CHAPTER 11
A Biography of Earth
Learning Objectives
1. Students should know the approximate age of Earth, how this date was obtained, and
why it is substantially older than the oldest known rocks.
2. Students should know that the Hadean is an informal eon that predates Earth’s rock
record. Hadean Earth was very hot, with many sources of heat. They should be able to
distinguish the earliest, volcanically dominated atmosphere from today’s atmosphere.
3. Students should know and understand the following: (a) the contrast between early
Archean tectonics (microcontinents) and Proterozoic conditions (development of larger
oceanic plates and extensive stable cratons); (b) the appearance of the first life on Earth (3.8–
3.5 Ga), and the geological importance of cyanobacteria (stromatolite formation; oxygenation
of oceans and, ultimately, atmosphere); (c) the great oxygenation event at 2.4 Ga; (d) the
formation of supercontinents Rodinia (1 Ga) and Pangaea (roughly 300 Ma), and evidence of
worldwide glaciation in the late Proterozoic; (e) the Cambrian explosion of animal life, and
the development of a complex food web with filter feeders, bottom (deposit) feeders, and
predators; (f) the timing of orogenic events that produced the Appalachians and Rockies; and
(g) major events in the history of life.
Summary from the Text
The Earth formed about 4.57 billion years ago. For part of the first 600 million years,
the Hadean Eon, the planet was so hot that its surface was a magma ocean.
The Archean Eon began about 3.85 Ga, when the first continental crust that still remains
formed. This crust assembled out of volcanic arcs and hot-spot volcanoes that were too
buoyant to subduct. The atmosphere contained very little oxygen, and the first life forms—
bacteria and archaea—appeared.
In the Proterozoic Eon, which began at 2.5 Ga, Archean cratons sutured together to
form large cratons. Photosynthesis added oxygen to the atmosphere. By the end of the
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Proterozoic, soft-bodied marine invertebrates populated the planet, and continental crust had
accumulated to form a supercontinent.
As the Paleozoic Era began, rifting yielded several separate continents. Sea level rose
and fell, depositing sequences of strata in continental interiors. Continents coalesced again,
to form another supercontinent, Pangaea. Early Paleozoic evolution produced many
invertebrates with shells, and jawless fish. Land plants and insects appeared in the middle
Paleozoic, and by the end of the eon, there were land reptiles and gymnosperm trees.
In the Mesozoic Era, Pangaea broke apart, and the Atlantic Ocean formed. Convergentboundary tectonics dominated along the western margin of North America. Dinosaurs
became prominent land animals through the Mesozoic Era. During the Cretaceous Period,
the continents flooded. Angiosperms appeared, along with modern fish. A huge massextinction event wiped out the dinosaurs at the end of the Cretaceous Period, probably due to
the impact of a large meteorite.
In the Cenozoic Era, the collision of Africa and India with Asia and Europe formed the
Alpine-Himalayan orogen. Convergent tectonics persisted along the margin of South
America, creating the Andes, but ceased in North America when the San Andreas Fault
formed. Rifting in the western United States produced the Basin and Range Province.
Various kinds of mammals filled niches left vacant by the dinosaurs, and the human genus,
Homo, appeared and evolved through the Pleistocene Ice Age.
Answers to Review Questions
1. Why are there no whole rocks on Earth that yield isotropic dates older than 4 billion
years?
ANS: Earth is a geologically active planet. Subduction, erosion, metamorphism, and
melting destroy rocks.
2. Why can we find mineral grains that are older than the oldest rocks?
ANS: These oldest minerals grains are zircons found in sedimentary rocks. Isotopic
dates for the zircons give the age of the original igneous rocks in which they formed. These
original igneous rocks have been destroyed by the weathering process. The sandstones in
which the zircon grains are found are younger than these isotopic dates (principle of
inclusions), and no rocks have been found that are as old as these zircon grains.
3. Describe the condition of the crust, atmosphere, and oceans during the Hadean Eon.
ANS: The crust would have been largely ultramafic magma and rock (which may have
been subject to remelting). The atmosphere lacked free oxygen and would have been
dominated by volcanically emitted gases (water vapor, nitrogen, and carbon dioxide). For at
least part of the Hadean, temperatures were likely too hot to sustain a liquid ocean.
4. Describe how the first continental crust might have formed.
ANS: Volcanic island arcs and hot-spot volcanoes may have become too buoyant to
subduct, particularly after magmatic differentiation processes.
5. How did the atmosphere and tectonic conditions change during the Proterozoic Eon?
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ANS: The atmosphere became oxygenated (and depleted of some carbon dioxide) as a
result of photosynthesis. Plate-tectonic conditions became more similar to what is observed
today, with larger oceanic plates and the development of broad, stable cratons in continental
interiors.
