Classroom presentations
to accompany
Understanding Earth, 3rd edition
prepared by
Peter Copeland and William Dupré
University of Houston
Chapter 1
Building a Planet
Understanding the Earth System
Geology is a science...
...just like chemistry and physics!
Geologists face the special
challenge of not being able to do
experiments in the sense that
chemists and physicists do.
Scientific principles
The universe is sensible
and governed by
immutable rules.
The scientific method
1) Make an observation about
the sensible world.
2) Develop an explanation
(hypothesis) that predicts the
outcome of other
observations or experiments.
The scientific method (cont.)
3a) Make new
experiments.
3b) Make new
observations.
The scientific method (cont.)
return to
step 2
Reject the
hypothesis
no
return to
step 3
4) Do
the results
match the
predictions of the
hypothesis
?
yes
The scientific method
Simply put:
Always ask yourself,
“How will I know if I’m wrong?”
Hypothesis - Theory - Law
A hypothesis is an explanation initially
offered for a set of observations.
When a hypothesis withstands many
tests it may be called a theory.
A theory for which it seems there to be
no sensible reasons to challenge is
called a law.
A good theory is...
• Parsimonious
(is the simplest explanation available)
• Consilient
(explains a wide range of
phenomena)
What are the data used in geology?
Observational:
•
maps
» rock types, distributions,
structures
•
microscopic investigations
Experimental:
•
chemical
•
geophysical
The problem of experiments
Since geologists are interested in
systems that are very big (hundreds
of km) and that have evolved over
long periods of time (millions of
years), they cannot conduct
controlled experiments. They must
observe the results of Nature’s
experiments that are already
complete.
Uniformitarianism
The present is the key to the
past
— James Hutton
Natural laws do not change —
however, rates and intensity
of processes may.
TIME
• The big difference between geology
and other sciences: TIME
(Geologically speaking, not much
happens in a human lifetime!)
• Rates of geologic processes:
µm/year to cm/year
• Big earthquakes may displace the
ground several meters in a few
seconds, but they occur only every
500 years or so.
TIME
The rates of geologic
processes are almost always
slower than the rates of human
effects on the environment.
TIME
The official SI unit of time is the
second, but it would be very
inconvenient to use this unit in
geology; even the year is too small
in most cases.
Therefore, geologists use millions of
years as the standard unit of time:
10 Ma = 10 million years ago
10 m.y. = an interval of time lasting
10 million years
Some
geologic
features take
millions of
years to
form.
Carr Clifton
Fig. 1.1
Others take
seconds!
Fig. 1.1
John Sanford/Photo Researchers
Origin of solar system
Pick a theory, any theory, but it must be
consistent with these facts:
1) Planets all revolve around the Sun in the
same direction in nearly circular orbits.
2) The angle between the axis of rotation and
the plane of orbit is small (except Uranus).
3) All planets (except Venus and Uranus) rotate
in the same direction as their revolution; their
moons do, too.
Origin of solar system
4) Each planet is roughly twice as far as the next inner
planet is from the Sun (the Titus-Bode rule).
5) 99.9 % of mass is in the Sun; 99 % of angular momentum
is in the planets.
6) Planets in two groups:
•
terrestrial (inner): Mercury, Venus, Earth, Mars Mercury
is mostly Fe ( = 5.4)
•
Jovian (outer): Jupiter, Saturn, Uranus, Neptune. Jupiter
mostly gas and ice (  = 0.7) Pluto ????
7) Terrestrial planets are mostly O, Si, Fe, Mg. The Sun is
almost entirely H & He (also important in Jovian planets).
Nebular hypothesis
Primeval slowly rotating gas cloud
(nebula) condensed into several
discrete blobs.
fits
rotation
mass
doesn't fit
angular momentum
Collision hypothesis
Portions of the Sun were torn off
by a passing star: planetesimals
then collided to form planets.
Problems: gases coming from
Sun would be too hot to
condense; stellar collision
exceedingly rare.
Protoplanet hypothesis
• Large gas cloud begins to condense.
• Most mass in the center, turbulence in outer
parts.
• Turbulent eddies collect matter meters across;
small chunks grow and collide, eventually
becoming large aggregates of gas and solid
chunks.
• Protoplanets, much bigger than present planets,
eventually contracted due to their own gravity.
Evolution of the Solar System
Fig. 1.2a
Evolution of the Solar System
Fig. 1.2b
Evolution of the Solar System
Fig. 1.2c
Evolution of the Solar System
Fig. 1.2d
Our Solar System
Fig. 1.3
The Moon
• Only a little smaller than Mercury
(small planet in two-planet system).
• Surface of the moon very different
from the surface of Earth.
• No atmosphere, therefore, no
weathering.
Formation of the Moon
Alfred T. Kamajian
Fig. 1.4
Timeline for the
Sun, Earth, and Moon
Extensively modified from D.J. DePaolo, Nature
Fig. 1.5
Why worry about the beginning?
• The evolutionary course is
significantly influenced by the initial
state.
• We know the state of the Earth today
relatively well; knowing the
beginning will help constrain the in
between.
A Differentiating Planet
Fig. 1.6
An Early Homogeneous Earth
Fig. 1.6a
Differentiation
Begins
Fig. 1.6b
Fig. 1.6c
Relative Abundance of Elements
Fig. 1.7
Lithosphere
Hydrosphere
Atmosphere
Biosphere
Fig. 1.9
Interacting
Earth Systems
Fig. 1.8
Plate tectonics
The unifying concept of
the Earth sciences.
• The outer portion of the Earth
is made up of about 20 distinct
“plates” (~ 100 km thick) that
move relative to each other.
• This motion is what causes
earthquakes and forms
mountain ranges.
Plate tectonics
• lithosphere: the outer rigid
shell of the earth (~ 100 km).
The plates are composed of
this material.
• asthenosphere: part of
mantle beneath lithosphere.
• The lithosphere rides ON
TOP of the asthenosphere.
Earth’s Crust, Lithosphere,
and Asthenosphere
Fig. 1.11
Present-day Plates
Pete W. Sloss, NOAA-NESDIS-NGDC
Fig. 1.12
Convection
in the Kitchen
Fig. 1.13a
Convection
in the Earth
Fig. 1.13b
Three types of plate boundaries
1. Divergent
2. Convergent
3. Strike-slip
(transform,conservative)
Three Types of Plate Boundaries
Fig. 1.14
Fig. 1.14a
Fig. 1.14b
Mid-Atlantic
Ridge
Peter W. Sloss, NOAA-NESDIS-NGDC
Iceland is
being pulled
apart as it sits
astride the MidAtlantic Ridge.
Gudmundur E. Sigvaldason,
Nordic Volcanological Institute
Fig. 1.15
San Andreas Fault
John Sheldon
Fig. 1.17
Fig. 1.16
The rejection and acceptance
of Continental Drift
• First suggested by Alfred Wegener in
1912.
• Rejected by most geologists.
• New data after WWII led to the “plate
tectonic revolution” in 1960’s.
• Now embraced by essentially everybody.
• Today’s geology textbooks radically
different than those of 40 years ago.
The Supercontinent of Pangaea
(200 million years ago)
Fig. 1.18
Present-day Plates
Pete W. Sloss, NOAA-NESDIS-NGDC
Fig. 1.12
Caution: Geologist at Work
David R. Frazier/ Photo Researchers
Fig. 1.19