Earth Formation Theory Discredited by New Findings

Earth Formation Theory Discredited by New Findings
Late-veneer hypothesis no longer valid
By Gabriel Gache, Science News Editor
5th of May 2008, 14:27 GMT
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It is widely believed even today that most of the water on our planet along with other 'iron-loving' elements were brought to Earth during the last couple of hundred million years by asteroids, meteorites, comets and other such objects passing through the inner regions of the solar system. FSU's Department of Geological Sciences and National High Magnetic Field Laboratory researcher, Munir Humayun on the other hand thinks otherwise.

"For 30 years, the late-veneer hypothesis has been the dominant paradigm for understanding Earth's early history, and our ultimate origins. Now, with our latest research, we're suggesting that the late-veneer hypothesis may not be the only way of explaining the presence of certain elements in the Earth's crust and mantle", says Humayun.To better explain the claims made by the study Humayun had co-authored, he is first detailing the current knowledge of the composition of our planet: "We know that the Earth has an iron-rich core that accounts for about one-third of its total mass. Surrounding this core is a rocky mantle that accounts for most of the remaining two-thirds. According to the late-veneer hypothesis, most of the original iron-loving, or siderophile, elements would have been drawn down to the core over tens of millions of years and thereby removed from the Earth's crust and mantle. The amounts of siderophile elements that we see today, then, would have been supplied after the core was formed by later meteorite bombardment. This bombardment also would have brought in water, carbon and other materials essential for life, the oceans and the atmosphere", he said.By pressing samples of rock containing palladium with the help of the Johnson Space Center 880 ton press, Humayun along with Kevin Righter and Lisa Danielson from NASA, recreated the exact conditions experienced by matter while 480 kilometers deep into the Earth. Later investigations with the inductively coupled plasma mass spectrometer from the space center's magnet laboratory revealed the exact distribution of palladium inside the sample."At the highest pressures and temperatures, our experiments found palladium in the same relative proportions between rock and metal as is observed in the natural world. Put another way, the distribution of palladium and other siderophile elements in the Earth's mantle can be explained by means other than millions of years of meteorite bombardment", he said."This work will have important consequences for geologists thinking about core formation, the core's present relation to the mantle, and the bombardment history of the early Earth. It also could lead us to rethink the origins of life on our planet", said Humayun.

Earth Sciences Study

MANY things that scientists study are invisible to them. Announcements about the discovery of planets around other stars do not come because astronomers have seen the planets, although they would certainly love to. The discovery is more likely based on observations of the star's motion that indicate strong nearby gravitational forces. So it is with discoveries about the Earth's core and mantle. Because the deepest well ever drilled extends down just 12 kilometers, not even pricking the mantle, researchers have to employ indirect methods to study the Earth.

Using meteorites and seismological evidence as clues, scientists have known almost since the beginning of the century that the Earth has a solid, mostly iron, inner core and a molten outer core with a mantle and crust of rocky, silicate material. But for just as long they have been puzzled about how the core and mantle separated. The primordial planet Earth grew out of bits of gas and dust, aggregating over time into a larger, more solid body. Was there then some cataclysmic event billions of years ago that melted much of the planet, prompting the metals and silicates to separate as oil and water do? Or was the separation the result of a more gradual process, a trickling down of the denser molten metals between solid silicate mineral grains to the center of the Earth? Recent research at Lawrence Livermore National Laboratory by geochemist William Minarik has helped to dispel the second, "trickle down," theory. Using the larger of Livermore's two multi-anvil presses to mimic the pressures and temperatures that exist deep in the Earth, he has shown that metals like those in the Earth's core could not have trickled down (see figure below).

The materials used in the experiments were olivine, a silicate mineral that makes up much of the Earth's upper mantle, and an iron-nickel-sulfur-oxygen combination to represent the core.