6. What evidence do we have that Earth nearly froze over twice during the Proterozoic
Eon?
ANS: Glacial deposits have been found in sedimentary rocks representing sea-level
environments along the paleoequator.
7. Did supercontinents form in the Proterozoic?
ANS: Yes; Rodinia formed late in the Proterozoic.
8. How did the Cambrian explosion of life change the nature of the living world?
ANS: These first abundant animals complicated the world food web through the
introduction of more abundant filter and bottom (deposit) feeders, as well as large, mobile
predators and the first biogenic reefs. Carnivores likely induced selection pressure favoring
numerous defensive structures (thick shells, spines) and behaviors (burrowing, active swimming).
9. Were there multicellular organisms before the Cambrian?
ANS: Yes, multicellular animals and other multicellular life have been found in the late
Proterozoic Ediacaran faunas.
10. How did the Alleghenian and Ancestral Rocky orogenies affect North America?
ANS: The Alleghenian orogeny provided the final uplift to produce the Appalachian
Mountains. This powerful collision between Laurentia and Gondwana influenced fault
movement in what is now western North America. Uplifted western blocks (the Ancestral
Rockies) are the first tectonic events in the region of the modern Rocky Mountains.
11. What are the major types of organisms that appeared during the Paleozoic?
ANS: Conodonts, trilobites, mollusks, brachiopods, and echinoderms.
12. What supercontinent formed at the end of the Paleozoic, and what ocean formed when it
broke apart?
ANS: Pangaea formed at the end of the Paleozoic. When it broke up, the Atlantic Ocean
basin formed.
13. Describe the plate-tectonic conditions that led to the formation of the Sierran arc and
the Sevier thrust belt. What happened during the Laramide orogeny?
ANS: North America had a convergent-plate boundary to the west that induced
collisions with microcontinents and caused subduction of oceanic crust, which partially
melted to form the Sierran volcanic arc. Compression induced the folds and thrust faults just
interior to the arc itself. Faulting associated with the Laramide orogeny uplifted deep
basement rock to produce the Rocky Mountains, whereas earlier orogenies produced only the
more superficial faults of the fold-thrust belts behind the arc. Changes from the Sevier-style
orogeny may have been a result of a shallowing dip of the subducting slab.
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14. What life forms appeared during the Mesozoic?
ANS: Dinosaurs, mammals, birds, large swimming reptiles, and pterosaurs are a few
examples.
15. What may have caused the flooding of the continents during the Cretaceous Period?
ANS: Rapid Cretaceous rates of submarine volcanism thickened the mid-ocean
ridges and produced numerous hot spots, which displaced water onto the continents.
Additionally, glacial ice could not accumulate anywhere on Earth in the associated
greenhouse warmth.
16. What could have caused the K-T mass extinctions?
ANS: The extraterrestrial impact that produced the Chicxulub crater was likely a factor.
(The Deccan Traps eruption is another, unmentioned in the text.)
17. What might have caused the mass extinction at the end of the Paleozoic?
ANS: At this time, the Siberian traps formed. These flood basalts represent a massive
volcanic eruption that could have triggered drastic and sudden environmental changes.
18. What continents formed as a result of the breakup of Pangaea?
ANS: North America, South America, Africa, Eurasia, Australia, Antarctica, and the
microcontinent Madagascar are examples (answers may vary).
19. What caused the Himalayas and the Alps to form?
ANS: The Alps arose from Europe’s collision with Africa; the Himalayas arose from
India’s collision with Asia.
20. What major tectonic provinces formed in the western United States during the
Cenozoic?
ANS: The Basin and Range Province and the Snake River Plain Province are two
important examples.
21. What major climatic and biologic events happened during the Pleistocene?
ANS: Much of the northern hemisphere was in the grip of a glaciation (ice age), and
there were numerous extinctions of large animals toward the end of the glaciation.
On Further Thought
22. During intervals of the Paleozoic, large areas of continents were submerged by shallow
seas. Using Google Earth™, tour North America from space. Do any present-day regions
within North America consist of continental crust that was submerged by seawater? (Hint:
Look at the region just east of Florida.)
ANS: The broad continental shelf off the Atlantic and Gulf coasts represents continental
crust that was once above sea level, but was later stretched and thinned and now lies below
sea level. Conversely, the Bahamas to the east of Florida are composed of shallow- marine
carbonate deposits that are currently above sea level. Many regions of the midsection
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of the United States have been submerged at times beneath shallow seas, but this cannot be
readily inferred from modern satellite images.
23. Geologists have concluded that 80% to 90% of Earth’s continental crust had formed by
2.5 Ga. But if you look at a geological map of the world, you find that only about 10% of
Earth’s continental crustal surface is labeled “Precambrian.” Why?
ANS: In most places, the Precambrian is overlain by younger rocks and is not visible at
the surface.