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Changing Earth
Exploring Geology and Evolution
5th Edition
James S. Monroe | Reed Wicander
academic.cengage.com/earthsci
Chapter 1
Understanding
Earth
A Dynamic and
Evolving Planet
Introduction
Geology
Geology is a complex, integrated system of related
parts, components, or sub-systems that interact
in an organized fashion, affecting one another in
various ways.
Introduction
The principal subsystems
of the earth are the:
Atmosphere
Biosphere
Hydrosphere
Lithosphere
Mantle
Core
Introduction
The interaction of these subsystems has resulted in a
dynamically changing planet in which matter and
energy are continuously recycled into different forms.
What is geology?
Geology is the study of the Earth.
Physical geology is concerned with the
materials and processes which compose and
operate on the surface of, and within, Earth.
Historical geology is concerned with the
origin and evolution of Earth's continents,
oceans, atmosphere, and life.
What is geology?
Geologists are employed in
diverse occupations.
 Principle occupations
include:
 Mineral and
energy resource
exploration
 Solving
environmental
problems
 Predicting
natural disasters
Geology and the Formulation
of Theories
What is a theory?
 It is arrived at through the scientific method, which
involves
gathering and analyzing facts
formulating hypotheses to explain the phenomenon
testing the hypotheses
and finally proposing a theory.
 The hypotheses is a tentative explanation.
 A scientific theory is a testable explanation for some
natural phenomenon, that is supported by a large body
of evidence.
How Does Geology Relate to the
Human Experience?
 Geology pervades our everyday lives
and is a part of many aspects of
human experience, including the arts
and literature.
 The range of environmental problems
and issues of concern to society
require a basic understanding of
geology.
How does geology affect
our daily lives?
Natural Events
Economics and Politics
Our Role as Decision
makers
Consumers and Citizens
Sustainable Development
Global Geologic and Environmental
Issues Facing Humankind
Most scientists would argue
that overpopulation is the
greatest problem facing the
world today.
Increasingly large numbers of
people must be fed, housed,
and clothed, with a minimal
impact on the environment.
Global Geologic and Environmental
Issues Facing Humankind
 The greenhouse effect is the retention of heat in the
atmosphere, which results in an increase in the
temperature of Earth’s surface and atmosphere, thus
producing global warming.
Origin of the Universe
Did it begin with a Big Bang?
In the Big Bang theory, the
universe began approximately
15 billion years ago.
An extremely dense, hot body
of matter expanded and
cooled
Origin of the Universe
How do we know? Evidence for the Big Bang:
the universe is expanding
from a central point.
The entire universe has a
pervasive and constant
background radiation,
thought to be the faint
afterglow of the Big Bang.
Our Solar System
Its Origin and Evolution
The Solar System formed from a rotating cloud of
interstellar matter about 4.6 billion years ago.
This cloud, upon condensing, collapsed under the
influence of gravity and flattened into a rotating disk.
The sun, planets, and moons formed within this disk.
Earth
Its Place in Our Solar System
Earth formed from a swirling eddy of nebular
material 4.6 billion years ago, accreting as a solid
body and soon thereafter differentiated into a
layered planet during a period of internal heating.
Why Earth is a Dynamic
and Evolving Planet
Earth has continuously changed during its 4.6
billion year existence as a result of
interactions between its various subsystems
and cycles.
Why Earth is a Dynamic
and Evolving Planet
Earth is composed of 3 concentric layers.
Core
Mantle
Crust.
Why Earth is a Dynamic
and Evolving Planet
The Core
The core consists of
 a small, solid inner region
 a larger, liquid, outer portion
Composed of iron and a small amount of nickel.
Why Earth is a Dynamic
and Evolving Planet
The Mantle
The mantle surrounds the core and
is divided into:
 a solid lower mantle
 an asthenosphere that behaves
plastically and flows slowly
 a solid upper mantle.
Composed primarily of peridotite, an igneous rock
made of olivine.
Why Earth is a Dynamic
and Evolving Planet
The Crust
The outermost layer, the crust,
is divided into:
 thick continental crust
 thin oceanic crust
Why Earth is a Dynamic
and Evolving Planet
The Asthenosphere
 Surrounds the lower mantle
 Behaves plastically and slowly
flows
 Partial melting in the
asthenosphere generates magma
(molten rock) that rises to the
earth’s surface.
