The start of the Archean Eon is defined by the isotopic ages of the oldest rocks. Prior to the Archean, the Earth was in the Astronomical stage of its development. The oldest materials we have are not rocks, but minerals. To be precise, they are Australian zircons grains that are about 4.2-4.4 billion years old. They probably got into the Archean rock formation through a river.
During the Archean, volcanic activity produced rare rocks like Komalite, a rock low in silicon, potassium, and aluminum, with a high magnesium content. Granite rocks dominated in the remnants of the surviving Archean crust. Rocks from the Archean Eon are often highly deformed metaporphized deep water sediments, including gray wackes, mudstone, volcanic sediments, and Banded Iron formations. Carbonate rocks were also present in the Archean, which indicates a more acidic ocean than now. Greenstone belts are also typical of the eon. They consist of alternating layers of sedimentary and igneous rock. Rock from this age only makes up for 7% of the world's cratons, and Archean rocks can mainly be found in Greenland, Canada, the Baltic Shield, Scotland, India, Brazil, Australia, and Western Africa.
Banded Iron Formations (or BIFs), are an important type of rock formation from this eon. The oldest ones date back to the Mesoarchean. Basically, a Banded Iron Formation is a rock formation consisting of layers of iron-poor layers and iron-rich layers. This indicates oxygen "cycles" or "pulses", the causes of which have never been discovered. BIFs formed until about 1.8 billion years ago, and they contain 20 times the amount of oxygen we have in our atmosphere today.
When the Archean began, the Earth's heat flow was nearly three times higher than it is today, and was still twice that level when the eon ended. The heat mostly came from planetary accretion, heat from the formation of the Earth's core, and heat produced by radioactive elements. The early crust was very unstable and melted and re-melted a lot. Any surviving Hadean rocks were sucked back into the mantle and melted down, or "erased" them and all their isotopic records. All rocks could be sucked back into the mantle if they were heavy enough.
There are lots of theories surrounding Archean geology. One area where opinions are divided is the formation of the Archean and how it grew. Scientists who doubt plate tectonics in the Archean existed argue that protocontinents formed at hotspots (areas with abnormal heat), rather than subduction zones (areas where plates slide into the mantle). Partial melting in downward directed diapirs produced intermediate and felsic rocks. Others accept that continent formation is part of the plate tectonic process, which is thought to have operated since the start of the Archean.
Plate tectonics in the Archean would have been different from today. Because the Earth is thought to have been hotter, there would be more vigorous tectonic activity than today, resulting in faster recycling of the Earth's crust. This may have prevented cratonisation and continent formation until the mantle cooled and convection slowed down. Another theory is that the oceanic lithosphere was too buoyant to subduct, and the tectonic events caused the loss of most Archean rocks.
Understanding what cratons are is a big part of understanding Archean geology. They appear when the (mostly volcanic) rock is buried deep, but not deep enough to melt. Instead, the heat and the pressure make it metamorphic rock. These areas have fresh igneous rock on top and metamorphic rock on the bottom. For some reason, there was a lot of cratonisation towards the end of the Archean, which has not been replicated since. Cratons mainly sit on the continental crusts.
Monday, January 23, 2012
Geography
Earth was a much more colorful place in the early Archean. There were orange skies because of high methane content in the atmosphere, the seas were green because of dissolved iron compounds, there were tons of volcanoes, and the shorelines were marked by stromatolites.
That much we can agree on. We can also all agree on the fact that continents with plates and modern plate tectonics like the ones we have today developed towards the end of the Archean.
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| An artist's idea of the Archean Image courtesy of: http://www.etsu.edu/physics/plntrm/dino/stromat.gif |
It is under debate when the first continents appeared. During the Archean, areas of land increased in size because of a more stable crust. In the Middle Archean, the first continent-sized land masses appeared. These protocontinents only exist as traces in cratons (which will be explained in more depth when reading about Geology) or resting on continental plates. While we are not sure exactly when, the Archean is the eon in which the continent first began, because it was the first time the Earth was stable enough for continents to exist.
There are two major theories about continents in the Archean:
- The first theory says that no large continents existed until late in the Archean, instead small "protocontinents" were the norm, prevented from growing until the late Archean because of huge amounts of tectonic activity. Eventually, these protocontinents would grow and band together in the late Archean to make larger continents that eventually led to Pangaea, and our seven continents today.
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| The darkest orange are the protocontinents. Image: http://media-2.web.britannica.com/eb-media/89/789-004-B50C80D0.gif |
The other theory follows the ideas of Richard Armstrong, who thought continents grew the way they did in the first 500 million years of Earth's history, and have stayed constant ever since- for most of history, the recycling of continental material keeping growth in check.
