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Lecture 13:
The Dynamic Planet: Geology of the World
GEOLOGY
STRATIGRAPHY
GEOLOGICAL TIME SCALE
Geog 1000 Introduction to Physical Geography Fall 2013 : Dr. Hester Jiskoot, University of Lethbridge
GEOLOGY (or “Earth Science”)
Æ Study of the origin, structure, composition &
physical history of Earth, and the processes
which have led to its present state.
Æ The science that deals with the dynamics
and physical history of Earth, the rocks of
which it is composed, and the physical,
chemical, and biological changes that it has
undergone and is undergoing.
Fundamental stratigraphic principles
1. superposition of layers (strata)
2. original horizontality
STRATIGRAPHY
Æ description of all rocks forming the Earth's crust and
their classification into units based on their properties
Æ establish rock unit distribution & relationship in space
and their succession in time, and to interpret geologic
history.
Rocks have many tangible and measurable properties
and may be classified according to any of them.
Rocks may be classified by their time of origin or by their
interpreted attributes, such as environment or
formation/genesis.
versus
Uniformitarianism
“Present is key to the past”
Continuing uniformity of existing
processes/physical laws responsible
for present & past conditions on earth
Earth created through supernatural
means and affected by a series of
catastrophic events
Earth is old
Processes take time to form earth
Earth is young
Catastrophic events form earth
James Hutton (1726-97)
Cuvier (1769-1832)
- Father of geology
Earth is in perpetual process of
formation & denudation
Catastrophes are events with natural
causes
Charles Lyell (1797-1875)
- Wrote “Principles of Geology”
Evidence from geologic record for
Uniformitarianism
3. Guiding principle = uniformitarianism
Catastrophism
Charles Darwin (1809 –1882)
Biological evolution through natural
selection
Geologists now know that natural
catastrophes have some role in
forming the Earth's landforms and
rocks/sediments.
Earth formed through gradual
processes, punctuated by
dramatic events (volcanic
eruptions, earthquakes, floods,
landslides)
Pre 2006: Oldest dated rock on earth
Acasta gneiss found in Northwest Territories, Canada
Dating methods: radioactive decay of Uranium and Thorium to Lead
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The Earth is ~4.6 billion years old
This vast time span encompasses:
¾ our planet’s formation (its oceans, continents and atmosphere)
¾ the origin of life and the evolution of the biosphere to its present
complexity.
To cope with such long geological time, we divide it into manageable time
intervals based on natural transitions
Eons hundreds of millions to billions of years
Eras many millions of years: distinctive fossil records
Periods
millions of years: distinctive rock units
Epochs
few million years
Ages thousands of years
3.96 billion years
Rock in National Museum for Natural Sciences, Smithsonian institute (Washington, DC)
The fossil record in rocks deposited in the shorter time intervals is well-constrained:
Æ we can correlate these units globally
Æ we can reconstruct the appearance & conditions of our planet for many hundreds
of different time slices
The geological time scale
Boundaries of eons, eras, periods and epochs
based on
evolvement/extinction of animal life forms
&
global climate variations
&
rock units
Eras
Periods
Eon
Era
today
Global climate
and
continental plate
distribution
over
geological time
544 M yr
New in 2004:
International Cmtee on Stratigraphy
http://www.stratigraphy.org
PRECAMBRIAN deleted
4,500 M yr
PROTEROZOIC
ARCHAEAN
HADEAN
Now EONS
Hadean eon
Eon
4.5 to 3.8 billion yrs ago
Era
today
Solar System was forming (planetisemal hypothesis)
Earth starts cooling
First Rocks form
Archaean eon
3.8 to 2.5 billion yrs ago
Earth's crust cooled enough Æ continental plates began to form.
Oldest fossils date ~ 3.5 billion yrs ago (bacteria microfossils).
544 million yrs
Proterozoic eon
2.5 billion to 544 million yrs ago
First stable continents
First abundant fossils of living organisms (bacteria, archaeans, eukaryotic cells)
4,500 million yrs
First evidence of oxygen build-up in the atmosphere
Paleozoic Era
544 to 245 million yrs
Two of the most important events in the history of animal life:
1)
Multi-celled animals underwent a dramatic "explosion" in diversity
2)
the largest mass extinction in history wiped out approximately 90% of
all marine animal species
Seas Æ Limestone/coal
Mesozoic era
245 to 65 million yrs
Albertosaur
Culmination of world fauna (except for insects): very different from previous eras.
