Name: _____________________
Date: ______________________
Flynt- ___Period
___th Grade Science
Questions
Understanding Stratigraphy
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strata/strato/stratum = Latin root word for layers
o
stratosphere – calm layer of the atmosphere
o
stratovolcano – made of alternating layers of ash and lava
“ography” = Latin root word for describing and mapping
o
Geography – describe and map the earth
o
Oceanography – describing and mapping the ocean.
Stratigraphy = scientifically mapping, describing, and interpreting the layers found in
rocks.
o
The study of. the changes in strata or bedding of sedimentary rocks.
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Changes in rock types reflect changing environments of deposition.
o
While stratigraphy involves identification and interpretation of ALL rocks
present in a particular sequence, many stratigraphers specialize in sedimentary
petrology (the study of sedimentary rocks).
o
Involves determining the type, origin of, and relative and/or absolute age of
rock layers in order to infer the geologic history of a particular area.
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Relative Dating: Use the Laws of Stratigraphy to determine relative age
of rocks and the sequence of events that led to their formation
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Absolute Dating: Use radio isotope data (radiometric dating) from
fossils, igneous intrusions, or other inclusions found in strata to help
correlate and assign specific times to the events and changing
environments represented by changes in rock type.
Formation of Beds/Strata
Summary:
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beds/bedding = the layers or strata visible in a rock outcrop (bedrock)
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erosion = to wear away or carry away sediments, usually by wind, water and/or gravity
(mass wasting)
o
Wind, water, and gravity are “agents of erosion.”
o
The rocks that erode are called the source material.
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deposition = the settling of transported sediments after being suspended or carried in a
fluid (wind or water).
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Erosion/deposition and formation of new sedimentary rocks from older rock material
is part of the ROCK CYCLE!
Notes (cont.)
Questions
Steno’s Laws/Principles of Stratigraphy
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Nicholas Steno = father of stratigraphy; a Danish anatomist, geologist, and priest
(1636–1686); credited with describing a set of geologic observations that are the basis
of relative dating; published his laws/principles of stratigraphy in 1669, providing the
foundation for RELATIVE AGE DATING.
o
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Relative dating means identifying which rock units formed first, second, third,
and so on. Relative dating tells us the sequence in which events occurred, not
how long ago they occurred.
Law of Superposition = Oldest layer on bottom/youngest layer on top
o
"...at the time when any given stratum was being formed, all the matter resting
upon it was fluid [liquid or gas], and, therefore, at the time when the lower
stratum was being formed, none of the upper strata existed." Steno, 1669.
o
In an undeformed sequence of sedimentary rocks, each bed is older than the
one above it and younger than the one below it.
Although it may seem obvious that a rock layer could not be deposited unless it
had something older beneath it for support, it was not until 1669 that Steno
stated the principle.
This rule also applies to other surface-deposited materials, such as lava flows
and beds of ash from volcanic eruptions.
 Note that this rule does NOT apply to ALL igneous rocks; igneous
intrusions (batholiths, plutons, dikes, and sills) do NOT obey this rule.
Applying the law of superposition to the beds exposed in the upper portion of
the Grand Canyon, shown in the figure below, you can easily place the layers in
their proper order.
o
o
Older
Younger
o
Which layer is the oldest? The youngest?
What came first—the layers or the Colorado River?
Summary
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Law of Original Horizontality = most sedimentary and surface-deposited strata are
originally deposited or laid down in horizontal layers. If we find them at an angle or
folded, this happened sometime AFTER they were deposited.
o
"Strata either perpendicular to the horizon or inclined to the horizon were at one
time parallel to the horizon." Steno, 1669
o
o
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Summary
Rock layers that are folded or
tilted—like the layers shown here
exposed in the Namib Desert of
Africa—must have been moved into
that position by major crustal
disturbances—usually major
tectonic forces like regional faulting
and folding—long after the rocks
were deposited.
