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Diamond

This article is about the mineral. For the gemstone, see Diamond (gemstone). For other uses, including the shape ? Diamond (disambiguation).
Diamond
The slightly misshapen octahedral shape of this rough diamond crystal in matrix is typical of the mineral. Its lustrous faces also indicate that this crystal is from a primary deposit.
General
Category Native minerals
Formula
(repeating unit) C
Strunz classification 1. CB.
Dana classification 1.
Crystal system Cubic
Crystal class Hexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space group F d3m (No.
Structure
Jmol (3D) Interactive image
Identification
Formula mass 12.
Color Typically yellow, brown, or gray to colorless. Less often blue, green, black, translucent white, pink, violet, orange, purple, and red.
Crystal habit Octahedral
Twinning Spinel law common (yielding “macle”)
Cleavage 111 (perfect in four directions)
Fracture Irregular/Uneven
Mohs scale hardness 10 (defining mineral)
Luster Adamantine
Streak Colorless
Diaphaneity Transparent to subtransparent to translucent
Specific gravity 3.
Density 3.
Polish luster Adamantine
Optical properties Isotropic
Refractive index 2.
Birefringence None
Pleochroism None
Dispersion 0.
Melting point Pressure dependent
References [1] [2]

Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. At room temperature and pressure, another solid form of carbon known as graphite is the chemically stable form of carbon, but diamond almost never converts to it. Diamond has the highest hardness and thermal conductivity of any natural material, properties that are utilized in major industrial applications such as cutting and polishing tools. They are also the reason that diamond anvil cells can subject materials to pressures found deep in the Earth.

Because the arrangement of atoms in diamond is extremely rigid, few types of impurity can contaminate it (two exceptions being boron and nitrogen). Small numbers of defects or impurities (about one per million of lattice atoms) color diamond blue (boron), yellow (nitrogen), brown (defects), green (radiation exposure), purple, pink, orange or red. Diamond also has relatively high optical dispersion (ability to disperse light of different colors).

Most natural diamonds have ages between 1 billion and 3. Most were formed at depths between 150 and 250 kilometres (93 and 155 mi) in the Earth’s mantle, although a few have come from as deep as 800 kilometres (500 mi). Under high pressure and temperature, carbon-containing fluids dissolved various minerals and replaced them with diamonds. Much more recently (tens to hundreds of million years ago), they were carried to the surface in volcanic eruptions and deposited in igneous rocks known as kimberlites and lamproites.

Synthetic diamonds can be grown from high-purity carbon under high pressures and temperatures or from hydrocarbon gas by chemical vapor deposition (CVD). Imitation diamonds can also be made out of materials such as cubic zirconia and silicon carbide. Natural, synthetic and imitation diamonds are most commonly distinguished using optical techniques or thermal conductivity measurements.

Contents

1 Material properties

1.1 Thermodynamics

1.2 Crystal structure

1.3 Crystal habit

1.4 Mechanical properties

1.4.1 Hardness

1.4.2 Toughness

1.4.3 Yield strength

1.4.4 Elasticity and tensile strength

1.5 Electrical conductivity

1.6 Surface property

1.7 Chemical stability

1.8 Color

1.9 Identification

2 Geology

2.1 Surface distribution

2.2 Exploration

2.3 Ages

2.4 Origin in mantle

2.5 Carbon sources

2.6 Formation and growth

2.7 Transport to the surface

2.8 In space

3 Industry

3.1 Gem-grade diamonds

3.1.1 Cutting

3.1.2 Marketing

3.2 Industrial-grade diamonds

3.3 Mining

3.4 Environmental effects

3.4.1 Political issues

4 Synthetics, simulants, and enhancements

4.1 Synthetics

4.2 Simulants

4.3 Enhancements

4.4 Identification

5 Theft

6 Etymology, earliest use and composition discovery

7 See also

8 References

9 Books

10 External links

Material properties

Diamond is a solid form of pure carbon with its atoms arranged in a crystal. Solid carbon comes in different forms known as allotropes depending on the type of chemical bond. The two most common allotropes of pure carbon are diamond and graphite. In graphite the bonds are sp 2 orbital hybrids and the atoms form in planes with each bound to three nearest neighbors 120 degrees apart. In diamond they are sp 3 and the atoms form tetrahedra with each bound to four nearest neighbors. Tetrahedra are rigid, the bonds are strong, and of all known substances diamond has the greatest number of atoms per unit volume, which is why it is both the hardest and the least compressible. It also has a high density, ranging from 3150 to 3530 kilograms per cubic metre (over three times the density of water) in natural diamonds and 3520 kg/m 3 in pure diamond. In graphite, the bonds between nearest neighbors are even stronger but the bonds between planes are weak, so the planes can easily slip past each other. Thus, graphite is much softer than diamond. However, the stronger bonds make graphite less flammable.