The multi-anvil press is a relatively rare research tool. Livermore's two presses have been used for a variety of material property studies, including diffusion and deformation of ceramics and metals, deep-focus earthquakes, and the high-pressure stability of mineral phases. The larger, 1,200-ton hydraulic press can produce pressures of 25 gigapascals (GPa), which is equivalent to 250,000 times the atmospheric pressure at sea level, or the pressure that occurs 700 kilometers deep in the Earth. In addition to pressing on the sample, the experiment passes an electric current through a furnace within the assembly to generate temperatures up to 2,200°C.

These experiments using the multi-anvil press generated a high pressure of 11 GPa, or 110,000 times the atmospheric pressure at sea level. This corresponds to 380 kilometers deep into the Earth, or pressures at the center of moons or asteroids 2,500 kilometers in radius (about the size of Mercury). The sample was also heated to a temperature of 1,500°C. Under those conditions, the metal melts and the olivine remains solid. The geometry of the press is key to creating these enormous pressures. For the 11-GPa experiment, a ceramic octahedron had a 10.3-millimeter-long hole with a tiny rhenium furnace, a thermocouple to measure temperatures, and a graphite capsule containing the olivine and iron-nickel-sulfur-oxygen sample inserted in it. The octahedron rested in the center of eight 32-millimeter tungsten carbide cubes whose inside corners were truncated to accommodate the sample. (Tungsten carbide is used for the cubes, or anvils, because of its hardness, which is close to that of diamonds but at a much lower cost.) Tiny ceramic gaskets were placed at the edges of the carbide cubes to contain the pressure. This assembly of an inner octahedron and eight carbide anvil cubes was put in the press's split-cone, steel buckets as shown in the figure above. In several stages, the steel buckets pushed on the carbide cubic anvils, which pushed on the octahedral volume inside. The multi-anvil press is not Livermore's only device for studying the behavior of the Earth's innards, but in many ways it is the best for this type of study. The diamond-anvil cell can produce 100-GPa pressures, comparable to the pressures at the center of the Earth (see Science & Technology Review, March 1996). But it can accommodate only a 20-micrometer sample, too small for much post-experiment evaluation. With the piston-cylinder press, the sample volume is about 500 millimeters3, but it has only a 4-GPa pressure capability, which is comparable to a depth of just 120 kilometers. The multi-anvil press is in the middle, providing pressures useful for studies of this type and accommodating a sample large enough for evaluation after the experiment.

In Minarik's experiments, the press took about 4 hours to bring the samples to full pressure, after which the samples were heated for periods ranging from 4 to 24 hours. During this time, the porosity of the sample collapsed, and the stable microstructure developed. Then the unit was cooled down and allowed to decompress for about 12 hours. During this process, the graphite capsule turned to diamond, which must be ground off before the sample could be sliced and polished for evaluation.

Despite being molten and much denser than the olivine, the metallic melt showed no signs of separating and draining to the bottom of the capsule. For the molten metal to drip down along the silicate grain edges, it has to be able to wet the edges. But in none of the experiments did wetting occur.1 Rather, the iron-nickel mixture beaded up at the corners of the silicate grains like water does on a waxed car, as shown in the figure below.

Livermore's findings agree with similar, lower-pressure studies that have melted meteorites and iron-nickel-sulfur- oxygen mixtures and failed to wet the silicate minerals. Together, these experiments indicate that much higher temperatures were required to separate the Earth's core and mantle--temperatures high enough to melt most of the Earth.

All of these data lend credence to the theory that the young, growing Earth was repeatedly bombarded by large planetoids, with some of these collisions generating temperatures high enough to form a magma ocean from which drops of dense molten metal separated. The largest collision may have been when a large celestial body, about the size of Mars, collided with Earth nearly 4.5 billion years ago, melting most of it and causing the core and mantle to separate. The leading theory today for the Moon's formation postulates that some material from that collision was ejected into orbit and condensed into the Earth's Moon. Livermore scientists have long studied material properties and the effects of high temperatures and pressures. Their work has resulted in some mighty big bangs but none as large as the ones Minarik has postulated.

"We plan to look next at the geochemical aspects of this project, the partitioning of trace elements between molten metal and silicates at the same high temperatures and pressures," says Minarik. "There are many scientists in this country and elsewhere studying the formation of the Earth, and all of us are in the same boat. With all of the direct evidence of the Earth's formation buried far beneath our feet, these laboratory experiments are our only way to recreate what might have happened."

