User:Marshallsumter/Rocks/Rocky objects/Earth

The Earth usually falls into three of the four types of astronomical objects: gaseous, liquid, and rocky. This lecture looks at the rocky object: Earth.

In addition to the Moon, there are Near-Earth objects (NEOs) that orbit the Earth briefly such as 1999 JM8 and 2006 RH120.

(53319) 1999 JM8 is an asteroid, slow rotator and tumbler, classified as a near-Earth object and potentially hazardous asteroid (PHA) of the Apollo group, approximately 7 km in diameter, making it the largest PHA known to exist. It was discovered on 13 May 1999, by Lincoln Near-Earth Asteroid Research at the Lincoln Laboratory's Experimental Test Site near Socorro, New Mexico. Its orbit has an eccentricity of 0.64 and an inclination of 14° with respect to the ecliptic. The body's observation arc begins with its first identification as 1990 HD1 at Palomar Observatory in April 1990, more than 9 years prior to its official discovery observation at Socorro.

In the SMASS classification and Tholen classification, 1999 JM8 is an X-type asteroid. It has also been characterized as a carbonaceous C-type asteroid, which seems more likely due to its exceptionally low albedo.

Radar imaging by Goldstone and Arecibo observatories revealed that 1999 JM8 has an unusually slow and possibly chaotic rotation period, similar to that of asteroid 4179 Toutatis.

In July 1999, a rotational lightcurve of 1999 JM8 was obtained from photometric observations that gave a period of $136$ hours with a brightness amplitude of 0.7 magnitude (LCDB quality code of U=2), and suggests that the body is in a non-principal axis rotation, commonly known as tumbling.

1999 JM8 measures between 5 and 7 kilometers in diameter and its surface has an exceptionally low astronomical albedo of 0.02. The Collaborative Asteroid Lightcurve Link derives an albedo of 0.03 and adopts a diameter of 7 kilometers based on an absolute magnitude of 15.2.

This minor planet was numbered by the Minor Planet Center on 16 February 2003. As of 2018, it has not been named.

Astronomy
The only current ice sheets are in Antarctica [second image on the right] and Greenland [the first image on the left]; during the last glacial period at Last Glacial Maximum (LGM) the Laurentide ice sheet covered much of North America, the Weichselian ice sheet covered northern Europe and the Patagonian Ice Sheet covered southern South America.

Shown in the northern hemisphere images is the rock distribution of sea ice and ice sheets [first image on the right].

At the right is a satellite composite image of the ice sheet over Greenland.

At the south pole, Antactica, there is also an extensive ice sheet [second image on the right]. Apparently, when the North polar sea ice and ice sheet has been contracting, the South polar sea ice and ice sheet has been expanding.

Often called the third pole, the third image on the right shows the rocky ice sheet over the top of the Himalayas.

Radiation
The first image on the right shows Mount Redoubt in Alaska erupting on April 21, 1990. The mushroom-shaped plume rose from avalanches of hot debris that cascaded down the north flank.

The second image is a view of the first high-temperature vent (380 °C) ever seen by scientists during a dive of the deep-sea submersible Alvin on the East Pacific Rise (latitude 21° north) in 1979.

"In 1977, scientists discovered hot springs at a depth of 2.5 km, on the Galapagos Rift (spreading ridge) off the coast of Ecuador."

"Such geothermal vents--called smokers because they resemble chimneys--spew dark, mineral-rich, fluids heated by contact with the newly formed, still-hot oceanic crust. This photograph shows a black smoker, but smokers can also be white, grey, or clear depending on the material being ejected."

Theoretical Earth
Def. the "third planet of the" "Sun and all the heavenly bodies that orbit around it, including the eight planets, their moons, the asteroids and comets" is called the Earth.

Entities
The image on the right shows a portion of the San Andreas Fault in California USA on Earth.

Def. a "fracture in a rock formation causing a discontinuity" is called a fault.

Strong forces
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption. They are sometimes confused with volcanic craters. The word comes from Spanish caldera, and this from Latin CALDARIA, meaning "cooking pot". In some texts the English term cauldron is also used.

About 75,000 years ago, this Indonesian volcano released about 2,800 km3 DRE of ejecta, the largest known eruption within the Quaternary Period (last 1.8 million years) and the largest known explosive eruption within the last 25 million years. In the late 1990s, anthropologist Stanley Ambrose proposed that a volcanic winter induced by this eruption reduced the human population to about 2,000 - 20,000 individuals, resulting in a population bottleneck (see Toba catastrophe theory). More recently several geneticists, including Lynn Jorde and Henry Harpending have proposed that the human race was reduced to approximately five to ten thousand people. Whichever figure is right, the fact remains that the human race seemingly came close to extinction about 75,000 years ago.

Objects
Centered above is the basin underneath the Pacific Ocean. Just below it is the geographic feature map for the floor of the Indian Ocean. Off on the right is the sea floor of the Atlantic Ocean. And, on the left is the basin around Antarctica of the Southern Ocean. At the bottom is the basin of the Arctic Ocean.

Surface
The image at right shows what the Earth looks like topographically under the water cover that constitutes more than 50 % of the Earth's actual surface. Based on this sea floor topography, the Earth is a rocky object to depths much less than its radius.

Emissions
A resurgent dome forms the island of Samosir within the caldera of Lake Toba. The image at the left. It is 100 km/62 mi long and 30 km/19 mi wide, a caldera of the world's largest class.

