The mantle is the cream filling between a silicon-rich crustal meringue and an iron-rich core base. Unlike a common pie, however, it’s composed of layers detected by seismic wave tests and is not some separate ingredient like lemon or coconut, but a transition from silicon to iron in mixtures that includes oxygen, magnesium, aluminum, calcium, sodium, potassium and hydrogen in combinations that change by layer.
Starting at the top, the beginning depths, temperatures and distinguishing characteristics of these layers are:
Crust
Moho - 7 km - 500 C - increase in seismic velocity
Lithosphere - 50 km - 900 C - low viscosity, rigid
Asthenosphere - 200 km - 1100 C - flowing, low velocity
Transition Zone - 410 km - 1800 C - distinctive seismic patterns
Lower Mantle - 660 km - no agreement - high viscosity
Anomalies - 1700 km - no agreement - lateral velocity anomalies
D” Layer - 2891 km - 4000 C - ultralow velocity, distinctive seismic patterns
Core
Olivine, a compound of iron, silicon, magnesium and oxygen [(Mg,Fe)2SiO4], is identified as the most important component in the asthenosphere. In the lithosphere, it combines with elements like calcium and aluminum to produce pyroxene. Above that, in the Moho, pyroxene replaces the calcium and aluminum with potassium to become amphibole. That, in turn, replaces the potassium with calcium, sodium and hydrogen (water) to create biotite.
The ratio of iron and magnesium in olivine can vary, which is why they are represented by a comma, rather than a more specific number in the formula. The permutations of both olivine and pyroxene produce many of the silicon rocks found on the surface that have created the perception that the mantle is predominately composed of silicon.
Below the asthenosphere, in the transition zone, the forsterite-fayalite form of olivine changes its crystalline structure into wadsleyite [(Mg,Fe2+)2(SiO4)], which in turn becomes ringwoodite [(Mg,Fe2+)2(SiO4)]. The important feature of their structures is that changes in oxygen create spaces for water molecules which are being released as parts of the crust sink into the transition zone. Eli Ohtani thinks the presence of water trapped in these two forms may explain the layer’s seismic properties.
In the lower mantle, the crystalline structure changes again, this time to magnesium silicate perovskite [(Mg,Fe)SiO3]. Water is lost and iron becomes more important. With an atomic weight of 26, it has a partially filled third orbit. In its ferrous form [F2+], it has two electrons available to bond with other atoms; in its ferric form [F3+] it has three.
As pressure increases, the spin speed within the atom changes from high to low. Ferric iron changes its spin at 70 gigapascals which is about 1700 kilometers down in the area where seismic anomalies have been detected. Ferrous iron changes its spin pattern at 120 gigapascals which is about 2600 kilometers down, just above the D” layer where the crystalline structure changes again to one called post-perovskite.
The two valences can appear in the same compound in different ratios, while the amount of iron relative to magnesium also varies. Such differences explain why detectable seismic patterns are hard to replicate with current technology: the deeper one goes into the mantle, the greater the amount of iron that has changed its spin cycle.
Neither of the transition zone compounds are known from samples from the mantle. Wadsleyite was found in the Peace River meteorite from Canada. Ringwoodite was first identified in the Tenham meteorite in Australia and has since been seen in other meteorites from other parts of the world. It’s the laboratory conditions under which the two were synthesized that suggests they are the primary components of the transition zone.
A team headed by James Brado put samples of magnesium silicate perovskite through laboratory tests to produce evidence of spin changes and the deep mantle conditions that could produce them. They didn’t indicate where they obtained their raw material.
Notes:
Brado, James, Guillaume Fiquet, and François Guyot. Thermochemical State of the Lower Mantle: New Insights from Mineral Physics.
Ohtani, Eli. Recent Progress in Experimental Mineral Physics: Phase Relations of Hydrous Systems and the Role of Water in Slab Dynamics.
Both appear in Earth’s Deep Mantle: Structure, Composition and Evolution, 2005, edited by Robert D. van der Hilst, Jay D. Bass, Jan Matas and Jean Trampert for the American Geophysical Union.
Showing posts with label Iron. Show all posts
Showing posts with label Iron. Show all posts
Thursday, December 22, 2011
Tuesday, November 22, 2011
Earthly Beginnings
The early history of the earth is more theory than fact, and those theories are taken more from astrophysics than other disciplines. Those with other interests tend to pick through the available information in hopes of arriving at some early history for their subject.
A great many look for the thread that explains the origin of life. Others want to know how the moon was made.
My focus has been the creation of the conditions that made possible the emergence of that part of the North American plate where New Mexico sits between 1710 and 1600 million years ago, with a certain inclination to pay some attention to the formation of Michigan, the state where I was raised.
For me, this means envisioning how a 4600 million year old cloud of gas and dust evolved into a stratified ball whose layers have been defined by the workings of heat and cold.
The most important element has been iron. According to Wikipedia, iron begins in stars hot enough to burn silicon and initiate a chain of reactions with helium that transform matter from silicon to calcium to titanium to chromium to an unstable iron. That iron fuses with a helium nucleus to create 56nickel, after which the star collapses and the 56nickle transforms into 56iron via 56cobalt.
