Precambrian is often used to refer both rocks and time. As a geologic term it includes all geologic time from the earth origin to the beginning of Phanerozoic eon 540 million years ago. (more than 21 hours of a 24 hours day).
It is extremely hard to work with Precambrian rocks because:
· The preserved rocks are metamorphosed, geologic principles are hard to use
· Relative timing and correlation is impossible
· Very few fossils are present, so correlation is extremely hard
This part of the earth history begins with the origin of the earth and ends with the oldest rocks, at which conventional geology begins.
The oldest crustal rocks are dated at 3962 million years (Northwest territory of Canada), and of course the Earth is older than that:
-Meteorites are dated to be about 4550 million years old, and they are believed to have the same origin as earth and to have the same composition as earth.
iron meteorites (representing core composition)
Stone meteorites representing crustal composition
iron-stone representing mantle composition
The earth’s origin was already discussed.
1.It most likely formed by the solar nebula, at the same time with the other planets (evidence is that they have the same plane of revolution around the Sun)
2. Later the earth was heated up by radioactive decay and it become differentiated (heavier elements in core, lighter ones on the top). Since in the deepest part of the earth is still hot, the core is still molten (magnetic field).
3. At the earliest time the mantle of the earth was on the surface, and probably it started to cool down by the mantle convection. Partial melting of this material started to form the earliest crust, (basaltic, andesitic).
There are two Precambrian eons
These divisions are geochronologic (based on absolute age dating), there is no connection with time stratigraphic units (no fossils, or stratotypes).
Some crust existed at least 3.8 billion years ago (rocks are metamorphosed, so they must have been even older) Minnesota, Greenland, South Africa. In Australia geologists found detrital zircon which dated 4.2 billion years indicating a source rock at least that old were present.
Its composition was ultramafic, and formed by partial melting of the mantle, and crystallization on the surface.
Partial melting of early basaltic, ultramafic crust along subduction zones could have produced andesitic island arc, and partial melting of andesite could result in granitic magma.
Shields and Cratons
Each present day continent has so called Precambrian shield consisting of exposed ancient rocks. Continuing outward from shields there are broad platform areas which are underlied by Precambrian rocks.
Shields and platforms together are called Craton.
The Cratons are relatively stable (They are stable since the beginning of the Phanerozoic), immobile parts of the continents and they form the basis upon which Phanerozoic sediments were deposited.
In the world:
In North America:
Includes large area of Canada, Large part of Greenland, Lake Superior. Archean and Proterozoic rocks are both present here. They are sometimes thinly covered by young glacial deposits.
There are some outcropping Precambrian rocks elsewhere such as Appalachian (Blue Ridge), Rocky Mountains, Blackhills of South Dakota. These are outcropped by erosion.
Two main rock types:
Greenstone belts mostly contain volcanic rocks lots of which consist of pillow lavas. Their color are greenish due to the chlorite which is a mineral forming during low grade metamorphism of mafic ultramafic igneous rocks. The slightly metamorphosed volcanic rocks are often overlaid by sedimentary rocks. The sedimentary rocks formed by the erosion of the previous volcanic rocks. Some of these sediments and the present of pillow lavas show clear evidence of being deposited in deep water. One of the most interesting igneous rock in the greenstone belts are the ultramafic lava flows. These rocks are very rare in younger rocks (The presence of ultramafic lava indicates higher mantle temperature(as much as by 300ºC)).
Tectonic Evolution of Greenstone belts
The most recent plate tectonic model for greenstone belts is rift basin model, or back arc basin model:
Back arc basin model:
Rift basin model:
Probably the plate movements in the Archean was much faster due to much more heat in the mantle.
During Archean there is no evidence for shallow warm sea water platform, and for the presence of long course river systems.
The end of Archean is represented by the Kenoran orogenesis which lasted from about 2700-2500 million years ago. During this orogenesis the core of North American continent was assembled by continental collisions.
At the formation of the earth the only atmosphere could be H and He. But most likely the earth lost that, because these elements are were very light. Before the earth had a differentiated core it did not have magnetic field, and magnetosphere. The absence of magnetosphere caused a very strong solar wind, an outflow of ions from the sun, which sweep away any gases.