When the material above the explosion is solid rock, then a mound may be formed by broken rock that has a greater volume [as in the image of the Whetstone explosion area at the right]. This type of mound has been called "retarc", "crater" spelled backwards.

"The name Panum Crater refers to a crater surrounded by [an] ejecta ring, with a dome in the middle. At Panum Crater [image at the second right] the dome didn't completely fill the crater or overrun the ring (as often happens) providing an opportunity to explore all three structures."

"At Panum, a pyroclastic eruption (new magma explosively fragmented into the air) followed the phreatic (steam) eruption. During a pyroclastic eruption, the gas within the magma continues to expand and escape as the magma is thrown into the air and cools. The resulting deposits included ash (particles <2mm in size) and pumice. The pumice is frothy preserving the frozen gas bubbles."

"The ejecta ring is made up of small bits of pumice, ash, obsidian fragments, and well-rounded granitic pebbles (which were part of the surrounding rock and not formed during the eruption) that were ejected during the final explosive stage of the eruption."

"The central lava dome was erupted from degassed material and is made up of pumice and obsidian of the same composition. The difference between the two has to do with gas escaping as the magma cooled. The magma that created the dome had dissolved gas in it, like a bottle of seltzer water. As the magma rose towards the surface where there was less pressure on it than at depth, the gas expanded producing the holes (bubbles) you see in the pumice. The magma that remained pressurized while it cooled quickly or that had already lost its gas, formed the obsidian."

"Flow banding containing both obsidian and pumice is common at Panum Crater. Another common texture, called breadcrust, can also be seen in the dome. Breadcrust textures form when the inside of a cooling rock is still hot with gas escaping from it while the outside surface has already cooled. As the gas expands from the inside, the outside surface cracks to allow the gas to escape."

Absorptions


The image at right shows the crater created by the Sedan shallow underground nuclear test explosion.

At left is a stylised cross-section of a crater formed by a below-ground explosion.

“A crater is formed by an explosive event through the displacement and ejection of material from the ground. It is typically bowl-shaped. High pressure gas and pressure waves are responsible for the creation of the crater by three processes
 * 1) plastic deformation of the ground
 * 2) projection of material (ejecta) from the ground by the expansion of gases in the ground
 * 3) spallation of the ground surface and two processes partially fill it back in
 * 4) fall-back of ejecta
 * 5) erosion and landslides of the crater lip and wall

The relative importance of the five processes varies depending on the height above or depth below the ground surface at which the explosion occurs, and the material composing the ground. A subsidence crater is a hole or depression left on the surface of an area which has had an underground (usually nuclear) explosion.

Subsidence craters are created as the roof of the cavity caused by the explosion collapses. This causes the surface to depress into a sink (which subsidence craters are sometimes called). It is possible for further collapse to occur from the sink into the explosion chamber. When this collapse reaches the surface, and the chamber is exposed atmospherically to the surface, it is referred to as a chimney.

When a drilling oil well encounters high-pressured gas which cannot be contained either by the weight of the drilling mud or by blow-out preventers, the resulting violent eruption can create a large crater which can swallow up a drilling rig. This phenomenon is called "cratering" in oil field slang.

Removal of material and rock beneath a surface may result in a collapse of material above into the cavern below.

"In accordance with its definition, a makhtesh (Hebrew for "mortar" or "crater"; plural, makhteshim) is an erosion structure incised into an anticline and having a single drainage system with one outlet."

"Erosional craters (Makhtesh) were formed by truncation and erosion of several of these anticlinal crests."

At lower left, the image is an oblique aerial photo of Makhtesh Hazera. The Makhtesh drainage divide is outlined by a bold black line, with both of its constituent features (the anticlinal valley and the Upper Basin) located.

Bands
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body. In most common usage, the term is used for the approximately circular depression in the surface of a planet, moon or other solid body in the Solar System, formed by the hypervelocity impact of a smaller body with the surface. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters range from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins. Meteor Crater is perhaps the best-known example of a small impact crater on the Earth.

Impact cratering involves high velocity collisions between solid objects, typically much greater than the velocity of sound in those objects. Such hyper-velocity impacts produce physical effects such as melting and vaporization that do not occur in familiar sub-sonic collisions. On Earth, ignoring the slowing effects of travel through the atmosphere, the lowest impact velocity with an object from space is equal to the gravitational escape velocity of about 11 km/s. The fastest impacts occur at more than 80 km/s in the "worst case" scenario which the asteroid hits the earth in a retrograde parabolic orbit (because kinetic energy scales as velocity squared, earth's gravity only contributes 1 km/s to this figure, not 11 km/s). The median impact velocity on Earth is about 20 to 25 km/s.

Impacts at these high speeds produce shock waves in solid materials, and both impactor and the material impacted are rapidly compressed to high density. Following initial compression, the high-density, over-compressed region rapidly depressurizes, exploding violently, to set in train the sequence of events that produces the impact crater. Impact-crater formation is therefore more closely analogous to cratering by high explosives than by mechanical displacement. Indeed, the energy density of some material involved in the formation of impact craters is many times higher than that generated by high explosives. Since craters are caused by explosions, they are nearly always circular – only very low-angle impacts cause significantly elliptical craters.

The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as shatter cones, melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in the uplifted center of a complex crater, however.

