria)
floating in the ocean. That's all there is! Just single-celled bacteria in the ocean. There
is as yet no life on land. Life began in the ocean near the beginning of this era. The
oldest known fossils--the remains of different types of bacteria--are in archean rocks
about 3.5 billion years old. The surface of Earth is still very active, but a few of the
rocks that are forming now will actually survive to the present in spite of wind, rain,
and remelting. In fact, geologists define the beginning of the Archean era as the age of
the oldest rocks on Earth we can still find today.
Proterozoic ("Early Life") Era
Well, here we are about 700 million years ago, near
the end of the longest time period in geologic history. It began about two billion years
after the formation of Earth and lasted about another two billion years! So what has
happened in all that time?
Hmmmmm. There is a lot more land to be seen. In fact, there are two supercontinents,
one visible across the equator on this side of Earth and another one on the other side.
These huge masses of land formed by collisions of the many, many islands made by
volcanoes during the Archaean and most of the Proterozoic eras. Earth's interior has
cooled some more, and there are fewer volcanoes than in the Archean. Even though
the movements of Earth's surface we call Plate Tectonics are still very fast and
continental collisions are frequent (every few hundred million years or so!), the centers
or cores of the continents are now quite large and stable. In fact, geologists date the
beginning of the Proterozoic Era by the age of the oldest continental rocks that have
not been reheated or chemically altered.
Life has not changed much during the last two billion years, but the few changes are
significant. Life is still found only in the ocean, but sometime around 1.7 billion years
ago, single-celled creatures appeared that had a real nucleus. Another important
change is about to happen: true multi-celled life is about to appear, some 30 million
years before the end of the Proterozoic. These multi-celled creatures will have no hard
parts like shells or teeth in their bodies, so their fossils will be hard to find.
The atmosphere is about the same, mostly nitrogen, with a little water vapor and
carbon dioxide. These single-celled plants have been producing oxygen for about two
billion years, but up until now the oxygen has been combining chemically with iron and
other elements to form great mineral deposits around the world. Paradoxically, this
oxygen, which we must have to live, is poisonous to most of the life forms living on
Earth during the Proterozoic, so another great change in the types of life is about to
occur. Earth at this time is also very cold, with huge, bluish glacial ice sheets visible
across the supercontinent, even in the normally warm equatorial regions!
Geologic Time
Cenozoic ("Recent Life") Era - Worksheet
1. What is the Cenozoic era called?__________________________
2. The Cenozoic period began about ___________________ years
ago with the _______________ of the ___________________
and continues through the _________________.
3. India moved across the Indian Ocean and collided with Asia to
form what?_______________________________
4. How were the Rocky and Andes Mountains formed?
___________________________________________________
___________________________________________________
5. How has the climate changed and what has resulted from it?
___________________________________________________
___________________________________________________
___________________________________________________
6. What happened after the extinction of the dinosaurs?
___________________________________________________
___________________________________________________
___________________________________________________
7. When did humans appear and how have their presence altered
the Earth?
___________________________________________________
___________________________________________________
Geologic Time
Mesozoic ("Middle Life") Era
1. When was the Mesozoic Era and how long did it
last?_____________________________________________
2. What are some of the things that people think happened
during the Mesozoic Era?
_________________________________________________
_________________________________________________
3. What 2 supercontinents collided to form Pangaea?
_________________________________________________
4. What does Pangaea mean?___________________________
5. What kinds of animals are present during the Mesozoic Era?
___________________________________________________
___________________________________________________
6. What is the Mesozoic Era referred to as?
_________________________________________________
7. Describe the climate during the Mesozoic Era.
_________________________________________________
_________________________________________________
8. What caused the extinction of the dinosaurs?
Geologic Time
Paleozoic ("Ancient Life") Era - Worksheet
1. When was the Paleozoic era and how long did it last?
_______________________________________________
_______________________________________________
2. Explain the climate of the Paleozoic era.
_______________________________________________
_______________________________________________
_______________________________________________
3. What kind of plant and animal life existed during the
Paleozoic era?
_______________________________________________
_______________________________________________
_______________________________________________
4. What is the Paleozoic era often called?
_______________________________________________
5. When did the mass extinctions occur during the Paleozoic
era and how were they probably caused?
_______________________________________________
_______________________________________________
6. Why can scientists follow the development of life during
this era?
Geologic Time
_______________________________________________
_______________________________________________
The Precambrian Eon- Worksheet
1. The Precambrian Eon spans what percent of the earth’s
history?
____________________________________________
2. What does Precambrian mean?
____________________________________________
3. List and describe the three eras that the Precambrian
eon has been divided into.
a. ________________________________________
________________________________________
________________________________________
________________________________________
________________________________________
________________________________________
b. ________________________________________
________________________________________
________________________________________
________________________________________
________________________________________
________________________________________
c. ________________________________________
________________________________________
Geologic Time
________________________________________
________________________________________
________________________________________
________________________________________
4. Describe the atmosphere and climate during this time
period.
____________________________________________
____________________________________________