Transcript Geology and the Earth (Con`t.)

Arthur’s Seat, Edinburgh, Scotland
Debbie Amato
Geology and the Methods of Science
Basic Assumption: The Earth works in an orderly fashion
in which natural phenomenon will recur given the same
set of conditions.
1. Question: We want to know how some part of the Earth
works.
2. Data Collection: Observe, measure, describe, compile.
3. Hypothesis! A logical but tentative explanation that fits
all the data collected and is expected to account for future
observations as well.
4. Testing. Testing, testing, and more testing.
5. Modification. Hypotheses are modified to include the
results of testing (see #4).
6. Theory! The hypothesis now withstands rigor of all the
testing (see #4) and consistently explains accumulating data
to become a THEORY. A theory is a generally accepted
explanation for a given set of data or observations.
7. Testing. The theory is tested, tested and tested again.
8. Scientific Law! If a theory meets rigorous testing over
a long period of time it may become a SCIENTIFIC LAW.
9. Principle. Generally a scientific law so fundamental to
geology that it provides the foundation of the science.
GEOLOGICAL CONCEPTS
1. Catastrophism - Observations led to the belief that the Earth
developed through a series of upheavals that were relatively
short-lived. A hypothesis that was proven inadequate and
discarded.
Through Geology, we have learned that the Earth is very
old (4.6 billion years). So….
Time, time, time is on my side, yes it is.
Siccar Point, Scotland
2. Principle of Uniformitarianism - James Hutton discovered
that Earth processes acted over a long period of time (1830s).
Hutton proposed that the physical, chemical and biological
processes observed on a daily basis, have also acted on the
Earth over very long periods of time.
Observations of current geological processes could be used to
interpret the rock record of very old geologic events.
BIRTH OF MODERN GEOLOGY
Other Important Geologic Principles
Superposition - The scientific law stating that in any
unaltered sequence of rock strata, each stratum is younger
than the one beneath it and older than the one above it, so
that the youngest stratum will be at the top of the sequence
and the oldest at the bottom.
Principle of Original Horizontality - The scientific law stating
that sediments settling out of a fluid (air and water) are
deposited horizontally or nearly horizontally in layers that lie
parallel or nearly parallel to the Earth’s surface.
Photo by Kevin Hefferan
Badlands National Park
Principle of Cross-Cutting Relationships - The scientific law
that states an igneous intrusion or geologic structure is always
younger than the rock that surrounds it.
Siccar Point, Scotland
Siccar Point, Scotland
Principle of Inclusions - The scientific law stating that rock
fragments contained within a larger body of rock are
always older than the surrounding body of rock.
So…
Where do we begin our study of
The Earth ?
The Creation of the Solar System
Begin with the “Big Bang” approximately 12 billion years ago.
Space expanded rapidly and then began to contract.
As temperatures cooled, Hydrogen and Helium gases formed.
Denser pockets of gas condensed further due to gravity.
Accumulations became galaxies.
Began to rotate to form discshaped clouds.
Center collapsed to form the Sun.
As heat increased in the Sun,
particles were blown away as
“solar wind”.
Particles collided and accreted
becoming planetesimals.
So how did we get to here?
As larger and larger particles collided,
larger planetesimals were formed.
Some of these continued to collide and
the largest became the planets, while the
smaller ones may have become moons.
Intense solar radiation heated the closest planets causing the
lighter elements to be vaporized and blown out into space.
This concentrated the heavier elements like iron and nickel on
the inner planets and the lighter elements on the outer planets.
The Earth’s Earliest History
Beginning of the Earth was
extremely violent.
Grew by planetesimal impact.
Became very hot, heated to the
melting point of iron.
Innermost rocks began to
become compressed, so more
heat.
Radiogenic heat was added
due to radioactive fission.
Earth underwent
differentiation into layers.
Early Differentiation of the Earth
What was the Earth’s early composition?
Need to consider meteorites that have struck the earth to get an
idea of composition.
Most are iron and nickel.
Some contain chondrules.
Small rocky bodies within the meteorites that may
represent matter condensing from the original solar
nebula.
Earth’s composition should be similar to these meteorites.
However Meteorites are 35 % iron, while Earth’s surface rocks only 6 %.
Early Differentiation of the Earth
Where did the iron go?
As Earth was still accreting, temperature
rose above melting point of iron.
Iron liquified.
Because of higher density, iron sank into
the proto-Earth’s center due to gravity.
Lighter elements rose to the surface.
Originally, Earth was homogeneous.
Due to heat and melting, Earth materials
separated forming concentric zones of
differing density.
Thus, Differentiation.
Differentiation and the Earth’s Interior
Earth’s Interior
Three Principal Layers
Each has different
Composition and density
(mass/volume).
CRUST - Outermost layer
Density = low
Composition is silicon and oxygen-based minerals
and rocks.
Crust is extremely thin.
Consistency is rocky.
Composed of two general types.
Continental crust
Oceanic crust
Earth’s Interior
Three Principal Layers
Each has different
Composition and density
(mass/volume).
CRUST - Outermost layer
Density = low
Composition is silicon and oxygen-based minerals
and rocks.
Crust is extremely thin.
Consistency is rocky.
Composed of two general types.
Continental crust
Oceanic crust
Earth’s Interior
MANTLE - Middle thin layer
Density = medium
Composition is silicon
and oxygen-based but
also includes iron
and magnesium.
Consistency is plastic.
Contains two parts, Upper and Lower Mantle.
CORE - Inner layer
Density = high
Composition is primarily iron and nickel.
Contains two parts
Inner core is solid.
Outer core is liquid.
Subdivisions of the Earth’s Interior
Within these three principal layers are subdivisions.
Crust consists of
OCEANIC CRUST (brown)
CONTINENTAL CRUST
(green).
Oceanic crust is thin
(8-10 km), dense,
and found below ocean
basins (blue).
Continental crust is thicker
(20-70 km), has low density and forms the bulk of continents.
The crust rides on the very upper most portion of the mantle.
The outermost sublayer is the most active geologically.
Large scale geological processes occur, including
earthquakes, volcanoes, mountain building and the
creation of ocean basins.
Contains parts of the upper mantle and all of the crust.
Called the LITHOSPHERE (rock layer).
Lithosphere is a strong layer, but brittle.
Represents the outer approximately 100 km of the Earth.
Thicker where continents exist, thinner under oceans.
Below the lithosphere resides the ASTHENOSPHERE
(weak layer).
Asthenosphere is part of the upper mantle.
Asthenosphere is heat softened and acts like a plastic.
It is weak, slow flowing, yet solid rock.
(Things that make you go, hmmm.)
Generally 100 to 350 km beneath Earth’s surface.
Overlying the lithosphere is the ATMOSPHERE.
Composed of gases released during volcanic eruptions and from
plant respiration.
Outgassing from volcanoes also helped produce the
water in the Earth’s ocean basins.
Led to the initial development of the HYDROSPHERE.
Together, the Lithosphere, Atmosphere and Hydrosphere
support the BIOSPHERE.
Earliest atmosphere was comprised of H, He,
ammonia (NH4) and methane (CH4).
Combination was deadly to life as we know it.
Modern atmosphere is composed of:
Nitrogen (N)
Oxygen (O)
Argon (Ar)
All other gases
79%
20%
1%
<1%
Atmosphere protects us from a variety of dangerous
particles.