Key Words: Earth core formation, multi-anvil press. Reference 1. W. G. Minarik, et al., "Textural Entrapment of Core-Forming Melts," Science 272, 530-533 (April 26, 1996). For further information contact William Minarik (510) 423-4130 Safdarhussan25@yahoo.com) or Frederick Ryerson (510) 422-6170

The structure of the Earth Test

The structure of the Earth

Imagine a Scotch egg......
1. The outer shell of the Earth is called the CRUST (breadcrumbs)
2. The next layer is called the MANTLE (sausagemeat)
3. The next layer is the liquid OUTER CORE (egg white)
4. The middle bit is called the solid INNER CORE (egg yolk)
DEAD EASY !
The deepest anyone has drilled into the earth is around 12 kilometres, we've only scratched the surface. How do we know what's going on deep underground?

There are lots of clues:
The overall density of the Earth is much higher than the density of the rocks we find in the crust. This tells us that the inside must be made of something much denser than rock.
Meteorites (created at the same time as the Earth, 4.6 billion years ago) have been analysed. The commonest type is called a chondrite and they contain iron, silicon, magnesium and oxygen (Others contain iron and nickel). A meteorite has roughly the same density as the whole earth. A meteorite minus its iron has a density roughly the same as Mantle rock (e.g. the mineral called olivine).
Iron and Nickel are both dense and magnetic.
Scientists can follow the path of seismic waves from earthquakes as they travel through the Earth. The inner core of the Earth appears to be solid whilst the outer core is liquid (s waves do not travel through liquids). The mantle is mainly solid as it is under extreme pressure (see below). We know that the mantle rocks are under extreme pressure, diamond is made from carbon deposits and is created in rocks that come from depths of 150-300 kilometres that have been squeezed under massive pressures.

The Earth is sphere (as is the scotch egg!) with a diameter of about 12,700Kilometres. As we go deeper and deeper into the earth the temperature and pressure rises. The core temperature is believed to be an incredible 5000-6000°c.
The crust is very thin (average 20Km). This does not sound very thin but if you were to imagine the Earth as a football, the crust would be about ½millimetre thick. The thinnest parts are under the oceans (OCEANIC CRUST) and go to a depth of roughly 10 kilometres. The thickest parts are the continents (CONTINENTAL CRUST) which extend down to 35 kilometres on average. The continental crust in the Himalayas is some 75 kilometres deep.
The mantle is the layer beneath the crust which extends about half way to the centre. It's made of solid rock and behaves like an extremely viscous liquid - (This is the tricky bit... the mantle is a solid which flows????) The convection of heat from the centre of the Earth is what ultimately drives the movement of the tectonic plates and cause mountains to rise. Click here for more details
The outer core is the layer beneath the mantle. It is made of liquid iron and nickel. Complex convection currents give rise to a dynamo effect which is responsible for the Earth's magnetic field.
The inner core is the bit in the middle!. It is made of solid iron and nickel. Temperatures in the core are thought to be in the region of 5000-6000°c and it's solid due to the massive pressure.
THAT'S ALL WE REALLY NEED TO KNOW!
(If you haven't seen a solid that flows then go back here and have a look)

EARTH STRUCTURE TEST
HERE IS SOME EXTRA STUFF (IN A LOT MORE DETAIL THAN WE NEED FOR GCSE):
This diagram shows a detailed picture of the Earth's interior. Crust is being created at the mid ocean ridges and being eaten at the subduction zones. The movement processes are driven by the convection currents created by the heat produced by natural radioactive processes deep within the Earth.

Inner core: depth of 5,150-6,370 kilometresThe inner core is made of solid iron and nickel and is unattached to the mantle, suspended in the molten outer core. It is believed to have solidified as a result of pressure-freezing which occurs to most liquids under extreme pressure.