Diamonds have been adapted for many uses because of the material’s exceptional physical characteristics. Of all known substances, it is the hardest and least compressible. It has the highest thermal conductivity and the highest sound velocity. It has low adhesion and friction, and its coefficient of thermal expansion is extremely low. Its optical transparency extends from the far infrared to the deep ultraviolet and it has high optical dispersion. It also has high electrical resistance. It is chemically inert, not reacting with most corrosive substances, and has excellent biological compatibility.

Thermodynamics

Theoretically predicted phase diagram of carbon

The equilibrium pressure and temperature conditions for a transition between graphite and diamond are well established theoretically and experimentally. The pressure changes linearly between 1. GPa at 0 K and 12 GPa at 5000 K (the diamond/graphite/liquid triple point). However, the phases have a wide region about this line where they can coexist. At normal temperature and pressure, 20 �C (293 K) and 1 standard atmosphere (0. MPa), the stable phase of carbon is graphite, but diamond is metastable and its rate of conversion to graphite is negligible. However, at temperatures above about 4500 K, diamond rapidly converts to graphite. Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at 2000 K, a pressure of 35 GPa is needed.

Above the triple point, the melting point of diamond increases slowly with increasing pressure

GPa, it decreases. At high pressures, silicon and germanium have a BC8 body-centered cubic crystal structure, and a similar structure is predicted for carbon at high pressures. At 0 K, the transition is predicted to occur at 1100 GPa.

Research results published in an article in the scientific journal Nature in 2010 suggest that at ultrahigh pressures and temperatures (about 10 million atmospheres or 1 TPa and 50,000 �C) diamond behaves as a metallic fluid. The extreme conditions required for this to occur are present in the gas giants of Neptune and Uranus. Both planets are made up of approximately 10 percent carbon and could hypothetically contain oceans of liquid carbon. Since large quantities of metallic fluid can affect the magnetic field, this could serve as an explanation as to why the geographic and magnetic poles of the two planets are unaligned.

Crystal structure

See also: Crystallographic defects in diamond

Diamond unit cell, showing the tetrahedral structure.

The most common crystal structure of diamond is called diamond cubic. It is formed of unit cells (see the figure) stacked together. Although there are 18 atoms in the figure, each corner atom is shared by eight unit cells and each atom in the center of a face is shared by two, so there are a total of eight atoms per unit cell. Each side of the unit cell is 3.

A diamond cubic lattice can be thought of as two interpenetrating face-centered cubic lattices with one displaced by 1/4 of the diagonal along a cubic cell, or as one lattice with two atoms associated with each lattice point. Viewed from a <1 1 1> crystallographic direction, it is formed of layers stacked in a repeating ABCABC . Diamonds can also form an ABAB .

Crystal habit

One face of an uncut octahedral diamond, showing trigons (of positive and negative relief) formed by natural chemical etching

Diamonds occur most often as euhedral or rounded octahedra and twinned octahedra known as macles. As diamond’s crystal structure has a cubic arrangement of the atoms, they have many facets that belong to a cube, octahedron, rhombicosidodecahedron, tetrakis hexahedron or disdyakis dodecahedron. The crystals can have rounded off and unexpressive edges and can be elongated. Diamonds (especially those with rounded crystal faces) are commonly found coated in nyf, an opaque gum-like skin.

Some diamonds have opaque fibers. They are referred to as opaque if the fibers grow from a clear substrate or fibrous if they occupy the entire crystal. Their colors range from yellow to green or gray, sometimes with cloud-like white to gray impurities. Their most common shape is cuboidal, but they can also form octahedra, dodecahedra, macles or combined shapes. The structure is the result of numerous impurities with sizes between 1 and 5 microns. These diamonds probably formed in kimberlite magma and sampled the volatiles.

