Igneous rock

• 1. Introduction

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  Igneous rock forms the skeleton of worlds. Beneath the continents, at depths stretching to about 35 kilometres, a crystalline basement of igneous and metamorphic rock underpins virtually everything we stand on. And those two rock types together account for 90 to 95 percent of the top 16 kilometres of the Earth's crust by volume. Yet most of us live our entire lives without ever thinking about what lies below.

  The name itself is a clue. Igneous comes from the Latin igni-, meaning fire. These are rocks born in heat so extreme that solid stone melts, flows, and is remade. The process begins deep in the Earth's mantle or crust, where existing rock partially melts to produce magma. That magma rises, cools, and solidifies, either underground or at the surface. The result can be anything from the pale, coarse granite beneath a mountain range to the dark, glassy basalt of a mid-ocean ridge, or the steep-sided rhyolite flows that pour from some of the world's most violent volcanoes.

  How does a rock melt without anyone adding heat? What does the texture of a grain of crystal reveal about where a rock has been? And why did geologists in 1902 propose throwing out every existing classification system and starting over from scratch? Those are the threads this documentary follows.

• 2. Where Magma Is Born

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  At a depth of about 100 kilometres, peridotite, the dominant rock of the Earth's mantle, begins to melt near 800 degrees Celsius when water is present. Without water, the same rock needs temperatures near or above 1,500 degrees Celsius before it yields. That single comparison captures how profoundly chemistry shapes the birth of magma.

  Three mechanisms can trigger melting. A rise in temperature is the most familiar, but it is not always the most important. Decompression, a decrease in pressure, drives the creation of oceanic crust at mid-ocean ridges: as solid mantle rises during convection, it cools at a rate of only about 0.3 degrees Celsius per kilometre, far slower than the 3 to 4 degrees per kilometre increase in solidus temperature as rock ascends. If the rock rises far enough, it crosses its own melting point and begins to liquefy.

  The addition of water is the change in composition most responsible for magma generation at subduction zones. As oceanic lithosphere sinks beneath a continent or island arc, water is driven out of the descending slab and into the mantle above it. The resulting hydrous magmas build island arcs along the Pacific Ring of Fire and produce rocks of the calc-alkaline series, a cornerstone of the continental crust.

  Carbon dioxide plays a narrower but distinctive role. In a narrow pressure interval corresponding to roughly 70 kilometres depth, adding carbon dioxide lowers the peridotite solidus temperature by about 200 degrees Celsius. At depths approaching 200 kilometres, the effect is even greater: temperatures of initial melting drop by 450 to 600 degrees Celsius compared with the same rock without carbon dioxide. Magmas of nephelinite, carbonatite, and kimberlite are among those that can emerge from such conditions.

  Meteor impacts also deserve a mention. They are a minor cause of melting today, but during the accretion of the early Earth, large impacts were common enough that the outer several hundred kilometres may have been an ocean of magma.

• 3. Underground and Overground

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  Not all magma reaches daylight. A large portion solidifies within the crust itself, forming what geologists call intrusive rocks. Country rock, the pre-existing material surrounding an intrusion, acts as an excellent thermal insulator, so the magma cools slowly. Slow cooling produces coarse-grained, or phaneritic, rock, where individual mineral grains are large enough to be identified with the naked eye. Granite, gabbro, and diorite are common products of this process, and the central cores of major mountain ranges are built from them. When erosion strips away overlying material, these cores, called batholiths, can cover huge areas of the Earth's surface.

  When magma breaks through to the surface through fissures or volcanic eruptions, the story changes entirely. Cooling is rapid, grains have little time to grow, and the resulting rock is fine-grained, or aphanitic, or even glassy. Basalt is the most abundant extrusive igneous rock. It forms lava flows, lava sheets, and lava plateaus across the ocean floor and on land. Some basalt flows cool into long, regular polygonal columns. The Giant's Causeway in Antrim, Northern Ireland stands as one well-known example.

  Viscosity governs how lava behaves on the surface. High-temperature basaltic magma behaves like thick oil and, as it cools, like treacle. Rhyolite, a felsic magma, can be up to 10,000 times as viscous as basalt and is typically erupted at lower temperature. That resistance to flow means rhyolitic volcanoes tend to erupt explosively, and rhyolitic flows are usually short and steep-sided.

  There is a middle category as well. Hypabyssal, or subvolcanic, rocks form near the surface rather than deep within the crust. They are finer-grained than plutonic rocks but often resemble volcanic rock, and they commonly appear as dikes, sills, laccoliths, lopoliths, or phacoliths. They are less common than either their deep or surface counterparts.

• 4. Crystal Clocks and Chemical Fingerprints

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  Igneous rocks are more than structural material. They are records. The minerals locked inside them carry information about conditions that existed deep in the Earth's mantle or lower crust, far beyond any drill or direct observation.