Why Earth is a Dynamic
and Evolving Planet
The Lithosphere
 The crust and upper mantle make
up the lithosphere which forms the
solid outer layers of the Earth.
Why Earth is a Dynamic
and Evolving Planet
Plate Tectonic Theory
 The lithosphere is composed of rigid plates that
diverge, converge, or slide sideways past one
another as they move over the asthenosphere
Why Earth is a Dynamic
and Evolving Planet
Plate Tectonic Theory
Why Earth is a Dynamic
and Evolving Planet
Plate Tectonic Theory
 Volcanoes and earthquakes occur at the
boundaries between the plates.
Plate Techtonics
Why Earth is a Dynamic
and Evolving Planet
Plate Tectonic Theory
Plate tectonic theory is
a unifying explanation
for many geologic
features and events,
helping us understand
the composition and
internal processes of
Earth on a global scale.
Continental Drift and Plate Tectonics
From the time maps of the globe became available, people wondered about the
arrangement of the continents and oceans. Hundreds of years later, valid explanations
were constructed.
Early Observations
Leonardo da Vinci and Francis Bacon wondered about the possibility of the American
and African continents having broken apart, based on their shapes.
This thinking continued up into the early 20th century, to a meteorologist named Alfred
Wegener.
Pangaea
Wegener revived the early idea of continental drift, contending that all of the present-day
continents were connected, side-by-side, as long ago as the Carboniferous (~300 Myr).
He called the supercontinental mass Pangaea,
Greek for ‘all lands’.
Wegener’s Evidence
Wegener’s summary was based on a number of careful observations:
-- matching rock, fossil, glacier, and structural relations
among different parts of different continents
Continental Drift: Fossil Evidence
Mesosaurus: purely freshwater reptile
Glossopteris: seeds too large to be effectively wind-transported
Continenta
l Drift:
Glacial
Evidence
Large ice masses
carve grooves in the
rocks over which flow.
Such masses tend to
flow outward
(generally downhill)
from a central locality.
Continental Drift: Rock
Ages
Even before geochronology, the
relative framework of rock ages showed strong correlation across the Atlantic, as did
mountain ranges of similar age.
Mechanism of Continental
Wegener never lived to see the general acceptance of continental drift, largely because
Drift?
of the lack of a mechanism. Wegener considered the buoyant continents to be ‘plowing’
through the mantle, resulting in mountain belts on continental edges.
Mantle Convection
Beginning just after Wegener’s end, Arthur Holmes began to
describe mantle heat flow in terms of convection.
Deep materials, hotter than their surroundings (and hence
buoyant), would tend to flow upward. In approaching the cool
surface of the Earth, the material would lose its thermal
energy, cool and sink, having lost buoyancy.
The motion of mantle material put into action by convection
thus becomes a plausible mechanism for moving rigid pieces
of the crust over some more actively flowing mantle material.
Mantle
Convection
Materials that can flow
tend to lose thermal energy
by the convection process.
This explains circulation in
a pot of water that is being
heated from below in the
same way it describes the
cooling of the Earth.
Sea Floor Spreading
Hess combined his observations with the earlier ideas of Wegener and the mechanism
of Holmes into the concept of
sea floor spreading, which lead to plate tectonics.
*This hypothesis
makes a number of
testable predictions.*
The Keys Features of Plate
Tectonics
(1) The Earth’s crust is constantly being created
and destroyed (recycled).
(2) Ocean crust, formed at divergent margins, is mafic and
dense.
(3) As ocean crust ages and cools, its great density relative
to the continents results in subduction
as plates converge.
[As a result, old ocean crust cannot persist, whereas old
parts of the buoyant continents can survive for eons.]
(4) The other kind of plate margins, transforms, are parallel
to the current motion of the plates.
Testing Plate Tectonics
Like any theory, plate tectonics has been rigorously tested,
and from a startling array of disciplines.
This model is consistent with the key tests thus far, including:
* sea floor spreading
* paleomagnetic ‘paths’
* age structure of the sea floor and continents
* locations and focal depths of earthquakes
* seismic tomography
* hotspot tracks
Mechanisms of Plate Tectonics:
1
RidgePush
2
3 Mantle
drag
convective flow of mantle
Mechanisms of Plate Tectonics:
4
PlumeDriven