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| A map showing Richard Armstrong's theory. http://www.innovateus.net/science/what-archean-eon |
Either way, major continent formation existed during the Archean. An Archean supercontinent called Ur is well known for consisting of many cratons. It is thought to have survived for a long time, joining to create Rodina, and eventually Pangaea.
Life
While some theories place the origin of life in the Hadean eon, the first incontrovertible evidence of life evolved in the Archean. Some of the oldest fossils include the Apex chert (a type of bacteria) and stromatolites. Stromatolies have been shown to be as old as 3.5 billion years old, but it is hard to date the very oldest fossils because they're just so old. The earliest chemical markers of life are 3.8 billion years old, but the oldest fossils, the only conclusive proof of life, are about 3.43 billion years ago.
The oldest life on earth lived anaerobically, using no oxygen and relying on sulfates of possibly methane for their energy needs. These life forms were very, very simple and simply existed. The Archean is the eon in which the last common ancestor of all living things existed. Fossils of archebacteria (a type of life which still exists today) have been found from this time. The time between the first life and the end of the Archean is occasionally called the 'Boring Billion' by scientists, because very little actually happened.
Cyanobacteria are the most important form of Archean life. They were the first life to photosynthesize, perhaps as early as 3.5 billion years ago. They were the first life to create oxygen, and were the ones responsible for the oxygenation of the Earth. However, much of the early oxygen produced by cyanobacteria actually sunk into the Earth and became part of oxygen-rich iron settlements. Colonies of cyanobacteria, also called stromatolites, became widespread in the Paleoarchean.
So, what are stromatolites, anyway? Spelled STRO-MA-TO-LITES, they are formations of colonies of photosynthesizing cyanobacteria (blue-green algae). They are small, cylindrical layered structures made of blue-green algae cells. Stromatolies were extremely common in the Archean eon that formed in shallow waters. These stromatolites helped create oxygen in the atmosphere and make more oxygen.
The oldest life on earth lived anaerobically, using no oxygen and relying on sulfates of possibly methane for their energy needs. These life forms were very, very simple and simply existed. The Archean is the eon in which the last common ancestor of all living things existed. Fossils of archebacteria (a type of life which still exists today) have been found from this time. The time between the first life and the end of the Archean is occasionally called the 'Boring Billion' by scientists, because very little actually happened.
Cyanobacteria are the most important form of Archean life. They were the first life to photosynthesize, perhaps as early as 3.5 billion years ago. They were the first life to create oxygen, and were the ones responsible for the oxygenation of the Earth. However, much of the early oxygen produced by cyanobacteria actually sunk into the Earth and became part of oxygen-rich iron settlements. Colonies of cyanobacteria, also called stromatolites, became widespread in the Paleoarchean.
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| Thank this for being able to breathe. Image courtesy of : http://www.globalhealingcenter.com/media/algae_closeup.jpg |
So, what are stromatolites, anyway? Spelled STRO-MA-TO-LITES, they are formations of colonies of photosynthesizing cyanobacteria (blue-green algae). They are small, cylindrical layered structures made of blue-green algae cells. Stromatolies were extremely common in the Archean eon that formed in shallow waters. These stromatolites helped create oxygen in the atmosphere and make more oxygen.
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| An artistic rendering of stromatolites (based on modern photographs). |
Atmosphere
At the beginning of the Archean, the atmosphere was completely toxic to life today. It was mostly methane, CO2, ammonia, hydrogen, carbon monoxide, and other greenhouse gases. While these gases formed a greenhouse and kept the Earth warm, they weren't very breathable. Even though volcanic eruptions were extremely common and released lots of water vapor (among other things) into the air, there was little to no free oxygen in the atmosphere.
The story of how oxygen came to be in Earth's atmosphere is as much a story about life as it is about air. While the first life forms were centered around methane and other sulfates and simply lived out their lives, along came cyanobacteria, otherwise known as blue-green algae or some of the first photosynthesizing life. These cyanobacteria, living in settlements called stromatolites, produced oxygen through photosynthesis. While this did not dramatically increase atmospheric oxygen at first, it steadily rose for millions of years until the Great Oxygenization Event.
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| This is one of the reasons inter-eon time travel is generally a bad idea. |
The story of how oxygen came to be in Earth's atmosphere is as much a story about life as it is about air. While the first life forms were centered around methane and other sulfates and simply lived out their lives, along came cyanobacteria, otherwise known as blue-green algae or some of the first photosynthesizing life. These cyanobacteria, living in settlements called stromatolites, produced oxygen through photosynthesis. While this did not dramatically increase atmospheric oxygen at first, it steadily rose for millions of years until the Great Oxygenization Event.