  Radiometric dating can extract absolute ages from igneous rocks. Those ages can then be compared with adjacent geological strata, allowing geologists to calibrate the geological time scale with precision that no other method provides. The characteristic features of an igneous rock also tend to reflect the tectonic setting where it formed, making it possible to reconstruct ancient plate configurations by studying rocks in the field.

  Some igneous rocks host economically important mineral deposits. Tungsten, tin, and uranium are commonly associated with granites and diorites. Ores of chromium and platinum tend to be associated with gabbros. These associations are not coincidental; they reflect the chemistry of fractional crystallization, the process by which minerals crystallize from a cooling melt at different temperatures. As early crystals settle out or otherwise separate from the remaining liquid, the residual melt changes composition. A magma that begins with a gabbroic composition can, through this process, evolve a residual melt of granitic composition. Gabbro may have a liquidus temperature near 1,200 degrees Celsius; the derivative granite-composition melt may crystallize as low as about 700 degrees Celsius.

  Incompatible elements, those that are excluded from the crystal structures of the common early-forming minerals, become concentrated in the last dregs of magma during fractional crystallization. Those late-stage melts can crystallize into pegmatite, a rock type prized precisely because it is enriched in elements found nowhere else in such concentrations.

  A database of over 230,000 rock analyses, accessible through a site sponsored by the U.S. National Science Foundation and known as EarthChem, captures the full chemical diversity that emerges from these processes.

• 5. The Battle Over Classification

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  Basalt as a specific rock name dates to Georgius Agricola, who used it in 1546 in his work De Natura Fossilium. Granite goes back at least to the 1640s, derived from French granit or Italian granito, both meaning simply granulate rock. The term rhyolite was introduced in 1860 by German geologist and traveler Ferdinand von Richthofen. By the early twentieth century, the naming of new rock types had peaked after accelerating sharply through the nineteenth century.

  In 1902, the American petrologists Charles Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry Stephens Washington put forward a proposal that shook the geological community: discard every existing classification of igneous rocks and replace them with a single quantitative system grounded in chemical analysis. They argued that geological age, mode of occurrence, and mineralogical constitution, the accepted bases of classification, were vague and often unscientific. Chemical composition, they insisted, was the most fundamental property of an igneous rock and should take precedence. The proposal created a sensation. It was also criticized sharply for being impractical in the field, and the scheme was abandoned by the 1960s. The concept of normative mineralogy that it introduced, however, endured and still shapes how geologists think about rock chemistry.

  By 1958 the proliferation had reached a point where at least 1,637 rock type names were in active use across twelve separate classification schemes. That year, Albert Streckeisen published a review article that eventually led to the formation of the IUGG Subcommission of the Systematics of Igneous Rocks. By 1989, the subcommission had agreed on a single classification system. A further revision in 2005 reduced the list of recommended rock names to 316.

  Among the schemes considered along the way was that of M.A. Peacock, whose division of igneous rocks into the alkalic, alkali-calcic, calc-alkali, and calcic series remains partially alive in the widely used Irvine-Barager classification, together with the tholeiitic series defined by W.Q. Kennedy.

• 6. Reading Texture and Composition

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  Grain size is the first thing a geologist notices about an igneous rock, and it carries a direct message about cooling history. Phaneritic rocks cooled slowly underground; aphanitic rocks cooled quickly at the surface. A rock with large crystals embedded in a fine-grained matrix has its own name: porphyry. The large crystals, called phenocrysts, grew during an early, slower phase of cooling. The fine-grained material surrounding them, called groundmass, formed later when cooling accelerated. Two distinct cooling phases produce porphyritic texture.

  Composition divides igneous rocks into broad families. Felsic rocks, dominated by quartz, plagioclase, alkali feldspar, and muscovite, contain more than 63 percent silica by weight and tend to be light in colour. Granite and rhyolite belong here. Intermediate rocks, such as diorite and andesite, carry silica contents between 52 and 63 percent and are typically darker. Mafic rocks, including basalt and gabbro, fall between 45 and 52 percent silica, are dark coloured, and are denser than felsic rocks. Ultramafic rocks drop below 45 percent silica and consist of more than 90 percent iron- and magnesium-rich minerals such as olivine and pyroxene.

  The silica content of a volcanic rock, combined with its total alkali metal oxide content, places it on the TAS diagram, a widely used tool for classifying rock when its mineral grains are too fine to identify directly. Alkali content is the second most important chemical variable after silica. Where the TAS diagram leaves ambiguity, additional criteria such as the ratio of potassium to sodium, or normative mineralogy, resolve the classification. Some volcanic rocks classified this way include basanite, distinguished from tephrite by its high normative olivine content.

  The tholeiitic series, the calc-alkaline series, and the alkaline series represent the three major magma lineages. Tholeiitic rocks are common at mid-ocean ridges, back-arc basins, and oceanic hotspot islands. The calc-alkaline and alkaline series are characteristic of mature subduction zones. In the Japanese island arc system, the rock types shift from tholeiite to calc-alkaline to alkaline with increasing distance from the trench, a spatial record of the depth and age of subduction preserved in the landscape itself.