Download Precambrian Time

Precambrian Time
(Prekambríum)
Archean Eon
(Upphafsöld)
>4.600 MY-2.500 MY
Proterozoic Eon
(Frumlífsöld)
2.500 MY-540 MY
Jarðsaga 1
- Þróun Lífs og Lands –
Ólafur Ingólfsson
Eon
Era
Time units of the Geologic time scale
Period
Epoch
Holocene
Pleistocene
Mesozoic
Tertiary
Cretaceous
Jurassic
Triassic
Devonian
Silurian
Ordovician
Cambrian
2500
Hadean
Eocene
Paleocene
3800
4600
0.01
1.6 Ice ages
5.3
23.7
36.6
“Age of Reptiles”
“Age of Amphibians”
360
408
“Age of Fishes”
438
505
“Age of Invertebrates”
Collectively called Precambrian,
comprises about 87% of the
geologic time scale
Humans develop
“Age of Mammals”
57.8
66.4
144 warm
208
570
Snowball earth ??
Proterozoic
Archean
Pliocene
Miocene
Oligocene
245
Permian
286
Pennsylvanian
320
Mississippian
Carboniferous
Paleozoic
Phanerozoic
Cenozoic
Quarternary
Development of plants and animals
Extinction of Dinosaurs and many
other species
First Flowering Plants,
First birds,
Dinosaurs dominant
Extinction of trilobites and many other
marine animals
First reptiles
Large coal swamps
Amphibian dominant
First insect fossils
Fishes dominant
First land plants
First fishes
Trilobites dominant
First organisms with shells
First multicelled organism
First one-celled organism
Age of oldest rock
Origin of the earth
The Precambrian – most of Earth´s History
Simplified timeline
4,600 million years ago. The Earth has formed, along with
the other bodies in the solar system, from a cloud of dust
and gas swirling around the sun. The Precambrian era has
begun.
3,750 million years ago. The oldest still-existing rocks
have just formed (4.000 million years ago). The very first
life is just about to appear in the form of simple, singlecelled organisms (Eubacteria, Prokaryotae).
3000 million years ago. The first photosynthetic bacteria
have appeared. All life is restricted to the sea, which
provides a fairly constant environment and protection
from the sun's ultraviolet rays. With photosynthesis, the
levels of atmospheric oxygen begin to rise.
2,250 million years ago. Half of Earth history has elapsed,
and the first multicellular organisms are just appearing.
The first Eukaryotes arise, beginning with the protists.
Simplified timeline, continued
1,500 million years ago. World-wide radiation of photosynthetic aquatic life has significantly altered the composition of the atmosphere. Free oxygen forms ozone, blocking ultraviolet light and paving the way for life on land.
750 million years ago. Brown and red alagae have formed,
and the first animals (Annelida) are immediately around
the corner.
374 million years ago. The Devonian period, the Age of
Fishes, is here. In the last hour, plants and insects have
begun the colonization of the land, and the first
amphibians have pulled themselves out of the water. The
great Carboniferous forests will shortly arise.
Now. The first reptiles appeared early in the hour, the
dinosaurs lived for about 26 minutes later in the hour.
The first hominids appeared about 39 seconds ago.
Modern humans have been on Earth for the past 6
seconds or so, or about 0.043% of Earth’s history...
In the beginning, Earth and all its systems
(including us!) looked like this, just a huge
cloud of gas in space….
Hadean time:
>4.600 to ~3.800 million years ago
Hadean time is not a geological period as such. Almost no
rocks on the Earth are this old - except for meteorites.
Hadean (“hellish”) Earth
• Hadean Earth
– Heavy Bombardment of meteors
– Outgassing from inner layers of
Earth
– Atmosphere and Oceans form
– Origin of the Continents
Creation of the Solar System
The relative abundance of
heavy elements in the Solar
System suggests that it was
formed from gas and dust
derived from a supernova the explosion of an old,
massive star.
Heavier elements are
generated within stars
by nuclear fusion of
hydrogen, and are
otherwise uncommon.
The Nine Planets: A Multimedia Tour of the Solar System
http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html
You are composed of:
Creation of the Solar System – the brief version
• A cloud of interstellar gas and dust is
disturbed and collapses under its own gravity. A
rotating primordial gas disk forms.
• The trails give way to well-defined spheres of
dust and gas; the protosun glows dim red.
