Transcript Geological History (1): Strata

4, 6
Warm up
Wrap up
5,7
Geological History (1): Strata
© Howard Falcon-Lang
Youngest strata
http://en.wikipedia.org/wiki/Nicolas_Steno
Nicolas Steno
(1638-1686)
Oldest strata
So far we’ve looked at early attempts to determine the age of the Earth, let’s now consider how
other scientists were trying to work out its history. The term geological history refers to the
sequence of historical events from the formation of the Earth to the present day.
One of first people to try and understand the history of our planet was a Danish priest called
Nicolas Steno (1638-1686). In 1669 he argued that sediment gradually built up on the sea floor as
layers which are both laterally continuous and horizontal. The sediments at the bottom of the pile
are the oldest and the sediments at the top are the youngest. Steno showed that in much the same
way that the pages of a book are read from left to right, the history of the Earth must be read from
bottom to top in a layered succession of rocks.
Background note: Steno’s insight that sediments accumulate in continuous horizontal layers is
known as the Principle of Horizontality and the Principle of Lateral Continuity. His insight that
the oldest sediments are at the bottom and the youngest at the top is known as the Principle of
Superposition. These three principles are fundamental to understanding the order that events
occurred through Geological Time.
geological
history
sequence of historical
events from the
formation of the Earth
to the present day
strata
Rock layer
Principle of
Horizontality
and
Principle of
Lateral
Continuity
sediments accumulate
in continuous
horizontal layers
Principle of
oldest sediments are at the
Superposition. bottom and the youngest
at the top
Geological History (2): Neptunism
http://en.wikipedia.org/wiki/Image:Abraham_
Gottlob_Werner.jpg
• Werner argued all rocks
had been deposited in
a worldwide ocean
(think Noah’s Flood)
• Geologists could figure
out the order in which
rocks formed
• Divided geological record
into four main divisions
Abraham Werner
(1749-1817)
Granite
© Howard Falcon-Lang
A century later, Abraham Werner (1749-1817), a mining
engineer from Saxony, started to apply some of Steno’s
principles to understand the history of the Earth. He thought
that all the Earth’s rocks had been deposited from a worldwide
ocean (think Noah’s Flood in the Bible). This idea was later
dubbed ‘Neptunism’ after the Roman god of the sea
(Neptune).
Neptunism
Earth’s rocks had been
deposited from a worldwide
ocean (think Noah’s Flood in
the Bible).
Geological History (3): Gaps
Copyright © Marli Miller, University of Oregon
http://upload.wikimedia.org/wikipedia/
commons/c/c6/James_Hutton.jpg
Siccar Point
• Hutton’s unconformity showed that there
were big gaps in the geological record
James Hutton
(1726-1797)
Hot on the heels of Werner came another influential geologist called James Hutton
(1726-1797). In the late eighteenth century, Hutton realized that there were ‘big gaps’
in the geological record of time.
At several places like Siccar Point in Berwickshire, Scotland, Hutton’s noticed beds of
layered rocks that stood on their end overlain by other layered rocks that dipped away
at a shallow angle. Hutton was quick to realize what this meant. In the real world,
sediment is not deposited continuously. Rather through convulsions of the Earth’s crust,
sediments laid down as horizontal layers in the sea might be pushed up to form
mountains. The layers might then be tilted at an angle and partly eroded away before
new horizontal layers accumulated on top.
Hutton’s name for this kind of angular contact between two sets of sedimentary layers
was an unconformity. In the time between the formation of the lower and upper layers
of rocks, a mountain had been born and eroded down. This would have taken tens of
millions of years. Therefore the unconformity represented a massive time gap in the
geological record. Hutton referred to this phenomenon as ‘the abyss of time’.
James Hutton
(1726–1797)
unconformity
“father
of modern geology”
Believed Earth to be ever
changing and very old
angular contact
between two sets of
sedimentary layers
Unconformities
Movements in the Earth’s crust can lift up rock layer
that were buried and expose them to erosion. If
layers of new sedimentary rock are laid down on
this eroded surfaces, an unconformity forms.
Unconformities are boundaries between old rock
and new rock and they indicate an interruption in
the rock record. The old rock may be igneous,
metamorphic or sedimentary. The new rock is
always sedimentary. According to the Law of
Superposition, all rocks beneath the unconformity
are older than the rocks above it.
When rock layers are tilted or
Angular
nonconformity folded, then eroded and new
sedimentary rock is laid on top of
the eroded surface.
Disconformity When sedimentary rock is eroded
and new sedimentary rock is laid
over top.
When igneous or metamorphic
Nonconformity
rock is exposed and eroded and
new sedimentary rock is laid
over top.