Law of Lateral Continuity = Sedimentary rocks are laterally continuous over large areas.
o
Rocks that are otherwise similar, but are now separated by a valley or other
erosional feature, can be assumed to have been originally continuous.
o
Think of snowfall. As snow falls, it is not limited to just the roads; it does not fall
in one backyard, but not another yard. Instead, it falls over large regional areas.
Sediments also "rain" down in a similar fashion such that sedimentary layers are
laterally continuous over large areas.
o
"Material forming any stratum were continuous over the surface of the Earth
unless some other solid bodies stood in the way." Steno, 1669
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Law of Cross-Cutting Relationships = Canyons,
impact craters, faults, and igneous intrusions (dikes,
plutons, and batholiths) that cut through or across
sedimentary layers or other features must have
occurred AFTER the layers were deposited.
o
"If a body or discontinuity cuts across a
stratum, it must have formed after that
stratum." Steno, 1669.
If younger features cut across older features, then which occurred first— the batholith (B), the
dike (D), the thrust fault (A), or the normal fault (F)? Folded, metamorphic rock strata (also
called basement rocks) are cut by the thrust fault (A). This was followed by a large igneous
intrusion (the batholiths cuts off fault A). An erosional unconformity (C), on which rock strata
were later deposited, cuts off the folded, metamorphic strata, the thrust fault (A), and the
batholith (B). A volcanic dyke
(D) cuts through A, B & C,
but layer E is an even
younger layer of rock
deposited after the uplift
and erosion that caused
the unconformity (C)—
notice that that volcanic
dike (D) ends abruptly at
layer E. Finally, a normal fault (F)
cuts through B, C & E.
Summary
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Law of Inclusions = This law, developed by James Hutton, states that the rock
fragments (sediments and clasts—broken pieces of rock) that make up sedimentary
rocks must be older than the sedimentary rock containing the fragments.
o
The sediments and clasts that make up clastic
sedimentary rocks (like the conglomerate
shown here) were once part of an entirely
different rock.
o
In figure A below, the parent rock (granite) had
to become exposed and eroded; the granite
clasts mixed with other sediments and clasts
were later deposited in the same general area,
where they became compacted and cemented, forming a new sedimentary rock
(in this case conglomerate) with large granite inclusions. The granite inclusions
in the conglomerate indicate that the granite parent rock must be older.

o
Summary
Sediments and other clasts may move mere inches from their parent
rock or hundreds of miles via wind, water, and gravity before final
deposition!
The law of inclusions also relates to igneous rocks (like granite) with igneous
inclusions (like xenoliths, tephra, etc.), which are foreign rock fragments that
get caught up along the edges of intruding bodies of magma (plutons,
batholiths, dikes).
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While many igneous inclusions are destroyed as they melt and become
part of the magma, occasionally the magma cools and solidifies before
some of the inclusions completely melt.
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Igneous inclusions are often similar in composition to the “country
rock” invaded by the body of magma.
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In the case of Figure B above, magma has intruded into conglomerate
country rock. Pieces of the conglomerate country rock broke off and got
caught up in the magma. When the magma cooled and hardened, the
chunks of conglomerate were trapped, forming igneous inclusions
called xenoliths (“foreign rock”).
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Law of Faunal Succession- This law, developed by William "Strata" Smith, say that
fossils with more archaic biological features and organisms are succeeded in the fossil
record (followed by) by fossils of more modern versions/organisms.
o
Smith recognized that fossil groups were succeeded (followed/replaced) by
other fossil groups through time.
o
In a sequence of rocks, more archaic forms would be near the bottom, while
more modern organisms or forms would be near the top.
o
Example: A fossilized Neanderthal bone will never be found in the same
stratum as a fossilized Megalosaurus (a type of dinosaur) because Neanderthals
and Megalosauruses lived during different geological periods, separated by
many millions of years.
o
Law of faunal succession allows for strata to be identified and dated by the
fossils found within, which in turn allowed geologists to develop a fossil
stratigraphy and provided a means to correlate rocks throughout the world.
Steno was honored with a Google Doodle on January 11, 2012, his 374th birthday:
Which law is illustrated in each picture below?
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