• Once the larger of the dust particles get big
enough to have a nontrivial gravity, their growth
accelerates. Their gravity pulls in more, smaller
particles. This goes on until the particles get to
the size of boulders and asteroids.
Eventually, after ten to a hundred million
years, the sun has blazed to life, driving away
the residual cloud material and revealing solid
planets in stable orbits.
•
Origin of the Solar System
The Nebular Hypothesis (Stjörnuþokukenningin)
Hubble Space Telescape
painting by William K. Hartmann
Palomar Observatory
Origins of the Planets
– nebula formed of dust and gas (of
previous star(s))
– collapse due to disturbance
– slow rotation increases as nebula
collapses
– mass collects at center of system
• hot, dense gas begins fusion (sun
ignites)
– additional material collects around
smaller centers of mass (planetesimals)
• higher density elements condense
near primary center of mass
• lower density material cleared from
center by solar wind
– planetesimals coalesce into planets
Planet Building:
the Condensation of Solids
• Different materials condense from the gas cloud
onto grains of elements at different
temperatures.
• The temperature due to the Sun varied with
distance, so different materials condensed at
different distances from the Sun.
Close to the Sun: metal oxides and pure metals.
• Farther out: silicates and rocky material.
• Outer regions: ices (water, methane & ammonia).
Planet-building processes
• Dust grains stick together Æ planetesimals
• Planetesimals stick together Æ protoplanets
– Terrestrial:
• metallic / rocky
• but small – not much material
– Jovian:
• LOTS OF ICES, so quickly grew more massive
• When ~15 x Earth’s mass, gravity strong enough to
attract lots of H/He from solar nebula
• got really really big – but not dense
The Nine
Planets
Saturn
Neptune
Pluto
Uranu
s
Jovian
planets
Jupiter
Mars
Earth
Venus
Mercury
asteroid belt
Terrestrial
planets
The planets formed and differentiated into
Terrestrial and Jovian Planets
Terrestrial Planets
Jovian Planets
Small size, low mass
Large and massive
Dense, rocky solid surfaces
Low density, huge gaseous
atmospheres
Heavy gas atmospheres (N2,
O2, CO2)
Slow rotators
Few satellites (3)
Close to the Sun (within 1.5
AU)
No ring system
Lighter elements, H and He
Faster rotators,
differential rotation
Many moons (over 60)
Farther away (from 5.2 to
30 AU)
Planetary rings
The Hadean
Differentiation
Heavier materials in
Earth’s core, lighter in
mantle and crust
Origin of the Earth
Geological Differentiation
Earth melted by:
gravitational energy left from formation
of the planet
meteor bombardment
Radioactive decay
Gravity concentrated denser materials (Ni
& Fe) in center
Less dense materials (silicates) forced to
outer layers
Crust became stable after 1 billion years
Compositional Zones
Crust: Continental and Oceanic
Mantle
Core: Outer (Liquid), Inner (Solid)
Hadean Period of Heavy Bombardment
As Earth accreted, frequency of impacts
decreased. BUT size of impacts increased due to
increasing size of impacting bodies
Period of heavy bombardment from about 4.5 to 4
billion years ago
- “magma ocean” due to widepread melting, so no
rock record
Oldest rocks around 4 billion years old (Canada,
Greenland, Australia) indicating presence of solid
crust by that time.
Formation of the Moon
The Moon formed as the result
of a collision between Earth and
a large meteorite. Moon rocks
date to 4.4-4.6 billion years
Evidence of Bombardment by Other Objects
Obliquity of Axis
Venus “overturned”
Uranus tipped on its side
All planets have at least some axial tilt
Forming of Earth’s
Atmosphere
Earth’s early atmosphere
Early composition of Earth’s
Atmosphere:
The first atmosphere may have
consisted of carbon oxides,
water vapor, nitrogen, hydrogen
cloride, and nobel gases.
Today’s composition of Earths
Atmosphere:
21% Oxygen (O2)
78% Nitrogen (N2)
0.04% Carbon Dioxide (CO2)
~0.9% Argon (Ar)
0-7% water (H2O)
0.01% Ozone (O)
Earth probably did not inherit
its atmosphere from the
asteroids that coalesced to
form it; the gases that formed
the early atmosphere must
partly have been emitted from
within the Earth after it
formed, by extensive degassing.