After the formation of stable magnetosphere, internally derived volcanic gases began to accumulate such as Co2, SO2, HCl, CO, H2, N2. This process is called Outgassing.
The early atmosphere could also contain some Methane and ammonia from volcanic gases reacted with atmosphere.
The whole Archean had an oxygen deficit atmosphere.
1. Detrital dep. containing pyrite, uraninite (UO2)
(They would have oxydized quickly in the presence of oxygen)
2. There was no Fe present in oxidized form.
Since there was no oxygen in the atmosphere, there was no ozone layer to protect life. The only way for life was the ocean.
As soon as earth cooled down to let the water to be liquid, the formation of oceans started.
Archean sedimentary rocks show evidence of being deposited in deep water, however the distribution of water on earth was definitely different. Archean sediments show no evidence for shallow water platforms, and well developed continental water systems.
Origin of Life
Perhaps the most significant event to have occurred in the Archean was the origin of life. We know very little about how life came to be, however, recent studies of living bacteria have radically changed our ideas about where life evolved and what the first organisms were like.
For sure, life has started very early, almost as soon as the first crust developed on the surface, and the earth cooled down enough to condense water vapor to form the ocean.
It looks very possible, that life could not have started in an environment with free oxygen. (Amino acids would have been destroyed by oxidation).
The origin of life would have required the spontaneous organization of self-replicating organic molecules. The basic minimum requirements would have been the following:
1.A membrane-enclosed capsule to contain the bioactive chemicals.
2.Energy-capturing chemical reactions capable of promoting other reactions.
3.Some chemical system for replication.
Oparin I. Russian microbiologist was working in Russia in 1920’s speculated that life must have been started in and oxygen free environment, and he made an experiment in a tube containing the elements needed for amino acids such as ammonia, methane, hydrogen and water vapor.
Stanley Miller in 1953, put the same materials into a closed container in which an electric spark could be caused (representing lightening). He obtained a number of amino acids, suggesting, a possible origin of life this way.
Archaebacteria and deep sea vents
Recently, biologists have discovered a variety of extremely simple bacteria (the Archaebacteria) that appear to be the most primitive life forms on Earth and the closest to the ancestors of all life.
Amazingly, almost all of these primitive bacteria are hyperthermophiles meaning that they thrive in extremely hot temperatures, up to the boiling point of water. These bacteria are found living today in hot springs and in volcanic vents deep in the ocean. Also interesting is the fact that many of them can feed directly on elemental sulfur and sulfur compounds for their metabolic energy - they are chemoautotrophs.
The characteristics of these bacteria strongly argue that
life arose deep in the oceans of the Archean, at the many hydrothermal, volcanic
vents that would have dotted the ocean floor near rifting zones. These vents
would have provided chemical and heat energy, abundant chemical and mineral
compounds, including sulfur, and protection from oxygen and ultraviolet
The Fossil Record of Early Life
The Earliest Indirect Evidence of Life
There is reason to believe that life may have already evolved by the time the oldest known rocks were deposited. Some of the oldest rocks contain graphite (carbon) with an isotopic ratio of C12 to C13 that is identical to that produced by living systems.
The first fossil considered to be organic origin is 3.5 billion years old and was found in chert in Australia Pilbara shield. These fossils are cyanobacteria or blue-green algae, which are photosynthesizing,
however life must have been existed before and the living things probably were tiny bacteria. They must have been anaerobic, meaning that they did not need oxygen. Their food had to be dissolved material from the sea water so they were heterotrophic.
All these organisms were unicellular and they lacked a cell nucleus, and their DNA was not formed as chromosomes. Cells of this type is called PROCARYOTIC.
Cyanobacteria or blue-green algae, on the other hand, which are photosynthesizing, would represent organisms which are unicelled, autotrophic (making their own food) and procaryotic.
Johannesburg, South Africa (from conglomerate)
Hydrothermal deposits related to volcanism in greenstone belts.