Diamonds can also form polycrystalline aggregates. There have been attempts to classify them into groups with names such as boart, ballas, stewartite and framesite, but there is no widely accepted set of criteria. Carbonado, a type in which the diamond grains were sintered (fused without melting by the application of heat and pressure), is black in color and tougher than single crystal diamond. It has never been observed in a volcanic rock. There are many theories for its origin, including formation in a star, but no consensus.

Mechanical properties

Hardness

Diamond is the hardest known natural material on both the Vickers scale and the Mohs scale. Diamond’s great hardness relative to other materials has been known since antiquity, and is the source of its name. This does not mean that it is infinitely hard, indestructible, or unscratchable. Indeed, diamonds can be scratched by other diamonds [24] and worn down over time even by softer materials, such as vinyl records.

Diamond hardness depends on its purity, crystalline perfection and orientation: hardness is higher for flawless, pure crystals oriented to the <111> direction (along the longest diagonal of the cubic diamond lattice). Therefore, whereas it might be possible to scratch some diamonds with other materials, such as boron nitride, the hardest diamonds can only be scratched by other diamonds and nanocrystalline diamond aggregates.

The hardness of diamond contributes to its suitability as a gemstone. Because it can only be scratched by other diamonds, it maintains its polish extremely well. Unlike many other gems, it is well-suited to daily wear because of its resistance to scratching?

The hardest natural diamonds mostly originate from the Copeton and Bingara fields located in the New England area in New South Wales, Australia. These diamonds are generally small, perfect to semiperfect octahedra, and are used to polish other diamonds. Their hardness is associated with the crystal growth form, which is single-stage crystal growth. Most other diamonds show more evidence of multiple growth stages, which produce inclusions, flaws, and defect planes in the crystal lattice, all of which affect their hardness. It is possible to treat regular diamonds under a combination of high pressure and high temperature to produce diamonds that are harder than the diamonds used in hardness gauges.

Toughness

Somewhat related to hardness is another mechanical property toughness, which is a material’s ability to resist breakage from forceful impact. The toughness of natural diamond has been measured as 7. MPa �m 1/2. This value is good compared to other ceramic materials, but poor compared to most engineering materials such as engineering alloys, which typically exhibit toughnesses over 100 MPa�m 1/2. As with any material, the macroscopic geometry of a diamond contributes to its resistance to breakage. Diamond has a cleavage plane and is therefore more fragile in some orientations than others. Diamond cutters use this attribute to cleave some stones, prior to faceting. Impact toughness” is one of the main indexes to measure the quality of synthetic industrial diamonds.

Yield strength

Diamond has compressive yield strength of 130? GPa. This exceptionally high value, along with the hardness and transparency of diamond, are the reasons that diamond anvil cells are the main tool for high pressure experiments. These anvils have reached pressures of 600 GPa. Much higher pressures may be possible with nanocrystalline diamonds.

Elasticity and tensile strength

Usually, attempting to deform bulk diamond crystal by tension or bending results in brittle fracture. However, when single crystalline diamond is in the form of nanometer-sized wires or needles (~100? GPa, very close to the theoretical limit for this material.

Electrical conductivity

Other specialized applications also exist or are being developed, including use as semiconductors: some blue diamonds are natural semiconductors, in contrast to most diamonds, which are excellent electrical insulators. The conductivity and blue color originate from boron impurity. Boron substitutes for carbon atoms in the diamond lattice, donating a hole into the valence band.

Substantial conductivity is commonly observed in nominally undoped diamond grown by chemical vapor deposition. This conductivity is associated with hydrogen-related species adsorbed at the surface, and it can be removed by annealing or other surface treatments.

A 2020 paper reported that extremely thin needles of diamond can be made to vary their electronic bandgap from the normal 5. V to near zero by selective mechanical deformation.

Surface property

Diamonds are naturally lipophilic and hydrophobic, which means the diamonds’ surface cannot be wet by water, but can be easily wet and stuck by oil. This property can be utilized to extract diamonds using oil when making synthetic diamonds. However, when diamond surfaces are chemically modified with certain ions, they are expected to become so hydrophilic that they can stabilize multiple layers of water ice at human body temperature.