Impacts produce distinctive shock-metamorphic effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include:


 * A layer of shattered or "brecciated" rock under the floor of the crater. This layer is called a "breccia lens".
 * Shatter cones, which are chevron-shaped impressions in rocks. Such cones are formed most easily in fine-grained rocks.
 * High-temperature rock types, including laminated and welded blocks of sand, spherulites and tektites, or glassy spatters of molten rock. The impact origin of tektites has been questioned by some researchers; they have observed some volcanic features in tektites not found in impactites. Tektites are also drier (contain less water) than typical impactites. While rocks melted by the impact resemble volcanic rocks, they incorporate unmelted fragments of bedrock, form unusually large and unbroken fields, and have a much more mixed chemical composition than volcanic materials spewed up from within the Earth. They also may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt. Note: scientific literature has reported that some "shock" features, such as small shatter cones, which are often associated only with impact events, have been found also in terrestrial volcanic ejecta.
 * Microscopic pressure deformations of minerals. These include fracture patterns in crystals of quartz and feldspar, and formation of high-pressure materials such as diamond, derived from graphite and other carbon compounds, or stishovite and coesite, varieties of shocked quartz.
 * Buried craters can be identified through drill coring, aerial electromagnetic resistivity imaging, and airborne gravity gradiometry.

At right is a "[r]ecent airborne geophysical surveys near Decorah, Iowa [which is] providing an unprecedented look at a 470- million-year-old meteorite crater concealed beneath bedrock and sediments."

"Capturing images of an ancient meteorite impact was a huge bonus," said Dr. Paul Bedrosian, a USGS geophysicist in Denver who is leading the effort to model the recently acquired geophysical data. "These findings highlight the range of applications that these geophysical methods can address."

"In 2008-09, geologists from the Iowa Department of Natural Resources' (Iowa DNR) Iowa Geological and Water Survey hypothesized what has become known as the Decorah Impact Structure. The scientists examined water well drill-cuttings and recognized a unique shale unit preserved only beneath and near the city of Decorah. The extent of the shale, which was deposited after the impact by an ancient seaway, defines a "nice circular basin" of 5.5 km width, according to Robert McKay, a geologist at the Iowa Geological Survey."

"Bevan French, a scientist the Smithsonian's National Museum of Natural History, subsequently identified shocked quartz - considered strong evidence of an extra-terrestrial impact - in samples of sub-shale breccia from within the crater."

"The recent geophysical surveys include an airborne electromagnetic system, which is sensitive to how well rocks conduct electricity, and airborne gravity gradiometry, which measures subtle changes in rock density. The surveys both confirm the earlier work and provide a new view of the Decorah Impact Structure. Models of the electromagnetic data show a crater filled with electrically conductive shale and the underlying breccia, which is rock composed of broken fragments of rock cemented together by a fine-grained matrix."

"The shale is an ideal target and provides the electrical contrast that allows us to clearly image the geometry and internal structure of the crater," Bedrosian said.

The image at the right shows a crater produced by missile impact in silty sand and sandy silt photographed in an oblique view. The "[m]issile traveled along an oblique trajectory, 45.8° from the horizontal with a kinetic energy of 25.1 x 1014 ergs. The crater, about 6 metres across, and ejecta have bilateral symmetry because of the oblique trajectory. [The t]race of path of [the] missile is shown by [the] arrow. Small depressions in foreground are footprints."

"Craters in natural materials at White Sands Missile Range, N. Mex., were produced by the impact of high-velocity to hypervelocity missiles traveling along oblique trajectories with kinetic energies between 2.1 and 81 x 1014 ergs. The oblique impacts produce craters 2 to 10 m across with morphologies and ejecta that are bilaterally symmetrical with respect to the plane of the missile trajectory. Rims are high and the amount of ejecta large in down-trajectory and lateral directions, whereas rims are low to nonexistent and ejecta thin to absent up-trajectory. Symmetry development and modifications of the symmetry are a function of target material, local topography, and angle of impact."

Astrogeology
Def. the intellectual and practical activity encompassing the systematic study through observation and experiment of naturally occurring astronomical rocky objects, their physical structure and substance, history and origin, and the processes that act on them, especially by examination of their rocks, is called astrogeology.

Identifying the rocks, regoliths, and sediments on the solid surface of the Earth is often best accomplished from above the surface.

Geognosy
The astrognosy of the Earth is usually referred to as geognosy, the constitution of the Earth. Geology of the interior of Earth:

1. continental crust

2. oceanic crust

3. upper mantle

4. lower mantle

5. outer core

6. inner core

A: Mohorovičić discontinuity (Moho boundary)

B: Gutenberg Discontinuity

C: Lehmann discontinuity (inner core-outer core boundary)

The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The Inner Core of the earth is believed to be composed primarily of a nickel-iron alloy, with very small amounts of some other elements. The Outer Core of the earth is a liquid layer made of iron and nickel.

"A new dataset called Bedmap2 gives a clearer picture of Antarctica from the ice surface down to the bedrock below."

“In order to accurately simulate the dynamic response of ice sheets to changing environmental conditions, such as temperature and snow accumulation, we need to know the shape and structure of the bedrock below the ice sheets in great detail.”

Astrogony
The hominins of Earth may have observed and recorded a genealogy, or a begetting, of astronomical objects. Such a begetting may be called an astrogony.

Their astronomical observations may have suggested a genealogy, a progression from one astronomical object to another from the point of view of Earth. These objects may have been recorded and perhaps regarded based on what was observed.

Tectonics
"In the latest map, as many as 20,000 previously unknown seamounts, between 1.5 and 2 kilometres high, pop into view scattered along relatively young sections of the sea floor."

"The gravity model of the North Atlantic ocean basin [above] reveals tectonic history in sharp detail. [The red] dots show the [locations] of past earthquakes of magnitude 5.5 or higher."