HACHURES INDICATE
METAMORPHISM
Pg. 9
Construct the spider diagram. At
the end of each leg, there are 2
lines(toes). At the end of one
line/toe write the definition for the
concept and at the end of the
second line/toe, draw a picture
illustrating the concept.
Definition
Picture
Types of
unconformities
6
Warm up
Wrap up
7
8
Warm up
Wrap up
9
Geological History (4): Maps
http://en.wikipedia.org/wiki/Image:
Geological_map_of_Great_Britain.jpg
http://en.wikipedia.org/wiki/Image:Smith_fossils2.jpg
http://en.wikipedia.org/wiki/Image:
William_Smith.g.jpg
Some of
Smith’s fossils
William Smith
(1769-1839)
The Map that
changed the World
At the turn of the nineteenth century,
‘maps’ followed ‘gaps’ in the discovery of
geological history. In Britain railways and
canals were being constructed all over the
place. More than ever before, rocks were
being exposed all over the country.
William Smith (1769-1839) was a surveyor who saw
more than his fair share of rocks. He realized that
sequences of different rocks occurred in the same order
in different places. To be sure that a rock type in one
place was the same as a rock type in another place, he
compared the fossils that they contained.
Soon he started to draw maps of where the different rock
strata occurred and by 1815 he had produced a complete
geological map of Britain. Building on the insights of
Steno, Werner and Hutton, William Smith’s map showed
the relative order in which the rocks of Britain formed
and whether there were any big time gaps within the
succession. For the first time, the geological history of a
whole country was known.
Geological History (5): Fossils
en.wikipedia.org/wiki/Image:Georges_Cuvier.jpg
en.wikipedia.org/wiki/Image:Charles_Lyell.jpg
Cuvier
Georges Cuvier
(1769-1832)
Charles Lyell
(1797-1875)
• Cuvier showed that
some animals had
gone extinct
• Lyell used the
proportion of living
fossils to divide up
geological time
• Older rocks contained
more extinct types
than younger rocks
As we have just seen, William Smith used fossils to
check that the age of rocks in one area was the same
at the rocks in another. But why should rocks of
different ages contain the remains of different life
forms? It fell to two other scientists, Georges Cuvier
(1769-1832) and Charles Lyell (1797-1875) to answer
this question and in doing so discover the concept of
extinction.
Cuvier was a French naturalist. In 1796 he studied remains of
fossil and living elephants. He convincingly showed that animals
like the woolly mammoth had gone extinct. This was a pretty
radical idea because up to that point people thought extinction was
impossible. After all, if God’s creation was perfect why would He
let species like the mammoth die out?
A little later in 1828, Lyell studied fossil seashells in Tertiary rocks
in France. He showed that the oldest rocks contained mostly
extinct shells while the youngest rocks contained shells similar to
those living today. He divided up these Tertiary deposits into three
epochs which he named, from oldest to youngest, the Eocene,
Miocene, and Pliocene. At last here was an explanation as to why
the rocks of different time periods contained distinctively different
types of life forms
fossils
preserved remains or
traces of animals
Geological History (6): Stratigraphy
Sedgewick
Murchison
commons.wikimedia.org/wiki/Image
:Adam_Sedgwick.jpg
en.wikipedia.org/wiki/Image:Roderick
_Murchison.jpg
en.wikipedia.org/wiki/Image:Geological_map_of_Great_Britain.jpg
A tug-of-war as rocks got sorted into geological periods in the
new science of stratigraphy
One famous argument was between Adam
Sedgewick (1785-1873) and Roderick Murchison
(1792-1871). Both these men mapped the rocks in
Wales, which were thought at that time to be the
oldest rocks that contained fossils. Both wanted to be
the first to name this ancient geological period.
Sedgewick proposed the name Cambrian for the the
geological period represented by his rocks in Central
Wales. Murchison proposed the name Silurian for
the the geological period represented by his rocks in
North Wales.
As the Cambrian and Silurian strata overlapped, Murchison
wanted all of Sedgewick’s Cambrian to be included in his
Silurian. Based on the fossils they contained, Murchison didn’t
think there was much age difference between the two time
periods. Murchison and Sedgewick had a real tug-of-war over
the rocks of Wales and afterwards didn’t speak to each other for
years to come.