The surface of diamonds is partially oxidized. The oxidized surface can be reduced by heat treatment under hydrogen flow. That is to say, this heat treatment partially removes oxygen-containing functional groups. But diamonds (sp 3 C) are unstable against high temperature (above about 400 �C (752 �F)) under atmospheric pressure. The structure gradually changes into sp 2 C above this temperature. Thus, diamonds should be reduced under this temperature.

Chemical stability

At room temperature, diamonds do not react with any chemical reagents including strong acids and bases.

In an atmosphere of pure oxygen, diamond has an ignition point that ranges from 690 �C (1,274 �F) to 840 �C (1,540 �F)
It increases in temperature from red to white heat and burns with a pale blue flame, and continues to burn after the source of heat is removed. By contrast, in air the combustion will cease as soon as the heat is removed because the oxygen is diluted with nitrogen.
Heat generated from cutting a diamond will not start a fire, [44] and neither will a cigarette lighter, [45] but house fires and blow torches are hot enough. Jewelers must be careful when molding the metal in a diamond ring.

Diamond powder of an appropriate grain size (around 50 microns) burns with a shower of sparks after ignition from a flame. Consequently, pyrotechnic compositions based on synthetic diamond powder can be prepared. The resulting sparks are of the usual red-orange color, comparable to charcoal, but show a very linear trajectory which is explained by their high density. Diamond also reacts with fluorine gas above about 700 �C (1,292 �F).

Color

Brown diamonds at the National Museum of Natural History in Washington, D.

Diamond has a wide bandgap of 5. V corresponding to the deep ultraviolet wavelength of 225 nanometers. This means that pure diamond should transmit visible light and appear as a clear colorless crystal. Colors in diamond originate from lattice defects and impurities. The diamond crystal lattice is exceptionally strong, and only atoms of nitrogen, boron and hydrogen can be introduced into diamond during the growth at significant concentrations (up to atomic percents).
Virtually any element can be introduced to diamond by ion implantation.

Nitrogen is by far the most common impurity found in gem diamonds and is responsible for the yellow and brown color in diamonds. Boron is responsible for the blue color. Color in diamond has two additional sources: irradiation (usually by alpha particles), that causes the color in green diamonds, and plastic deformation of the diamond crystal lattice. Plastic deformation is the cause of color in some brown [51] and perhaps pink and red diamonds. In order of increasing rarity, yellow diamond is followed by brown, colorless, then by blue, green, black, pink, orange, purple, and red. Black”, or Carbonado, diamonds are not truly black, but rather contain numerous dark inclusions that give the gems their dark appearance. Colored diamonds contain impurities or structural defects that cause the coloration, while pure or nearly pure diamonds are transparent and colorless. Most diamond impurities replace a carbon atom in the crystal lattice, known as a carbon flaw. The most common impurity, nitrogen, causes a slight to intense yellow coloration depending upon the type and concentration of nitrogen present. The Gemological Institute of America (GIA) classifies low saturation yellow and brown diamonds as diamonds in the normal color range, and applies a grading scale from “D” (colorless) to “Z” (light yellow). Diamonds of a different color, such as blue, are called fancy colored diamonds and fall under a different grading scale.

In 2008, the Wittelsbach Diamond, a 35. King of Spain, fetched over US million at a Christie’s auction. In May 2009, a 7. Swiss francs (6. US. That record was, however, beaten the same year: a 5-carat (1. Hong Kong on December 1, 2009.

Identification

Diamonds can be identified by their high thermal conductivity (900? W�m ? K ? Their high refractive index is also indicative, but other materials have similar refractivity. Diamonds cut glass, but this does not positively identify a diamond because other materials, such as quartz, also lie above glass on the Mohs scale and can also cut it. Diamonds can scratch other diamonds, but this can result in damage to one or both stones. Hardness tests are infrequently used in practical gemology because of their potentially destructive nature.
Diamonds also possess an extremely high refractive index and fairly high dispersion. Taken together, these factors affect the overall appearance of a polished diamond and most diamantaires still rely upon skilled use of a loupe (magnifying glass) to identify diamonds “by eye”.

