Transcript Scientific Dating Methods of Fossils and Rock Sequences

Scientific Dating Methods
Unit 5
• Scientific dating methods of fossils and
rock sequences are used to construct a
chronology of Earth’s history expressed in
the geologic time scale.
James Ussher (1581-1656)
• Archbishop of Armagh,
Primate of all Ireland
• Constructed a chronology
of human and Earth
history in which he
determined that Earth
was only a few thousand
years old.
• Catastrophism—Earth
had been developed
primarily by great
catastrophes.
Relative Dating
• Placing rocks in their proper sequence of
formation—which formed first, second,
third, and so on
• Cannot tell us how long ago something
took place, only that it followed one event
and preceded another
Nicolaus Steno (1638-1686)
• Danish pioneer in
geology
• Three defining
principles of
stratigraphy
Original Horizontality
• sedimentary layers
are initially deposited
in horizontal layers in
a basin
• Forces break up rock
into small pieces and
these pieces wash
down rivers to the
ocean and settle on
the seafloor
Rock Superposition
• Younger rock layers
are on top of older
rock layers
Lateral Continuity
• Sediments are spread out
over continuous and
extensive geographic
areas
• They end by thinning out
at the edge of the basin
where they are deposited
–or• They abruptly stop at a
barrier –or• They grade into a different
type of sediment
James Hutton
• Scottish physician
and gentleman farmer
• Published Theory of
the Earth
• Principle of
Uniformitarianism—
the earth was shaped
by slow-moving
forces still in
operation today
Charles Lyell (1797-1875)
• British lawyer and
geologist
• Popularized
uniformitarianism
• Close and influential
friend of Charles
Darwin
Cross-Cutting Relationships
• Any event that cuts
across an existing
rock unit is younger
than that unit
• Common events that
can cut across
existing rock units are
unconformities,
intrusions and faults.
Unconformities
• Angular Unconformity
– Tilted or folded
sedimentary rocks are
overlaid by younger,
more flat-lying strata
– While layers were
being set down,
folding or tilting and
erosion occurred
Unconformities
• Disconformity
– Harder to identify
– Strata on either side
are parallel
Unconformities
• Nonconformity
– The break separates
older sedimentary
metamorphic or
intrusive igneous rocks
from younger
sedimentary strata