Outer core: depth of 2,890-5,150 kilometresThe outer core is a hot, electrically conducting liquid (mainly Iron and Nickel). This conductive layer combines with Earth's rotation to create a dynamo effect that maintains a system of electrical currents creating the Earth's magnetic field. It is also responsible for the subtle jerking of Earth's rotation. This layer is not as dense as pure molten iron, which indicates the presence of lighter elements. Scientists suspect that about 10% of the layer is composed of sulphur and oxygen because these elements are abundant in the cosmos and dissolve readily in molten iron.

D" layer: depth of 2,700-2,890 kilometresThis layer is 200 to 300 kilometres thick. Although it is often identified as part of the lower mantle, seismic evidence suggests the D" layer might differ chemically from the lower mantle lying above it. Scientists think that the material either dissolved in the core, or was able to sink through the mantle but not into the core because of its density.

Lower mantle: depth of 650-2,890 kilometresThe lower mantle is probably composed mainly of silicon, magnesium, and oxygen. It probably also contains some iron, calcium, and aluminium. Scientists make these deductions by assuming the Earth has a similar abundance and proportion of cosmic elements as found in the Sun and primitive meteorites.

Transition region: depth of 400-650 kilometresThe transition region or mesosphere (for middle mantle), sometimes called the fertile layer and is the source of basaltic magmas. It also contains calcium, aluminium, and garnet, which is a complex aluminium-bearing silicate mineral. This layer is dense when cold because of the garnet. It is buoyant when hot because these minerals melt easily to form basalt which can then rise through the upper layers as magma.

Upper mantle: depth of 10-400 kilometresSolid fragments of the upper mantle have been found in eroded mountain belts and volcanic eruptions. Olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3 have been found. These and other minerals are crystalline at high temperatures. Part of the upper mantle called the asthenosphere might be partially molten.

Oceanic crust: depth of 0-10 kilometresThe majority of the Earth's crust was made through volcanic activity. The oceanic ridge system, a 40,000 kilometre network of volcanoes, generates new oceanic crust at the rate of 17 km3 per year, covering the ocean floor with an igneous rock called basalt. Hawaii and Iceland are two examples of the accumulation of basalt islands.

Continental crust: depth of 0-75 kilometresThis is the outer part of the Earth composed essentially of crystalline rocks. These are low-density buoyant minerals dominated mostly by quartz (SiO2) and feldspars (metal-poor silicates). The crust is the surface of the Earth. Because cold rocks deform slowly, we refer to this rigid outer shell as the lithosphere (the rocky or strong layer).Back to the Earth Science zone

Structure of the Earth

Structure of the Earth

Geophysics, which studies the physics of the Earth, has led to many significant discoveries about the Earth and its make-up. Seismologic studies of the Earth have uncovered new information about the interior of the Earth that has helped to give credence to plate tectonic theory.

Geophysical studies have revealed that the Earth has several distinct layers. Each of these layers has its own properties. The outermost layer of the Earth is the crust. This comprises the continents and ocean basins. The crust has a variable thickness, being 35-70 km thick in the continents and 5-10 km thick in the ocean basins. The crust is composed mainly of alumino-silicates.

The next layer is the mantle, which is composed mainly of ferro-magnesium silicates. It is about 2900 km thick, and is separated into the upper and lower mantle. This is where most of the internal heat of the Earth is located. Large convective cells in the mantle circulate heat and may drive plate tectonic processes.

The last layer is the core, which is separated into the liquid outer core and the solid inner core. The outer core is 2300 km thick and the inner core is 1200 km thick. The outer core is composed mainly of a nickel-iron alloy, while the inner core is almost entirely composed of iron. Earth's magnetic field is believed to be controlled by the liquid outer core.

The Earth is separated into layers based on mechanical properties in addition to composition. The topmost layer is the lithosphere, which is comprised of the crust and solid portion of the upper mantle. The lithosphere is divided into many plates that move in relation to each other due to tectonic forces. The lithosphere essentially floats atop a semi-liquid layer known as the asthenosphere. This layer allows the solid lithosphere to move around since the asthenosphere is much weaker than the lithosphere,