Geology

Diamonds are extremely rare, with concentrations of at most parts per billion in source rock. Before the 20th century, most diamonds were found in alluvial deposits. Loose diamonds are also found along existing and ancient shorelines, where they tend to accumulate because of their size and density. Rarely, they have been found in glacial till (notably in Wisconsin and Indiana), but these deposits are not of commercial quality. These types of deposit were derived from localized igneous intrusions through weathering and transport by wind or water.

Most diamonds come from the Earth’s mantle, and most of this section discusses those diamonds. However, there are other sources. Some blocks of the crust, or terranes, have been buried deep enough as the crust thickened so they experienced ultra-high-pressure metamorphism. These have evenly distributed microdiamonds that show no sign of transport by magma. In addition, when meteorites strike the ground, the shock wave can produce high enough temperatures and pressures for microdiamonds and nanodiamonds to form. Impact-type microdiamonds can be used as an indicator of ancient impact craters. Popigai crater in Russia may have the world’s largest diamond deposit, estimated at trillions of carats, and formed by an asteroid impact.

A common misconception is that diamonds are formed from highly compressed coal. Coal is formed from buried prehistoric plants, and most diamonds that have been dated are far older than the first land plants. It is possible that diamonds can form from coal in subduction zones, but diamonds formed in this way are rare, and the carbon source is more likely carbonate rocks and organic carbon in sediments, rather than coal.

Surface distribution

Geologic provinces of the world. The pink and orange areas are shields and platforms, which together constitute cratons.

Diamonds are far from evenly distributed over the Earth. A rule of thumb known as Clifford’s rule states that they are almost always found in kimberlites on the oldest part of cratons, the stable cores of continents with typical ages of 2. However, there are exceptions. The Argyle diamond mine in Australia, the largest producer of diamonds by weight in the world, is located in a mobile belt, also known as an orogenic belt, [67] a weaker zone surrounding the central craton that has undergone compressional tectonics. Instead of kimberlite, the host rock is lamproite. Lamproites with diamonds that are not economically viable are also found in the United States, India and Australia. In addition, diamonds in the Wawa belt of the Superior province in Canada and microdiamonds in the island arc of Japan are found in a type of rock called lamprophyre.

Kimberlites can be found in narrow (1 to 4 meters) dikes and sills, and in pipes with diameters that range from about 75 m to 1. Fresh rock is dark bluish green to greenish gray, but after exposure rapidly turns brown and crumbles. It is hybrid rock with a chaotic mixture of small minerals and rock fragments (clasts) up to the size of watermelons. They are a mixture of xenocrysts and xenoliths (minerals and rocks carried up from the lower crust and mantle), pieces of surface rock, altered minerals such as serpentine, and new minerals that crystallized during the eruption. The texture varies with depth. The composition forms a continuum with carbonatites, but the latter have too much oxygen for carbon to exist in a pure form.
3). [61]

All three of the diamond-bearing rocks (kimberlite, lamproite and lamprophyre) lack certain minerals (melilite and kalsilite) that are incompatible with diamond formation. In kimberlite, olivine is large and conspicuous, while lamproite has Ti- phlogopite and lamprophyre has biotite and amphibole. They are all derived from magma types that erupt rapidly from small amounts of melt, are rich in volatiles and magnesium oxide, and are less oxidizing than more common mantle melts such as basalt. These characteristics allow the melts to carry diamonds to the surface before they dissolve.

Exploration

Kimberlite pipes can be difficult to find. They weather quickly (within a few years after exposure) and tend to have lower topographic relief than surrounding rock. If they are visible in outcrops, the diamonds are never visible because they are so rare. In any case, kimberlites are often covered with vegetation, sediments, soils or lakes. In modern searches, geophysical methods such as aeromagnetic surveys, electrical resistivity and gravimetry, help identify promising regions to explore. This is aided by isotopic dating and modeling of the geological history. Then surveyors must go to the area and collect samples, looking for kimberlite fragments or indicator minerals. The latter have compositions that reflect the conditions where diamonds form, such as extreme melt depletion or high pressures in eclogites.
a better approach is geothermobarometry, where the compositions of minerals are analyzed as if they were in equilibrium with mantle minerals. [61]

Finding kimberlites requires persistence, and only a small fraction contain diamonds that are commercially viable. The only major discoveries since about 1980 have been in Canada. Since existing mines have lifetimes of as little as 25 years, there could be a shortage of new diamonds in the future.