First fossils of modern mammal and bird groups
Dinosaurs (+ ammonites, etc) evolve and become extinct
Æ Lush terrestrial vegetation
Cenozoic era
65 million yrs to the present
Evolvement to modern times
>2.7 M yr Cyclic Ice Ages
K/T boundary: marker of one of the extinction events
Mesozoic era
Cretaceous Rocks
146 to 65 My BP
ERA
Holocene
PERIOD
Quaternary
ASTEROID IMPACTÆ Chicxulub crater at the bottom of Gulf of Mexico
Tertiary
Tertiary
Mass extinction
iridium-rich layer
Cretaceous
World-wide thin layer of same sediment:
Upper "Fireball layer" ~ 3 mm thick.
Lower "Ejecta layer“ ~ 2 cm thick.
Layers mostly clay, with soot in upper parts.
See a video on http://videos.howstuffworks.com/science-channel/29285-100-greatest-discoveries-chicxulub-crater-video.htm
K/T boundary
ERA
EPOCH
Holocene
PERIOD
Approximate order of origin and evolution of life
forms over geologic time compressed into 24 hrs
Quaternary
The Quaternary Period
-- began 2.7 million years ago (mya)
-- transition to colder climate
Two epochs:
HAD
The Pleistocene several ice ages &
interglacials
The Holocene the most recent interglacial,
began ~11 500 years ago
EAN
The Holocene is an epoch for practical purposes:
Æ most surface sediments on which we live
(soils, river deposits, deltas, coastal plains, etc.)
were formed during this time.
“THE ANTHROPOCENE” – new geological epoch?
Crutzen, 2002
Human activities are exerting increasing impacts on the environment on
all scales, in many ways outcompeting natural processes
THE ANTHROPOCENE
Clearly in the last 200 years.
But perhaps longer?
RECAP 1:
What are the main stratigraphic principles?
What are the geological time periods based on?
What are the main eras?
In which era did the dinosaurs live?
Why does the science of geology evolve so fast?
Excellent websites on geological time:
http://www.ucmp.berkeley.edu/exhibit/geology.html
http://www.stratigraphy.org/
COMPOSITION
OF THE
EARTH
Earth diameter = 12750 km Æ known by the ancient Greeks
In the 20th century we discovered that planet earth is made up of 3 main layers:
crust, mantle, core
Moho discontinuity
Think of an egg
MAGNETOSPHERE
Magnetic field &
Magnetic reversals
How do we know? Æ Seismic waves
MAGNETIC
REVERSALS
Earth’s magnetic field
periodically reverses
its polarity
Black = normal
•Earth is a geodynamo, generated
its magnetic field (dipole) due to earth
rotation, convection, and electrically
conducting fluid
•Reversals are mainly due to relative
changes in the solid and liquid parts
of the core
Reversals over the past >150 million and 2 million years.
Each polarity interval, or ‘chron’, is named after either a
famous palaeomagician (Brunhes, Matayama) or the
location where it was first identified (Olduvai).
THE EARTH’S CRUST
ISOSTACY & ISOSTATIC REBOUND
ISOSTACY & ISOSTATIC REBOUND
3.0 g/cm3
Assumed convection cells in the mantle.
Tectonic plates (~14)
Below ~ 700 km, the descending crustal slab softens,
becomes liquid, and loses its form.
PEOPLE who ‘discovered’ PLATE TECTONICS
1596
Abraham Ortelius, map maker: “Americas torn away from
Europe and Africa by earthquakes & floods"
1858
Antonio Snider-Pellegrini made ‘before’ and ‘after’ maps
1912-15 Alfred Wegener develops theory of continental drift
“understanding of how the Earth works requires input and
knowledge from all the earth sciences.”
1970s Theory of plate tectonics
Break-up
of
Pangaea
Distribution of continental
plates and orogens
important for world climate
How do we know how to fit the jig-saw?
Directions & rates of plate movement
Dated rocks/rocktypes/fossils - spatial distribution
Dated shorelines and marine deposits
Plate tectonics animations
http://www.ucmp.berkeley.edu/geology/anim1.html
http://www.scotese.com/sfsanim.htm
Plate convergence/divergence/faulting
Divergent plates
Seafloor surface map
Convergent plates
Transform boundary:
transverse movement
along a fault
Mid-Atlantic ridge
Magnetic time record
CRUSTAL FORMATION
Orogenesis related to plate convergence:
TECTONIC MOVEMENT
1) Oceanic plate
subducted under
continental plate
2) Continental plate
subducted under
continental plate
3) Oceanic plate
subducted under
oceanic plate
Reading for Friday
RECAP 2:
What is the earth’s inner/outer core composed of and why is this important?
What types of plate boundaries are there?
Why is plate tectonics important (what other processes does it influence?)
What is Pangaea and why did it break up?
Chapter 8