Craters
The Weaubleau structure is a probable meteorite impact site in western Missouri near the towns of Gerster, Iconium, Osceola, and Vista. It is believed to have been caused by a 1200-ft (366 m) meteoroid between 330 and 335 million years ago during the late Mississippian Period (Latest Osagean to Earliest Meramecian). It is listed by the Impact Field Studies Group as a "probable" impact structure.

The structure consists of an area of severe structural deformity and extensive brecciation that was poorly understood and had been thought to be the result of either thrusting over a dome or a cryptoexplosive event. A 19-km-diameter (12 mi) circular structure was discovered ... through examination of digital elevation data.

Because the site was covered by later Pennsylvanian Period sediments, and only partially exposed to erosion relatively recently, its structure is well preserved, and its age can be determined with fair accuracy. It is one of a series of known or suspected impact sites along the 38th parallel in the states of Illinois, Missouri, and Kansas.

The Weaubleau structure is one of the fifty largest known impact craters on earth and the fourth largest in the United States. The three larger ones in the US either have been glaciated and buried (Manson crater), are under water (Chesapeake Bay crater), or have been subjected to orogeny (Beaverhead crater). Therefore the Weaubleau structure is the largest exposed untectonized impact crater in the US.

In the image at left is an aerial view of the Barringer Meteor Crater about 69 km east of Flagstaff, Arizona USA. Although similar to the aerial view of the Soudan crater, the Barringer Meteor Crater appears angular at the farthest ends rather than round.

"Meteor Crater is a meteorite impact crater approximately 43 miles (69 km) east of Flagstaff, near Winslow in the northern Arizona desert of the United States. Because the US Department of the Interior Division of Names commonly recognizes names of natural features derived from the nearest post office, the feature acquired the name of "Meteor Crater" from the nearby post office named Meteor. The site was formerly known as the Canyon Diablo Crater, and fragments of the meteorite are officially called the Canyon Diablo Meteorite. Scientists refer to the crater as Barringer Crater in honor of Daniel Barringer, who was first to suggest that it was produced by meteorite impact.

From space the crater appears almost like a square. The image at right has a resolution of 2 meters per pixel, and illumination is from the right. Layers of exposed limestone and sandstone are visible just beneath the crater rim, as are large stone blocks excavated by the impact.

The Holsinger meteorite is the largest discovered fragment of the meteorite that created Meteor Crater and it is exhibited in the crater visitor center. The Canyon Diablo meteorite comprises many fragments of the asteroid that impacted at Barringer Crater (Meteor Crater), Arizona, USA. Meteorites have been found around the crater rim, and are named for nearby Canyon Diablo, which lies about three to four miles west of the crater. There are fragments in the collections of museums around the world including the Field Museum of Natural History in Chicago. The biggest fragment ever found is the Holsinger Meteorite, weighing 639 kg, now on display in the Meteor Crater Visitor Center on the rim of the crater.

Occasionally, objects fall from the sky. When and where this occurs, depending on the energy dumped into the atmosphere and the impact on the crust of the Earth, life forms nearby hear it, feel the vibrations from it, and recoil if the intensity is too high.

But asteroid impacts, though rare, occur once in a while, over very large areas, at aperiodic intervals such as the Chicxulub crater. Most scientists agree that this impact is the cause of the Cretatious-Tertiary Extinction, 65 million years ago (Ma), that marked the sudden extinction of the dinosaurs and the majority of life then on Earth. This shaded relief image of Mexico's Yucatan Peninsula shows a subtle, but unmistakable, indication of the Chicxulub impact crater.

At right is a natural color photograph of the Aurora Borealis or northern lights and the Manicouagan Impact Crater reservoir (foreground) in Quebec, Canada. They are featured in this photograph taken by astronaut Donald R. Pettit, Expedition Six NASA ISS science officer, on board the International Space Station (ISS).

"Collisions by fragmented objects result in multiple impacts that can lead to the formation of linear crater chains, or catenae, on planetary surfaces2."

"Five terrestrial impact structures have been found to possess comparable ages (214 Myr), coincident with the Norian stage of the Triassic period. These craters are Rochechouart (France), Manicouagan and Saint Martin (Canada), Obolon' (Ukraine) and Red Wing (USA). When these impact structures are plotted on a tectonic reconstruction of the North American and Eurasian plates for 214 Myr before present, the three largest structures (Rochechouart, Manicouagan and Saint Martin) are co-latitudinal at 22.8° (within 1.2°, 110 km), and span 43.5° of palaeolongitude. These structures may thus represent the remains of a crater chain at least 4,462 km long. The Obolon' and Red Wing craters, on the other hand, lie on great circles of identical declination with Rochechouart and Saint Martin, respectively. [...] the five impact structures were [likely] formed at the same time (within hours) during a multiple impact event caused by a fragmented comet or asteroid colliding with Earth."

Minerology
"Silicates are the dominant group minerals in the Earth's crust."

"Tcs are dominated by the volumetrically dominant Group A grains in the pillow interior and are generally between 130 and 190°C (sub-samples d–l at depths of 1.5 to 6.2 cm from the rim"

Much of the use of dominant group relative to the Earth (rocky object) is described in dominant group/Geology.

Magnetic field
From "stone samples from the bottom of the Atlantic Ocean that the magnetic field of Earth changes its direction on average every 780,000 years. And the last polar reversal, the Brunhes-Matuyama-Reversal, happened some 780,000 years ago."

"The Brunhes-Matuyama Reversal happened much faster than had been thought hitherto, as a team of scientists investigating a fossil lake in Italy recently found out. The change occurred in just about one hundred years."