Ages

Diamonds are dated by analyzing inclusions using the decay of radioactive isotopes. Depending on the elemental abundances, one can look at the decay of rubidium to strontium, samarium to neodymium, uranium to lead, argon-40 to argon-39, or rhenium to osmium. Those found in kimberlites have ages ranging from 1 to 3. The kimberlites themselves are much younger. Most of them have ages between tens of millions and 300 million years old, although there are some older exceptions (Argyle, Premier and Wawa). Thus, the kimberlites formed independently of the diamonds and served only to transport them to the surface. Kimberlites are also much younger than the cratons they have erupted through. The reason for the lack of older kimberlites is unknown, but it suggests there was some change in mantle chemistry or tectonics. No kimberlite has erupted in human history.

Origin in mantle

Eclogite with centimeter-size garnet crystals.

Most gem-quality diamonds come from depths of 150? Such depths occur below cratons in mantle keels, the thickest part of the lithosphere. These regions have high enough pressure and temperature to allow diamonds to form and they are not convecting, so diamonds can be stored for billions of years until a kimberlite eruption samples them.

Host rocks in a mantle keel include harzburgite and lherzolite, two type of peridotite.
However, diamonds in peridotite rarely survive the trip to the surface. Another common source that does keep diamonds intact is eclogite, a metamorphic rock that typically forms from basalt as an oceanic plate plunges into the mantle at a subduction zone.

A smaller fraction of diamonds (about 150 have been studied) come from depths of 330? They formed in eclogite but are distinguished from diamonds of shallower origin by inclusions of majorite (a form of garnet with excess silicon). A similar proportion of diamonds comes from the lower mantle at depths between 660 and 800 km.

Diamond is thermodynamically stable at high pressures and temperatures, with the phase transition from graphite occurring at greater temperatures as the pressure increases. Thus, underneath continents it becomes stable at temperatures of 950 degrees Celsius and pressures of 4. In subduction zones, which are colder, it becomes stable at temperatures of 800 �C and pressures of 3. At depths greater than 240 km, iron-nickel metal phases are present and carbon is likely to be either dissolved in them or in the form of carbides. Thus, the deeper origin of some diamonds may reflect unusual growth environments.

In 2018 the first known natural samples of a phase of ice called Ice VII were found as inclusions in diamond samples. The inclusions formed at depths between 400 and 800 km, straddling the upper and lower mantle, and provide evidence for water-rich fluid at these depths.

Carbon sources

The mantle has roughly one billion gigatonnes of carbon (for comparison, the atmosphere-ocean system has about 44,000 gigatonnes). The carbon has two stable isotopes, 12 C and 13 C, in a ratio of approximately 99:1 by mass. This ratio has a wide range in meteorites, which implies that it also varied a lot in the early Earth. It can also be altered by surface processes like photosynthesis. The fraction is generally compared to a standard sample using a ratio ? C expressed in parts per thousand. Common rocks from the mantle such as basalts, carbonatites and kimberlites have ratios between ? On the surface, organic sediments have an average of ?

Populations of diamonds from different sources have distributions of ? C that vary markedly.
This variability implies that they are not formed from carbon that is primordial (having resided in the mantle since the Earth formed). Instead, they are the result of tectonic processes, although (given the ages of diamonds) not necessarily the same tectonic processes that act in the present.

Formation and growth

Age zones in a diamond.

Diamonds in the mantle form through a metasomatic process where a C-O-H-N-S fluid or melt dissolves minerals in a rock and replaces them with new minerals. The vague term C-O-H-N-S is commonly used because the exact composition is not known. Diamonds form from this fluid either by reduction of oxidized carbon (e. CO 2 or CO 3) or oxidation of a reduced phase such as methane.

Using probes such as polarized light, photoluminescence and cathodoluminescence, a series of growth zones can be identified in diamonds. The characteristic pattern in diamonds from the lithosphere involves a nearly concentric series of zones with very thin oscillations in luminescence and alternating episodes where the carbon is resorbed by the fluid and then grown again. Diamonds from below the lithosphere have a more irregular, almost polycrystalline texture, reflecting the higher temperatures and pressures as well as the transport of the diamonds by convection.

Transport to the surface

Diagram of a volcanic pipe

Geological evidence supports a model