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Principles of Geology
Essential Questions
What is Uniformitarianism?
What is Catastrophism?
What is Steno’s Law?
What are the other geologic principles?
How is relative age determined using
these principles?
What is Uniformitarianism?
Development of the Theory
In the mid-seventeenth century, biblical scholar
and Archbishop James Ussher determined that
the earth had been created in the year 4004
BCE
Just over a century later James Hutton, known
as the father of geology, suggested that the
earth was much older.
The 19th century scholar Sir Charles Lyell whose
Principles of Geology (1830) popularized the
concept of uniformitarianism.
Uniformitarianism
can be summarized by the phrase "the
present is the key to the past."
based on the slow, natural processes
observed on the landscape
if given enough time, a stream could carve
a valley, ice could erode rock, sediment
could accumulate and form new landforms
What is Catastrophism?
Catastrophism
is the idea that many of Earth’s crustal
features (strata layers, erosion, polystrate
fossils, etc) formed as a result of past
cataclysmic activity.
What is Steno’s Law?
The 3 principles of Steno’s Law
The Principle of Superposition
The Principle of Original Horizontality
The Principle of Original Lateral Continuity
The Principle of Superposition
In any series of layered rocks, the older
rocks are on the bottom and they become
younger as you move up through the
beds.
The Principle of Original
Horizontality
All sediments are originally deposited in
flat layers.
The Principle of Original Lateral
Continuity
Sedimentary beds form over a large area in a continuous
sheet, gradually thinning toward the edges.
Often, coarser-grained material can no longer be
transported to an area because the transporting medium
has insufficient energy to carry it to that location. In its
place, the particles that settle from the transporting
medium will be finer-grained, and there will be a lateral
transition from coarser- to finer-grained material.
What are the other geologic
principles?
The Principles of Intrusive Relationships
The Principle of Cross-cutting
Relationships
The Principle of Inclusions
The Principle of Faunal Succession
Principle of Unconformities
The Principles of Intrusive
Relationships
The sedimentary rocks are older than the
igneous rock which intrudes them.
In other words, the sedimentary rocks had to be
there first, so that the igneous rocks would have
something to intrude.
Examples of types of igneous intrusions are:
Dikes
Sills
Laccoliths
Stocks
Batholiths
Discordant - a differing type
of rock cutting across a formation
Concordant – a differing type
of rock which lies parallel to a
formation
Dikes
is a type of sheet intrusion
referring to any geologic
body that cuts discordantly
across
Sill
is a tabular sheet
intrusion that has
intruded between
older layers of
sedimentary rock,
beds of volcanic
lava, or even along
the direction of
foliation in
metamorphic rock
Stocks
is a discordant igneous
intrusion having a
surface exposure of
less than 40 square
miles
may have been
feeders for volcanic
eruptions
Batholiths
is a large emplacement of
igneous intrusive rock that
forms from cooled magma
deep in the earth's crust
almost always made mostly
of felsic or intermediate
rock-types
very large intrusive bodies,
usually so large that there
bottoms are rarely exposed
Laccoliths
is a sheet intrusion that has
been injected between two
layers of sedimentary rock.
The pressure of the magma
is high enough that the
overlying strata are forced
upward, giving the laccolith
a dome or mushroom-like
form with a generally
planar base
Diagram (1): Dike B is younger than Sedimentary Rock
A. Erosion surface C is younger than Dike B.
Sedimentary Rock D is younger than Erosion Surface C.
Diagram (2) Sill B is younger than Sedimentary Rock A.
Dike C is younger than sill B.
Diagram (3) Stock B is younger than Sedimentary Rock
A. Dike C is the youngest.
The Principle of Cross-cutting
Relationships
Originally developed by James Hutton in Theory of the
Earth (1795)
embellished upon by Charles Lyell in Principles of
Geology (1830),
the principle states that the fault which cuts a rock is the
younger of the two features
if a fault is found that penetrates some formations but
not those on top of it, then the formations that were cut
are older than the fault, and the ones that are not cut
must be younger than the fault
Types of Cross cutting
Structural
Stratigraphic
Occur where currents have eroded or scoured older sediment in
a local area to produce, for example, a channel filled with sand.
Paleontologic
An erosional surface (or unconformity) cuts across older rock
layers, geological structures, or other geological features.
Sedimentological
A fault or fracture cuts through an older rock
Occur where animal activity or plant growth produces
truncation. This happens, for example, where animal burrows
penetrate into pre-existing sedimentary deposits.
Geomorphic
Occur where a surficial feature, such as a river, flows through a
gap in a ridge of rock. In a similar example, an impact crater
excavates into a subsurface layer of rock.
The Principle of Inclusions
developed by James Hutton
with sedimentary rocks, if inclusions (or
clasts) are found in a formation, then the
inclusions must be older than the
formation that contains them
The Principle of Faunal Succession
developed by the English geologist William Smith
is based on the observation that sedimentary
rock strata contain fossilized flora and fauna,
and that these fossils succeed each other
vertically in a specific, reliable order that can be
identified over wide horizontal distances
As organisms exist at the same time period
throughout the world, their presence or
(sometimes) absence may be used to provide a
relative age of the formations in which they are
found
Principle of Unconformities
Any place where there is missing time is
called an unconformity
Unconformities normally happen either
through erosion or non-deposition
An unconformity represents time during
which no sediments were preserved in a
region
The Great Unconformity
Grand Canyon, AZ
Types of Unconformities
Angular Unconformity
is where horizontally parallel strata of sedimentary rock
are deposited on tilted and eroded layers, producing an
angular discordance with the overlying horizontal layers.
younger sediments rest upon the eroded surface of tilted
or folded older rocks
Disconformity
is an unconformity between parallel layers
of sedimentary rocks which represents a
period of erosion or non-deposition
contact between younger and older beds is
marked by a visible, irregular or uneven
erosional surface
Disconformity continued…
Nonconformity
exists between sedimentary rocks and
metamorphic or igneous rocks when the
sedimentary rock lies above and was
deposited on the pre-existing and eroded
metamorphic or igneous rock
develops between sedimentary rock and older
igneous or metamorphic rock that has been
exposed to erosion
Nonconformity continued…
How is relative age determined
using these principles?
What is relative dating?
is the science determining the relative
order of past events, without necessarily
determining their absolute age
can only determine the sequential order in
which a series of events occurred, not
when they occur