"In the past one hundred years, [the Earth's magnetic field, illustrated in the first image at right] has diminished globally by a good five percent. In some areas, such as the Atlantic before the coast of Brazil [as seen in the second image at the right], it has lost even more of its strength. In the area of this so-called South-Atlantic anomaly, a minus of five percent has been observed within a mere ten or fifteen years."

"Almost as fast as during the short lived magnetic reversal 41,000 year ago, also known as the “geomagnetic excursion.” The magnetic North pole wandered within 200 years to the South pole, remained there for 440 years, then it moved back. These short excursions are even more frequent than the recurring long wanderings."

The "processes [in the Earth’s deep interior] are responsible for over 95 percent of the strength of the magnetic field: around a solid core of iron and nickel, there is a zone in which the metal is in a liquid state. The solid interior core of the Earth and its liquid surroundings rotate with different speeds, while convection currents are forming in the liquid zone which move by about ten kilometres a year."

The graph [at third right] shows a comparison of the observed magnetic profile for the seafloor across the East Pacific Rise against a profile calculated from the Earth's known magnetic reversals, assuming a constant rate of spreading.

"An observed magnetic profile (blue) for the ocean floor across the East Pacific Rise is matched quite well by a calculated profile (red) based on the Earth's magnetic reversals for the past 4 million years and an assumed constant rate of movement of ocean floor away from a hypothetical spreading center (bottom). The remarkable similarity of these two profiles provided one of the clinching arguments in support of the seafloor spreading hypothesis."

"A team of U.S. Geological Survey scientists -- geophysicists Allan Cox and Richard Doell, and isotope geochemist Brent Dalrymple -- reconstructed the history of magnetic reversals for the past 4 million years using a dating technique based on the isotopes of the chemical elements potassium and argon. The potassium-argon technique -- like other "isotopic clocks" -- works because certain elements, such as potassium, contain unstable, parent radioactive isotopes that decay at a steady rate over geologic time to produce daughter isotopes. The rate of decay is expressed in terms of an element's "half-life," the time it takes for half of the radioactive isotope of the element to decay. The decay of the radioactive potassium isotope (potassium-40) yields a stable daughter isotope (argon-40), which does not decay further. The age of a rock can be determined ("dated") by measuring the total amount of potassium in the rock, the amount of the remaining radioactive potassium-40 that has not decayed, and the amount of argon-40. Potassium is found in common rock-forming minerals, and because the potassium-40 isotope has a half-life of 1,310 million years, it can be used in dating rocks millions of years old."

Rocks "generally belong to two groups according to their magnetic properties. One group has so-called normal polarity, characterized by the magnetic minerals in the rock having the same polarity as that of the Earth's present magnetic field. This would result in the north end of the rock's "compass needle" pointing toward magnetic north. The other group, however, has reversed polarity, indicated by a polarity alignment opposite to that of the Earth's present magnetic field. In this case, the north end of the rock's compass needle would point south. How could this be? This answer lies in the magnetite in volcanic rock. Grains of magnetite -- behaving like little magnets -- can align themselves with the orientation of the Earth's magnetic field. When magma (molten rock containing minerals and gases) cools to form solid volcanic rock, the alignment of the magnetite grains is "locked in," recording the Earth's magnetic orientation or polarity (normal or reversed) at the time of cooling."

"As more and more of the seafloor was mapped during the 1950s, the magnetic variations turned out not to be random or isolated occurrences, but instead revealed recognizable patterns. When these magnetic patterns were mapped over a wide region, the ocean floor showed a zebra-like pattern. Alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge: one stripe with normal polarity and the adjoining stripe with reversed polarity. [And, in the Pacific Northwest, shown on the right in the fourth image.] The overall pattern, defined by these alternating bands of normally and reversely polarized rock, became known as magnetic striping."

Petrology of Earth
Def. a "naturally occurring aggregate of minerals" is called a rock.

Def. "one of the major groups of rock that makes up the crust of the Earth; formed by the cooling of molten rock, either below the surface (intrusive) or on the surface (extrusive)" is called an igneous rock.

Def. "one of the major groups of rock that makes up the crust of the Earth; consists of pre-existing rock mass in which new minerals or textures are formed at higher temperatures and greater pressures than those present on the Earth's surface" is called a metamorphic rock.

Def. "one of the major groups of rock that makes up the crust of the Earth; formed by the deposition of either the weathered remains of other rocks, the results of biological activity, or precipitation from solution" is called a sedimentary rock.

Def. the "study of the origin, composition and structure of rock" is called petrology.

Def. "the branch of petrology that deals with the scientific" "description and classification of rocks" is called petrography.

Def. the "study of rocks, with particular emphasis on their description and classification" or the "general composition of a rock or rock sequence" is called lithology.

Petrology is a branch of geology that studies rocks, and the conditions in which rocks form. Lithology focuses on macroscopic hand-sample or outcrop-scale description of rocks, while petrography deals with microscopic details. Petrology benefits from mineralogy, optical mineralogy, geochemistry, and geophysics. Three branches of petrology focus on the three major rock types: igneous (on the right), metamorphic (left), and sedimentary (center).

Aeolianites
Def. a "rock formed from dune sand, often calcareous" is called an aeolianite.

Amphibolites
Def. any "of a class of [...] rock composed mainly of amphibole with some quartz etc" is called an amphibolite.

