An eighteenth-century experiment turned chalk into fake marble inside a cannon barrel
To test whether pressure transforms rock, James Hall sealed chalk inside an old cannon barrel and cooked it in a foundry furnace. Instead of crumbly quicklime he got something very like marble. He had recreated metamorphism, the process that remakes buried rock into new kinds without ever fully melting it.
Metamorphic rock begins as some other rock, called the protolith, which may be igneous, sedimentary or already metamorphic. Heated beyond roughly 150 to 200 degrees Celsius and often squeezed at 100 megapascals or more, it stays essentially solid while its crystals reorganise into new textures and minerals. Deep burial, continental collisions and contact with intruding magma can all do the job. Such rocks make up much of the crust and about 12 percent of the land surface.
Scottish naturalist James Hutton, often called the father of modern geology, first argued in 1795 that some Highland rock beds had been sedimentary until great heat altered them. He suspected pressure mattered too, which his friend Hall set out to prove. French geologists later added metasomatism, in which hot fluids flowing through pores dissolve some minerals, carry substances away and deposit new ones.
Minerals can change even when the chemistry stays fixed, because each is stable only within certain conditions. Kyanite, andalusite and sillimanite share the identical formula, yet at normal atmospheric pressure kyanite becomes andalusite near 190 degrees and andalusite becomes sillimanite near 800. Geologists use such index minerals as built-in thermometers and pressure gauges, reading the depths a rock once reached from what it contains now.
Texture tells a story as well. Squeezing along one axis rotates flaky minerals such as mica into parallel bands, called foliation from the Latin for leaves. Heat a mudstone progressively and it passes through slate, phyllite and schist to coarse gneiss. Slate's cleavage lets it split into thin sheets for roofing, and schist's planes of weakness trouble engineers. Marble, lacking platy minerals, is usually unbanded, which is why sculptors prize it: limestone's tiny calcite grains grow into larger interlocking crystals, reducing surface energy as they coarsen.
Source: Metamorphic rock