The Great Oxygenization Event occurred 2.5 billion years ago for unclear reasons. While we are not sure of the specifics, cyanobacteria started pumping large amounts of oxygen into the air. In fact, there was more oxygen in the air 2.5 billion years ago than there is in the air today! This actually poisoned life in the early Proterozoic, because they were not used to all that oxygen!
Evidence of the Great Oxygenization event can be found in the isotopic ratio of sulfur in Archean rocks, as well as oxidized bands of iron on the sea floor and in the ground.
Oceanography, Meteorology, and Environment
During the Archean Eon, the Earth was very different from how it is today. In fact, the Archean atmosphere is thought to be devoid of oxygen! It was also a moderately warm and very wet climate; near constant volcanic eruptions due to increased tectonic activity caused ceaseless rain and violent lightning, and large amounts of greenhouse gases trapped heat in the atmosphere. This kept the climate at near-modern levels, despite the fact that the sun was about 30% dimmer than it is today.
Fun Fact: The Faint Young Sun paradox demonstrates the contradiction between liquid water in the Archean eon and the expectation that the sun would only be about 70% as intense as it is today. It was pointed out in 1972 by two scientists, Carl Sagan and George Mullen, and has been explained by greenhouse effects and/or astrophysical influences.
There is ample evidence for liquid water in the Archean Eon, which is thought to have existed as early as a few hundred million years after Earth's formation. Oceans were probably created by water condensation, a byproduct of volcanic eruption. At the beginning of the Archean, the Earth was entirely covered in water. Oceanic basins were known to have existed during the Archean, and the presence of liquid water can be shown by deformed gneisses produced by morphing of protoliths.
By the end of the Archean, the Earth was a lot more similar to what it is like today: it had modern continent structure and pate tectonics, there was much more oxygen in the air, and the oceans were less green than they were about a billion years before.
Fun Fact: The Faint Young Sun paradox demonstrates the contradiction between liquid water in the Archean eon and the expectation that the sun would only be about 70% as intense as it is today. It was pointed out in 1972 by two scientists, Carl Sagan and George Mullen, and has been explained by greenhouse effects and/or astrophysical influences.
There is ample evidence for liquid water in the Archean Eon, which is thought to have existed as early as a few hundred million years after Earth's formation. Oceans were probably created by water condensation, a byproduct of volcanic eruption. At the beginning of the Archean, the Earth was entirely covered in water. Oceanic basins were known to have existed during the Archean, and the presence of liquid water can be shown by deformed gneisses produced by morphing of protoliths.
By the end of the Archean, the Earth was a lot more similar to what it is like today: it had modern continent structure and pate tectonics, there was much more oxygen in the air, and the oceans were less green than they were about a billion years before.
Sunday, January 22, 2012
The Archean Eon Timeframe and Era Overview
Hello! Welcome to the Archean Eon, also referred to as the Archeozoic eon. Its name comes from the Greek word Arkhe, meaning "beginning, origin". In most texts, the beginning of this eon will be about 3.8 Billion years ago, or Ga, and the end about 2.5 years ago. However, the Archean eon has no strictly defined starting point; its beginning coincides with the oldest known rocks. Because rocks have been discovered that are 4 billion years old, the beginning of the Archean has technically been pushed back, but most sources still refer to it as beginning 3.8 billion years ago.
In older literature, the Archean Eon was a part of the Hadean eon, but that is no longer the case. The Archean is one of the four principal eons of Earth's history, and begins the second division of Precambrian time. It is directly followed by the Proterozoic.
The Archean Eon has been divided into several smaller eras. Read about each era in the section below:
The Eoarchean
The earliest era of the Archean eon was the Eoarchean. It began at the start of the Archean and ended 3.6 billion years ago. During the Eoarchean, the crust continued to form and stabilize from the end of the Hadean. No rocks survive from before the Eoarchean- they were either beaten away or recycled back into the mantle. However, rocks do survive from the Eoarchean itself. In fact, the era is best known through the Isua Greenstone Belt, a rock formation containing metamorphic and sedimentary rock located in Southwestern Greenland.
The Earth was a very different place during the Eoarchean. It was almost entirely water, with a volcano or volcanic island scattered here or there. Even the oceans were unrecognizable; because of dissolved iron compounds, they were green. During this time, Earth produces about 3 times as much heat internally than it does now, which made the planet very geoactive and compensated for a sun that was only 70-75% as bright as it is now.
Life, probably based around methane or other sulfates, may have first appeared at this time. While the first incontrovertible fossils came later, this is the era where we see the first signs of life.
The Paleoarchean
The Paleoarchean was the second era in the Archean, and was the time between 3.6 Ga and 3.2 Ga. This era is defined chronometrically, which means that there are no specific rock layers for this era. However, this was a very important era for life. It is thought that this is the era when the last universal common ancestor of all life on Earth lived. During this era, archea and eubacteria evolved, and the earliest stromatolites (oxygen-producing colonies of blue-green algae, or cyanobacteria) appeared.