On the left is foliated amphibolite, sample 81MW0005, a borehole sample from under Cape Cod in Massachusetts in USA. It is made of the minerals plagioclase (35%), hornblende (20%), biotite (20%), epidote (15%), quartz (9%), and trace oxides and sphene. Plagioclase is mostly fine grained and subhedral and occurs in the matrix. Fine-grained hornblende occurs as anhedral pleochroic green-tan crystals. Biotite is partly, but not entirely aligned in the foliation, suggesting that deformation took place before a secondary growth of biotite. Epidote is anhedral, and sometimes rimmed by biotite. Quartz occurs in 2 mm-thick aggregates and shows subgrain development.

Andesites
Def. a "class of fine-grained intermediate [..] rock [...] containing mostly plagioclase feldspar" is called an andesite.

"Andesite is a gray to black volcanic rock with between about 52 and 63 weight percent silica (SiO2). Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene (clinopyroxene and orthopyroxene) and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine. Andesite magma commonly erupts from stratovolcanoes as thick lava flows, some reaching several km in length. Andesite magma can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Andesites erupt at temperatures between 900 and 1100° C."

Anorthosites
Def. a "phaneritic, [...] rock characterized by a predominance of plagioclase feldspar" is called an anorthosite.

Anorthosite on Earth can be divided into five types:
 * 1) Archean-age anorthosites
 * 2) Proterozoic anorthosite (also known as massif or massif-type anorthosite) – the most abundant type of anorthosite on Earth
 * 3) Layers within Layered Intrusions (e.g., Bushveld Igneous Complex and Stillwater igneous complex intrusions)
 * 4) Mid-ocean ridge and transform fault anorthosites
 * 5) Anorthosite xenoliths in other rocks (often granites, kimberlites, or basalts).

Plagioclase crystals are usually less dense than magma; so, as plagioclase crystallizes in a magma chamber, the plagioclase crystals float to the top, concentrating there.

Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research.

Proterozoic anorthosites were emplaced during the Proterozoic Eon (ca. 2,500–542 Ma), though most were emplaced between 1,800 and 1,000 Ma.

Large volumes of ultramafic rocks are not found in association with Proterozoic anorthosites.

Anthracites
Def. a "form of carbonized ancient plants; the hardest and cleanest-burning of all the coals; hard coal" is called anthracite.

Def. a coal of a hard variety that contains relatively pure carbon is called an anthracite.

Aphanites
Def. certain "dark [...] rocks having grain so fine that the individual crystals cannot be seen with the naked eye" is called an aphanite.

Aplites
Def. a "fine-grained [...] rock composed mostly of quartz and feldspars" is called an aplite.

Argillites
Def. a "fine-grained [...] rock, intermediate between shale and slate" is called an argillite.

Basanites
Def. a "basaltic [...] rock, similar to chert" is called a basanite.

Basalts
Def. a "hard mafic [...] rock of varied mineral content" is called a basalt.

"Basalt is a hard, black volcanic rock with less than about 52 weight percent silica (SiO2). Because of basalt's low silica content, it has a low viscosity (resistance to flow). Therefore, basaltic lava can flow quickly and easily move > 20 km from a vent. The low viscosity typically allows volcanic gases to escape without generating enormous eruption columns. Basaltic lava fountains and fissure eruptions, however, still form explosive fountains hundreds of meters tall. Common minerals in basalt include olivine, pyroxene, and plagioclase. Basalt is erupted at temperatures between 1100 to 1250° C."

"Basalt is the most common rock type in the Earth's crust (the outer 10 to 50 km). In fact, most of the ocean floor is made of basalt."

"Huge outpourings of lava called "flood basalts" are found on many continents. The Columbia River basalts, erupted 15 to 17 million years ago, cover most of southeastern Washington and regions of adjacent Oregon and Idaho."

"Basaltic magma is commonly produced by direct melting of the Earth's mantle, the region of the Earth below the outer crust. On continents, the mantle begins at depths of 30 to 50 km."

"Shield volcanoes, such as those that make up the Islands of Hawai`i, are composed almost entirely of basalt."

Blueschists
Def. a "rock containing glaucophane" is called a blueschist. A metamorphic facies is a set of metamorphic mineral assemblages that were formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure (See diagram at right). Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in geological history of the area. The boundaries between facies (and corresponding areas on the temperature v. pressure graph), are wide, because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure, is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

Blueschist is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15 to 30 kilometers and 200 to ~500 degrees Celsius. The blue color of the rock comes from the presence of the mineral glaucophane. Blueschists are typically found within orogenic belts as terranes of lithology in faulted contact with greenschist or rarely eclogite facies rocks. ... Blueschist, as a rock type, is defined by the presence of the minerals glaucophane + ( lawsonite or epidote ) +/- jadeite +/- albite or chlorite +/- garnet +/- muscovite in a rock of roughly basaltic composition. Blueschist often has a lepidoblastic, nematoblastic or schistose rock microstructure defined primarily by chlorite, phengitic white mica, glaucophane, and other minerals with an elongate or platy shape. Grain size is rarely coarse, as mineral growth is retarded by the swiftness of the rock's metamorphic trajectory and perhaps more importantly, the low temperatures of metamorphism and in many cases the anhydrous state of the basalts. However, coarse varieties do occur. Blueschists may appear blue, black, gray, or blue-green in outcrop.

Biofacies
Def. a "body of rock with characteristic biological features, such as certain kinds of fossil" is called a biofacies.

Calcarenites
Def. a "form of limestone (or dolomite) composed of sand sized grains derived from the erosion of older rocks" is called a calcarenite.