Eventually, in 1879, Charles Lapworth (1842-1920) sorted out
the Welsh problem. He carried on using the names, Cambrian
and Silurian, but re-named the overlapping beds in the middle as
the Ordovician. Hence the oldest rocks in Britain that contain
‘visible fossils’ comprise three geological periods which are,
from oldest to youngest, the Cambrian, Ordovician, and Silurian.
stratigraphy The science of mapping
rocks and working out
their age relative to one
another
Geological History (7): Periods
en.wikipedia.org/wiki/Geologic_time_scale
Carboniferous
Triassic
Cambrian
Ordovician
Silurian
Permian
Devonian
Jurassic
Quaternary
Cretaceous
Tertiary
It wasn’t just in Wales that disputes raged about the
naming and relative age of different rock types.
Throughout the nineteenth century, geologists all
over Europe were describing the geological record,
eager to be the first to name the different geological
periods. The Devonian Period was named after
rocks in Devon that were a little younger than
Sedgewick’s Silurian. The Carboniferous Period
was named after the coal-rich rocks in Yorkshire.
The Jurassic Period was named after the rocks of the
Jura Mountains of Switzerland, and so on.
Geological History (8): The Column
http://en.wikipedia.org/wiki/Geologic_time_scale
Geological Time: Eons, Eras, Periods and Epochs
Geological History (9): Example
en.wikipedia.org/wikiImage:Palais_de_la_Decouvert
e_Tyrannosaurus_rex_p1050042.jpg
http://en.wikipedia.org/wiki/Image:KT_boundary_054.jpg
Impact
http://en.wikipedia.org/wiki/Image:Impact
_event.jpg
Paleogene
Extinction
Cretaceous
en.wikipedia.org/wiki/Image:Tyrannosaurus_BW.jpg
Radiometric dating (1): Discovery
http://en.wikipedia.org/wiki/Image:
Henri_Becquerel.jpg
• In 1896, Discovery of radioactivity paved
the way for the precise dating of events
in the geological record
Henri Becquerel
(1852-1908)
Radiometric dating (2): Decay
en.wikipedia.org/wiki/Image:Alpha_Decay.svg
• Radioactive ‘parent isotopes’ spontaneously emit protons and
neutrons and decay into ‘daughter isotopes’
• E.g., Uranium-238 decays into Lead-206
Radiometric dating (3): Half life
Half life: 0
Exponential
Half life: 1
Linear
Half life: 2
• The rate of decay from parent to daughter isotope depends on
its half life. The half life is the amount of time needed
for half the parent isotope to decay to daughter isotope
Radiometric dating (4): Clocks
Decay series
40K
Half life
to 40Ar
147Sm
to 143Nd
1250 Ma
Geological
timescales
1060 Ma
235U
to 207Pb
704 Ma
238U
to 206Pb
4468 Ma
14C
to 14N
5370 years
Archaeology
• Different radioactive isotopes have different half lives
• Isotopes with long half lives are useful for dating old rocks. It is
important to use the right tool for the right job
Radiometric dating (5): Pioneers
• Rutherford figured out a
technique to date the age
of rocks in 1904
• Holmes developed this
kind of ‘radiometric dating’
still further.
• In 1913 Holmes dated
en.wikipedia.org/wiki/Image:
en.wikipedia.org/wiki/Image:
A Holmes.jpg
Ernest_Rutherford2.jpg
some rocks from Ceylon
Ernest Rutherford Arthur Holmes to 1600 million years
(1890-1965)
(1871-1937)
Radiometric dating (6): Oldest Rock
Zircon mineral
en.wikipedia.org/wiki/Image:Zircon_microscope.jpg
© NASA
• Oldest rocks on Earth
are the Acasta Gniess of
northern Canada
• 4030 million years old
Acasta Gneiss
Radiometric dating (7): Oldest Grain
www.geology.wisc.edu/zircon/Earliest%20Piece/Images/2a-team.jpg
• Ancient mineral
grain found at
Jack Hills, Australia
• Mineral grain eroded
from first crust and
then deposited in a
new rock
© NASA
• Dates the Earth’s
first crust to around
4404 million years
4404 Ma
zircon grain
www.geology.wisc.edu/zircon/Earliest%20Piece/Images/5.jpg
Radiometric dating (8): Meteorites
http://upload.wikimedia.org/wikipedia/commons/0/00/Crab_Nebula.jpg
Canyon Diablo meteorite
en.wikipedia.org/wiki/Image:Canyon-diablo-meteorite.jpg
Crab Nebula
• Radiometric age of meteorites
date the formation of the
Solar System and Earth
(4550 million years old)
Radiometric Dating (9): History
http://en.wikipedia.org/wiki/Geologic_time_scale
© World Health Org.
first life
en.wikipedia.org/wiki/Image:Eopraptor_sketch5.png
dinosaurs
© NASA
origin of Earth
first complex cells
humans
Geological Time
© NASA