The Mesoarchean
The Mesoarchean is another chronometrically defined era from 3.2-2.8 Ga. Stromatolites were increasingly common in coastal waters, and there was oxygen flow in the air for the first time (not much, but some). This is also the era when the first incontrovertible fossils appear. While there is evidence of life, it cannot be completely proven at this time that it was in fact life and not some other abiotic factors.
Continent formation continued, and by the end of the era, the two cratons Kaapral (located in South Africa) and Polbara (located in Australia) were separated. Continental grown during this era eventually led to larger continents such as Pangea, and, eventually, our continents.
The Neoarchean
The final era in the Archean, the Neoarchean began 2.8 billion years ago and ended 2.5 billion years ago. In this era, cyanobacteria started producing huge amounts of oxygen, which finally started getting into the air. In fact, there was so much oxygen that it poisoned life in the early Proterozoic.
Large continents with modern plate tectonics appeared at this time. Continents grew by getting lighter and though through re-melting and reformation of old rocks. Lighter rocks are more buoyant, and so they can more easily float over the liquid mantle.
Not only did the end of the Archean end an era, it also ended an eon. But all joking aside, the end of the Archean marked the end of a time when geologic factors ruled the Earth, and the beginning of one where life took charge.
In older literature, the Archean Eon was a part of the Hadean eon, but that is no longer the case. The Archean is one of the four principal eons of Earth's history, and begins the second division of Precambrian time. It is directly followed by the Proterozoic.
The Archean Eon has been divided into several smaller eras. Read about each era in the section below:
The Eoarchean
The earliest era of the Archean eon was the Eoarchean. It began at the start of the Archean and ended 3.6 billion years ago. During the Eoarchean, the crust continued to form and stabilize from the end of the Hadean. No rocks survive from before the Eoarchean- they were either beaten away or recycled back into the mantle. However, rocks do survive from the Eoarchean itself. In fact, the era is best known through the Isua Greenstone Belt, a rock formation containing metamorphic and sedimentary rock located in Southwestern Greenland.
The Earth was a very different place during the Eoarchean. It was almost entirely water, with a volcano or volcanic island scattered here or there. Even the oceans were unrecognizable; because of dissolved iron compounds, they were green. During this time, Earth produces about 3 times as much heat internally than it does now, which made the planet very geoactive and compensated for a sun that was only 70-75% as bright as it is now.
Life, probably based around methane or other sulfates, may have first appeared at this time. While the first incontrovertible fossils came later, this is the era where we see the first signs of life.
The Paleoarchean
The Paleoarchean was the second era in the Archean, and was the time between 3.6 Ga and 3.2 Ga. This era is defined chronometrically, which means that there are no specific rock layers for this era. However, this was a very important era for life. It is thought that this is the era when the last universal common ancestor of all life on Earth lived. During this era, archea and eubacteria evolved, and the earliest stromatolites (oxygen-producing colonies of blue-green algae, or cyanobacteria) appeared.
The Mesoarchean
The Mesoarchean is another chronometrically defined era from 3.2-2.8 Ga. Stromatolites were increasingly common in coastal waters, and there was oxygen flow in the air for the first time (not much, but some). This is also the era when the first incontrovertible fossils appear. While there is evidence of life, it cannot be completely proven at this time that it was in fact life and not some other abiotic factors.
Continent formation continued, and by the end of the era, the two cratons Kaapral (located in South Africa) and Polbara (located in Australia) were separated. Continental grown during this era eventually led to larger continents such as Pangea, and, eventually, our continents.
The Neoarchean
The final era in the Archean, the Neoarchean began 2.8 billion years ago and ended 2.5 billion years ago. In this era, cyanobacteria started producing huge amounts of oxygen, which finally started getting into the air. In fact, there was so much oxygen that it poisoned life in the early Proterozoic.
Large continents with modern plate tectonics appeared at this time. Continents grew by getting lighter and though through re-melting and reformation of old rocks. Lighter rocks are more buoyant, and so they can more easily float over the liquid mantle.
Not only did the end of the Archean end an era, it also ended an eon. But all joking aside, the end of the Archean marked the end of a time when geologic factors ruled the Earth, and the beginning of one where life took charge.
Welcome!
Welcome to the Archean Eon Blog, the first and foremost area to find information on a lot of stuff from the Archean Eon. In these next posts, you will discover many things, which, if you are part of my teaching project, should be remembered, because they're going to be on the test. Enjoy the blog and don't forget to fill out your worksheet and answer the questions!
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