Carbonatites
Def. any "rock having a majority of carbonate minerals" is called a carbonatite.

Cinerites
Def. a "rock composed mostly of [...] ash" is called a cinerite.

Claystones
Def. a "rock composed of fine, clay particles" is called a claystone.

Coals
Def. a "black rock formed from prehistoric plant remains, composed largely of carbon and burned as a fuel" is called a coal.

Def. a black coal having a relatively high volatile content is called a bituminous coal.

Conglomerates
Def. a "rock consisting of gravel or pebbles embedded in a matrix" is called a conglomerate.

Dacites
Def. a rock with a high iron content is called a dacite.

"Dacite lava is most often light gray, but can be dark gray to black. Dacite lava consists of about 63 to 68 percent silica (SiO2). Common minerals include plagioclase feldspar, pyroxene, and amphibole. Dacite generally erupts at temperatures between 800 and 1000°C. It is one of the most common rock types associated with enormous Plinian-style eruptions. When relatively gas-poor dacite erupts onto a volcano's surface, it typically forms thick rounded lava flow in the shape of a dome."

"Even though it contains less silica than rhyolite, dacite can be even more viscous (resistant to flow) and just as dangerous as rhyolites. These characteristics are a result of the high crystal content of many dacites, within a relatively high-silica melt matrix. Dacite was erupted from Mount St. Helens 1980-86, Mount Pinatubo in 1991, and Mount Unzen 1991-1996."

Diabases
Def. a "fine-grained [...] rock composed mostly of pyroxene and feldspar" is called a diabase.

On the right is an image of a rock, a diabase with an aphanitic groundmass and plagioclase phenocrysts.

Diamictites
Def. "nonsorted, noncalcareous terrigenous deposits composed of sand and/or larger particles dispersed through a muddy matrix" are called diamictons.

Def. a lithified diamicton is called a diamictite.

"Such rocks have in common a mixed, ill-sorted, disperse-megaclastic lithology with a great to extreme range of size grades." The definitions of these rocks are "without regard to origin".

Diorites
Def. a speckled, coarse-grained rock consisting essentially of plagioclase, feldspar, and hornblende or other mafic minerals is called a diorite.

Def. a "grey [...] rock composed mostly of plagioclase feldspar, biotite, hornblende and/or pyroxene" is called a diorite.

Dolerites
Def. a "fine-grained basaltic rock" is called a dolerite.

Dolomites
Def. a "saline evaporite consisting of a mixed calcium and magnesium carbonate, with the chemical formula CaMg(CO3)2;" is called a dolomite.

Dolostones
Def. a "carbonate rock that contains a high percentage of the mineral dolomite" is called a dolostone.

Eclogites
Def. a "coarse-grained [...] rock, a mixture of pyroxene, quartz, and feldspar with inclusions of red garnet" is called an eclogite.

Eucrites
Def. an "achondritic meteoritic rock consisting chiefly of pigeonite and anorthite" is called a eucrite.

Eurites
Def. a "compact feldspathic rock" is called a eurite.

Felsites
Def. a "fine grained [...] rock, generally light in color, composed of felsic minerals" is called a felsite.

Gabbros
Def. a dark, coarse-grained plutonic rock of crystalline texture, consisting mainly of pyroxene, plagioclase feldspar, and often olivine is called a gabbro.

Def. "a coarsely crystalline, igneous rock consisting of lamellar pyroxene and labradorite" is called a gabbro.

As with diamictites, rock definitions should be without regard to origin.

Gneisses
Def. a "rock having bands or veins, but not schistose" is called a gneiss.

Granites
Def. a very hard, granular, crystalline, rock consisting mainly of quartz, mica, and feldspar is called a granite.

Granodiorites
Def. a "rock similar to granite, but containing more plagioclase than potassium feldspar" is called a granodiorite.

Granulites
Def. "fine-grained [...] rock composed chiefly of feldspar, quartz, and garnets" is called a granulite.

Greisens
Def. a "highly altered granitic rock containing quartz and mica" is called a greisen.

Greywackes
Def. a "hard dark sandstone with poorly sorted angular grains of quartz, feldspar, and small rock fragments in a compact, clay-fine matrix" is called a greywacke.

Hawaiites
Def. an "olivine basalt intermediate between alkali olivine and mugearite" is called a hawaiite.

Hornfels
The hornfels shown on the right were formed from the heating of sandstones and siltstones by the Insskoy series of granite intrusions.

Hyaloclastites
Def. a "rock containing glassy fragments" is called a hyaloclastite.

Jets
Def. a "hard, black form of coal", specifically lignite is called a jet.

Kimberlites
Def. a "variety of peridotite containing a high proportion of carbon dioxide; often contains diamonds" is called a kimberlite.

Lamproites
Def. any "of several volcanic rocks having a high potassium content" is called a lamproite.

Lamprophyres
Def. an "uncommon, small-volume ultrapotassic igneous rock primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions" is called a lamprophyre.

Lignites
Def. a "low-grade, brownish-black coal" is called a lignite.

Limestones
Def. a rock "primarily composed of calcite (CaCO₃); it occurs in a variety of forms, both crystalline and amorphous" is called a limestone.

Lithosomes
Def. a "uniform mass of rock that has different lithography to that of the surrounding rock" is called a lithosome.

Mafics
Def. "rocks, which contain relatively high concentrations of magnesium and iron" are called mafics.

Marbles
On the right is a block of white marble.

The left shows Mississippian marble in Big Cottonwood Canyon, Wasatch Mountains, Utah.