During the earliest millennia of the planet, radioactive decay continued in particles of nickle in the cloud which created conditions warm enough to heat the iron dust whose melting point is 1535 degrees centigrade. Following one basic law of physics, liquids are heavier than gases, the molten iron would have begun to isolate itself from the heat generating gases.
As it moved away from the source of heat, the molten iron would have gone through several structural phases. When the temperature fell below 1540c degrees, it would have begun to solidify and at 770c degrees it became magnetic.
Following another simple rule of physics, heat rises and cold falls, the cooler materials would have drifted towards the center of the ball, and the gases remained on the surface. Once enough iron had fallen below 770c degrees, the magnetic core could form. This process took about 50 million years and was complete around 4535 million years ago.
Once the nickle and iron coalesced into a ball divided into two parts, the magnetic, solid inner core and the molten outer core where temperatures today range from 5000c to 2200c degrees, silicon, magnesium and similar elements were segregated into an outer layer. Steve Kershaw suggests the mantle emerged around 4000 million years ago, and that it took about 2000 million years for the separation to be completed sometime around 2000 million years ago.
While the interior was still evolving, the outermost layer cooled into a skin that became the precursor of the crust. The oldest rocks found so far on the North American plate are from the Nuvvuagittuq greenstone belt near Hudson Bay. They are estimated to be between 3800 and 4280 years old.
The skin created a barrier between the warmer interior and the lighter gases which allowed water to condense without turning into steam. Oceans existed more than 3900 years ago, according to Kershaw. Scientists argue whether all the water was native or was increased by collisions with meteors and comets which could still easily penetrate the surface gases.
At this point, the history of the earth diverges into four separate narratives - the mantle and its skin, the oceans, the gases, and the crust and its plates - which rejoin sometime before New Mexico makes its appearance on the stage.
Notes:
Chandler, Harry. Metallurgy for the Non-Metallurgist, 1998, on properties of iron.
Kershaw, Steve. “Precambrian Ocean Change” in Oceanography: an Earth Science Perspective, 2000, with contributions from Andy Cundy.
Wikipedia entries on Earth’s history and iron.
A great many look for the thread that explains the origin of life. Others want to know how the moon was made.
My focus has been the creation of the conditions that made possible the emergence of that part of the North American plate where New Mexico sits between 1710 and 1600 million years ago, with a certain inclination to pay some attention to the formation of Michigan, the state where I was raised.
For me, this means envisioning how a 4600 million year old cloud of gas and dust evolved into a stratified ball whose layers have been defined by the workings of heat and cold.
The most important element has been iron. According to Wikipedia, iron begins in stars hot enough to burn silicon and initiate a chain of reactions with helium that transform matter from silicon to calcium to titanium to chromium to an unstable iron. That iron fuses with a helium nucleus to create 56nickel, after which the star collapses and the 56nickle transforms into 56iron via 56cobalt.
During the earliest millennia of the planet, radioactive decay continued in particles of nickle in the cloud which created conditions warm enough to heat the iron dust whose melting point is 1535 degrees centigrade. Following one basic law of physics, liquids are heavier than gases, the molten iron would have begun to isolate itself from the heat generating gases.
As it moved away from the source of heat, the molten iron would have gone through several structural phases. When the temperature fell below 1540c degrees, it would have begun to solidify and at 770c degrees it became magnetic.
Following another simple rule of physics, heat rises and cold falls, the cooler materials would have drifted towards the center of the ball, and the gases remained on the surface. Once enough iron had fallen below 770c degrees, the magnetic core could form. This process took about 50 million years and was complete around 4535 million years ago.
Once the nickle and iron coalesced into a ball divided into two parts, the magnetic, solid inner core and the molten outer core where temperatures today range from 5000c to 2200c degrees, silicon, magnesium and similar elements were segregated into an outer layer. Steve Kershaw suggests the mantle emerged around 4000 million years ago, and that it took about 2000 million years for the separation to be completed sometime around 2000 million years ago.
While the interior was still evolving, the outermost layer cooled into a skin that became the precursor of the crust. The oldest rocks found so far on the North American plate are from the Nuvvuagittuq greenstone belt near Hudson Bay. They are estimated to be between 3800 and 4280 years old.
The skin created a barrier between the warmer interior and the lighter gases which allowed water to condense without turning into steam. Oceans existed more than 3900 years ago, according to Kershaw. Scientists argue whether all the water was native or was increased by collisions with meteors and comets which could still easily penetrate the surface gases.
At this point, the history of the earth diverges into four separate narratives - the mantle and its skin, the oceans, the gases, and the crust and its plates - which rejoin sometime before New Mexico makes its appearance on the stage.
Notes:
Chandler, Harry. Metallurgy for the Non-Metallurgist, 1998, on properties of iron.
Kershaw, Steve. “Precambrian Ocean Change” in Oceanography: an Earth Science Perspective, 2000, with contributions from Andy Cundy.
Wikipedia entries on Earth’s history and iron.
Subscribe to:
Posts (Atom)