Possible Sources of Early Atmospheric Gases
Remnant gases from solar nebula: H2, He (these gases
too light to be held in atmosphere by gravitational field)
Outgassing from Earth’s interior (volcanoes):
H2O,
CO2,
SO2,
NH3 (ammonia), N2, NOx (nitric oxides)
Material from impacting comets:
H2O, CO (carbon monoxide), CH3OH (methanol), CH4
(methane), C2H2 (acetylene), C2H6 (ethane), HCN cyanide),
Ar (argon)
Early atmospheric composition
Early atmosphere
dominated by N2, CO2 and
H2O
Reduced gases from
volcanic emissions:
HCl & Cl
Acid rain led
to enhanced
weathering
HCl and Cl gases were
emitted and produce
acidity in water which
triggers weathering of
crust minerals (Na, Mg, &
Ca) which along with Cl
accumulated in early
oceans
Atmospheric Development
100
Methane &
Ammonia
75
50
unknown
Carbon Dioxide
25
Oxygen
Water Vapor
0
4.5
4
3.5
3
2.5
BYA
2
1.5
1
Evidence of Archean oxygen depletion
The mineral uraninite (UO2)
occurs in pebbly sediments
that formed by the accumulation of gravel in ancient
rivers, which are preserved
in South Africa. The mineral
uraninite cannot survive
prolonged contact with free
oxygen: it oxidizes.The
youngest uraninite-bearing
gravels are about 2.6 to 2.4
billion years.
Evidence of Archean oxygen depletion
Banded Iron Formation (BIF): these rocks are
not formed on the present earth. They are very
thick (thousands of m), widespread (hundreds of
km), and consist of finely-laminated rocks very
rich in iron, deposited in the Ocean. They started
to form about 2.500 MY.
• They are composed of
alternating layers of ironrich material (commonly
magnetite) and silica (chert).
•Each layer is relatively thin,
varying in thickness from a
millimeter or so up to several
centimeters.
Absence of banded-iron
formations and redbeds, and
the presence of abundant
uranite and sulfides in Early
Archean sediments indicate
that the atmosphere was
relatively depleted in oxygen.
Evolution of the Earth’s atmosphere
– Initially, hydrogen and helium from the solar nebula
• Expelled after few 10’s of millions of years by Sun’s
radiation
– 2nd generation, carbon dioxide (CO2), water (H20),
and nitrogen (N2) from volcanic activity of hot,
young Earth
• Formation of oceans, from extant water, and from water
delivered by comets, began absorbing CO2
– Evolution of plant life in oceans began processing
and transforming atmosphere--3rd generation
• carbon dioxide a component of seashells--limestone
• over billions of years, massive limestone bedrock form
• plants release oxygen which first caused oxidation of
surface (rust!), but eventually stabilized at present levels
– 4 to 1 mixture of nitrogen to oxygen
Evolution of Earth’s Atmosphere
1. Initial atmosphere much like Jupiter (rich in Hydrogen
and Helium derived from solar nebula)
- burned off by Solar Wind / escaped weak gravitational
field
2. Second atmosphere much like Venus (dominated by
carbon dioxide from the planets interior)
- “The Big Burp”
3. Third and present atmosphere (rich in oxygen)
- modified from second atmosphere due to rise of
anaerobic, photosynthesizing organisms
Origin of water and the
Oceans
• Total Volume of Earth’s H2O:
1,398,898,300 cubic kilometers
• Where did all this water come from?
The Early Oceans
• The Oceans formed by water contributed by
comets as well as volcanic emission of water vapor
from the planet’s interior.
• Salts were brought to early sea water by rivers
that carried the products of weathering on land.
There is an enormous
mass of comets “out
there”. Every year 15
million small comets
(<12 m in diameter)
pelt Earth’s atmosphere! Large comets
can contain several
km3 of ice. The ice
consists of various
frozen materials,
mainly water, carbon
dioxide, methane, and
ammonia.
Early Earth
Oceans and liquid water
• As the surface of the earth cooled, water
condensed out of the atmosphere. Oceans
may have formed and been re-vaporized
several times before a stable ocean
developed.
• There is evidence for the presence of water
on the Earth’s surface around 3.8 BYA.
• Total amount of water in the hydrological
circle has been relatively constant.
• Many reactive atmospheric gases (CO2, HCL,
SO2) are soluble & were deposited to oceans.
