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Lecture 20

EPSC201 Lecture 20 Notes.doc

Earth & Planetary Sciences
Course Code
EPSC 201
Anthony Williams- Jones

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EPSC201 - Lecture 20 Notes
How do we date the earth? In the 1600s we dated the Earth using biblical sources. They thought it
was around 6000 years old. In the 1800s, Lord Kelvin and other physicists tried to calculate how long
the Earth would take to cool, assuming the Earth started out as molten. Kelvin figured the Earth was
around 100 million years old (last lecture said 400 million years old). Wikipedia says the Kelvin initially
calculated the earth to be between 20 to 400 millions years old, but eventually settled on 20-40 mil-
lion years old.
Lord Kelvin, and other physicists, did not account for radioactive forces. As elements decay, they re-
lease heat in the form of alpha particles, which are absorbed by the rocks in the Earth, and are heat-
ed. This radioactivity heats the Earth.
Radioactivity started with Rutherford at McGill University. He showed that elements have a constant
rate of radiation decay. Rutherford made several estimates using uranium and lead decay. Different
rocks are different ages. The oldest rocks in Canada range from 3.8 to 4.2 billion years old. Mete-
orites can also be dated. In 1956, a meteorite in Kenya was dated to be 4.55 billion years old. In
1956, we established the age of the earth using this meteorite.
Not all rocks can be dated by these methods. They must have significant amounts of radioactive ele-
ments in them.
The first person to challenge the biblical date was a Danish bishop. He established this using the law
of Uniformitarism. A Scottish geologist named James Hutton introduced Uniformitarism. He is re-
garded as the father of modern geology. He said if we
could understand the present processes and the rate
at which they occur, we could use this to figure out the
age of rocks. His logic was that geological processes
have not changed over time. He noticed that mud
cracks now, have the same features as mud cracks
from 500 million years ago.
Uniformitarianism is the assumption that the same natural laws and process-
es that operate in the universe now, have always oper-
ated in the universe in the past and apply everywhere in
the universe. It
has included the concept that "the
present is the key to the past" and is functioning at the
same rates. Uniformitarianism has been a key principle of
geology and virtually all fields of science.
So this Danish bishop, Nicholas Steno, developed the prin-
ciple of superposition. He proposed that the top layer of
rocks were the youngest, and the deepest rocks were the
oldest. We now refer to this as stratigraphy. This was his
first principle.

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Steno’s second principle was that all sedimentary layers are formed in horizontal layers. He no-
ticed that canyons contained strata that traveled for long distances.
Steno’s third principle was the principle of original continuity. This states that if some amount of
sediment is worn away, you can pick up the same sediments some distance away, because at some
time, the sedimentary layer was continuous.
This seems fairly obvious now, but at the time it was not apparent.
Geological maps made nowadays, these principles are still applied.
Steno also established the crosscutting relationships. In the picture we have different sedimentary
layers. Limestone is shown by the brick symbol. The middle layer is shale, and the top layer is sand-
stone. Based on the law of superposition, we know the deepest layer is the oldest. Due to lateral
movment, we know the layers were once continuous. The lava crosscuts the layers by rising up
through them. The cross cutting relationship says that the cross cutting layers of magma are younger
then the rock they cut through. Using these relationships, we can tell the sequence they formed in.
Based on cross cutting relationships, we know the vertical crosscut is the youngest.
Xenoliths are pieces of foreign rock. The inclusions are broken off pieces of other layers. So the sill
must have came last.

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In the bottom left picture, we had magma introduced horizontally as a sill, and broke off pieces of the
limestone and sandstone. So the sill came in last, because the sandstone and limestone must have
already been present. The sill was inserted between two layers, and contains fragments of both. The
limestone and sandstone must have been an area of weakness because the lava was able to force
the two layers apart.
For the bottom right picture, the flow (lava) was laid down before the sandstone. We know this be-
cause there is a flow intrusion into the
sandstone, but no sandstone in the
flow. As the sandstone was deposited,
it broke chunks of the flow, which were
then included in the top layer.
We know from the law of horizontality
that the layers shown in the picture be-
low must have been horizontal at one
point. We see that the basalt dike is
the most resent formation in the pic-
ture. It has cut in after the fault. The
granite has intruded everything up to
layer five, so its younger then layer 5.
The granite has been moved by the
fault, so we know that the dip-slip fault
occurred after the granite intrusion.
Note how the dike includes layer of
shale and sandstone. This means it intruded after the sandstone layer formed.
There was a sill intrusion, which is younger then layer five, but older then the fault.
We can get the order of relative activity. We know that since layers 6 and 7 were affected by the fault,
that they happened before the fault occurred. The order of oldest to youngest is shown below.
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