Monzogranites
Rochovce granite, composing the coring on the right, is a coarse-grained biotite monzogranite.

Monzonites
Def. an "intrusive igneous rock composed mostly of plagioclase and orthoclase" is called a monzonite.

Mugearites
Def. a "kind of orthoclase-bearing basalt that is made up of olivine, apatite, and opaque oxides" is called a mugearite.

Peridotites
Def. a "rock consisting of small crystals of olivine, pyroxene and hornblende" is called a peridotite.

Phyllites
A sample of a phyllite is on the right.

Quartzites
Def. "a [...] rock consisting of interlocking grains of quartz" is called a quartzite.

In a quartzite, fractures occur through the quartz grains. In a sedimentary rock composed of quartz grains, the rock fractures around the quartz grains.

Rhyolites
Def. a rock "of felsic composition, with aphanitic to porphyritic texture" is called a rhyolite.

"Rhyolite is a light-colored rock with silica (SiO2) content greater than about 68 weight percent. Sodium and potassium oxides both can reach about 5 weight percent. Common mineral types include quartz, feldspar and biotite and are often found in a glassy matrix. Rhyolite is erupted at temperatures of 700 to 850° C."

"Rhyolite can look very different, depending on how it erupts. Explosive eruptions of rhyolite create pumice, which is white and full of bubbles. Effusive eruptions of rhyolite often produce obsidian, which is bubble-free and black."

"Some of the United States' largest and most active calderas formed during eruption of rhyolitic magmas (for example, Yellowstone in Wyoming, Long Valley in California and Valles in New Mexico)."

"Rhyolite often erupts explosively because its high silica content results in extremely high viscosity (resistance to flow), which hinders degassing. When bubbles form, they can cause the magma to explode, fragmenting the rock into pumice and tiny particles of volcanic ash."

Saprolites
Def. a chemically weathered rock is called a saprolite.

Schists
At right is an image of schist.

Shales
Def. a "fine-grained [...] rock of a thin, laminated, and often friable, structure" is called a shale.

Shergottites
Roughly three-quarters of all Martian meteorites can be classified as shergottites, named after the Shergotty meteorite, which fell at Sherghati, India in 1865. Shergottites are igneous rocks of mafic to ultramafic lithology that fall into three main groups, the basaltic, olivine-phyric (such as the Tissint group found in Morocco in 2011 ) and lherzolite, (lherzolitic) shergottites, based on their crystal size and mineral content, alternatively categorized into three or four groups based on their rare-earth element content.

Slates
Def. a "fine-grained homogeneous [...] rock composed of clay or [...] ash which [...] cleaves easily into thin layers" is called a slate.

Stocks
Def. a "pipe (vertical cylinder of ore)" is called a stock.

Syenites
Def. an "igneous rock composed of feldspar and hornblende" is called a syenite.

On the left is a leucocratic variety of nepheline syenite from Sweden called särnaite.

Tonalites
Def. an "igneous, plutonic rock composed mainly of plagioclase" is called a tonalite.

Travertines
Def. "light, porous form of concretionary limestone (or calcite)" is called a travertine.

Troctolites
Def. an "ultramafic [...] rock, consisting primarily of olivine and calcic plagioclase" is called a troctolite.

Ultramafics
Def. "rocks that contain magnesium and iron and only a very small amount of silica" are called ultramafics.

Technology
"Three [Swarm] satellites of the European Space Agency (ESA) have measured the magnetic field of Earth more precisely than ever before."

"Shinkai 6500, a Japanese research submarine built in 1989 [an artist's impression is on the left], can work at depths down to 6,400 m."

"Scientists discovered the hot-springs ecosystems with the help of Alvin [imaged on the right, third one], the world's first deep-sea submersible. Constructed in the early 1960s for the U.S. Navy, Alvin is a three-person, self-propelling capsule-like submarine nearly eight meters long. In 1975, scientists of Project FAMOUS (French-American Mid-Ocean Undersea Study) used Alvin to dive on a segment of the Mid-Atlantic Ridge in an attempt to make the first direct observation of seafloor spreading. No hot springs were observed on this expedition; it was during the next Alvin expedition, the one in 1977 to the Galapagos Rift, that the hot springs and strange creatures were discovered. Since the advent of Alvin, other manned submersibles have been built and used successfully to explore the deep ocean floor. Alvin has an operational maximum depth of about 4,000 m, more than four times greater than that of the deepest diving military submarine."

"The Glomar Challenger [imaged on the third right] was the first research vessel specifically designed in the late 1960s for the purpose of drilling into and taking core samples from the deep ocean floor."

"In the years following World War II, continental oil reserves were being depleted rapidly and the search for offshore oil was on. To conduct offshore exploration, oil companies built ships equipped with a special drilling rig and the capacity to carry many kilometers of drill pipe. This basic idea later was adapted in constructing a research vessel, named the Glomar Challenger, designed specifically for marine geology studies, including the collection of drill-core samples from the deep ocean floor. In 1968, the vessel embarked on a year-long scientific expedition, criss-crossing the Mid-Atlantic Ridge between South America and Africa and drilling core samples at specific locations. When the ages of the samples were determined by paleontologic and isotopic dating studies, they provided the clinching evidence that proved the seafloor spreading hypothesis."

"The JOIDES Resolution [in the fourth image on the right] is the deep-sea drilling ship of the 1990s (JOIDES= Joint Oceanographic Institutions for Deep Earth Sampling). This ship, which carries more than 9,000 m of drill pipe, is capable of more precise positioning and deeper drilling than the Glomar Challenger."