Origin of the Continents
- A hotter Earth and Smaller Plates -
Archean Earth much
hotter than present
The decline of heat production by
radioactive decay in Earth’s interior
Only after the final
magma ocean on Earth
had cooled to form a
basaltic crust, did
felsic material begin
to segregate from this
mafic crust and the
mantle to form the
nuclei of continental
crust.
Mafic vs Felsic Rocks
Igneous rocks can be placed into four groups based
on their chemical compositions:
1. Sialic (or granitic or felsic). Dominated by silicon
and aluminum (SiAl). Characteristic of continental
crust. (Granite)
2. Intermediate (or andesitic) Intermediate in
composition between sialic and mafic. (Diorite)
3. Mafic (or basaltic). Contains abundant
ferromagnesian minerals (magnesium and iron
silicates). Characteristic of Earth's oceanic crust
4. Ultramafic.
Almost entirely magnesium and iron silicates
(ferromagnesian minerals). Believed to be major
constituent of Earth's mantle. (Periodite)
Felsic rocks from mafic rocks – Hotspot
action needed
Iceland: a modern-day protocontinent:
Earliest continental crust probably formed by segregation of felsic rocks
from igneous material derived from the mantle, particularily in huge
central/stratovolcanoes. Presumably numerous protocontinets, resembling
Iceland, emerged during Archean time, and later sutured together to form
larger protocontinents. Removal of magenesium and iron from mafic rocks by
weathering produced sediments that became felsic metamorphic rocks
Archean Contients were small...
About 7% of the modern Continental crust is Archean
...the reason being an extensive network of rifting and
subduction zones and very active plate tectonic engine.
Archaean Eon - Cratons
craton - a part of the Earth's crust that has attained stability
and has been little deformed for a long time. The term is
restricted to continents, and includes both shield and platform.
shield - a large region of exposed basement rocks, commonly with
a very gently convex surface, surrounded by sediment covered
platforms.
platform - that part of the continent that is covered by flatlying or gently tilted sedimentary rocks, underlain by a complex
of rocks that were consolidated during earlier deformations.
Shields and Cratons
Archean
Continents
Protocontinents were apparently small
steep-sided island arcs which were
surrounded by deep ocean basins.
Evidence: The presence of abundant
greenstone belts with pillow structures
and the general absence of widespread
continental deposits.
Greenstone belts= Large, synclinal bodies
with successions of utrabasic, mafic and
felsic volcanics, and sediments
Greenstone Belts of the Superior Province
Hundreds of protocontinents...?
Plate Tectonic model for the Archaean
The Forming of Continental Crust
• Fragment by fragment, formed in the
beginning from island chains similar to modernday volcanic island arcs, the continental crust
was born, and so the external land cover of the
planet.
• This new type of crust had a unique feature of
fundamental importance: its low density kept it
riding on the surface. Thus it was able to
undergo intense transformations, such as
mechanical deformation (tectonics) or
metamorphism, but remain always in proximity
to the surface.
Large Cratons Appear
“Cratonization” about 2.700-2.300 MY, by assimilation of
small crustal elements into larger units and incorporation
of metamorphized sedimentary rocks (the Greenstones).
Precambrian Provinces of North America
Archean Sedimentary Rocks
- Composed dominantly of graywackes ("dirty sandstones"), conglomerates and sandstones.
- Deposited in deltas, tidal flats, shallow marine shelf
environments and continental shelf slopes.
- Characteristic for tectonically unstable environments
References, web site resources etc
Stanley, Earth System History, chapter 11
Mjög skemmtileg dagskrá um myndun jarðar:
http://www.scctv.net/annenberg/World_of_Chemistry_18.asx
Góð mynd (Earth Reveiled Series) um myndun djúpbergs:
http://www.scctv.net/annenberg/Earth_Revealed_14.asx
http://www.palaeos.com/Hadean/Hadean.htm
http://www.ucmp.berkeley.edu/precambrian/archaean.html
http://www.astro.washington.edu/labs/clearinghouse/labs/Formss/lab.html
http://facweb.bhc.edu/academics/science/harwoodr/Geol102/Study/Archean.htm
http://zebu.uoregon.edu/disted/astr121.html
http://www.palaeos.com/Archean/Archean.htm
http://www.platetectonics.com/index.asp
http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html
http://www.washlee.arlington.k12.va.us/staff/science/rymiller/21
http://www.solarviews.com/eng/comet.htm