Cemeteries not only provide a peaceful place to contemplate and commemorate the dead. They provide refuges for insects, wildlife, lichens and flowering plants, and are wonderful repositories for the study of local history and art. They are also great places for studying geology.
A human life is short if compared to the age of rocks. The oldest rock on earth is more than 4 billion years old, formed during the Hadean Eon (named after Hades, the Greek god of the dead and the king of the underworld) of Earth’s geological history. Maybe that’s one reason why rocks are so popular for memorials and gravestones.
Gravestones can be made from plutonic rocks, like gabbro and granite, metamorphic rocks, like slate and marble, and more rarely of sedimentary rocks, likes sandstone and limestone. The choice depends on aesthetic values and practical use. Granite can be of various colors, dotted with the black mica nests. Limestone is easy to work and sculpt and can display interesting bands or layers of colors.
Rocks seem to last forever, carrying with them the name and memory of the deceased. However, even rocks age and eventually erode into dust.
The minerals composing such rocks crystallize at high temperatures and pressures inside Earth. Under atmospheric conditions, such minerals are not stable and are very susceptible to physical and chemical weathering. Calcite and dolomite in carbonate rocks, limestone, dolostone and marble, are soluble in water and tend to erode quickly. Minerals like feldspar and mica, making up plutonic rocks like granite and metamorphic rocks like gneiss and schist, tend to react with water and oxygen, decaying to clay minerals. Carbonate minerals and feldspars are also vulnerable to air pollution. Sulfuric components or nitrogen from car exhaust fumes and industry will increase weathering and decay. Plutonic and metamorphic rocks are made of relatively large crystals of different minerals with different physical properties, and every mineral expands with the temperature at different rates. If the sun heats a gravestone, the mineral grains will expand, causing strain to build up. At night, with falling temperatures, the mineral grains will contract again. Repeated day by day, the thermal expansion and contraction will break apart the entire gravestone.
A long-lasting gravestone should be made of a monomineralic rock, a rock that is composed of only one mineral, so all mineral grains expand and contract at the same rate, minimizing the effects of physical weathering. Quartzite, a metamorphic rock which was originally pure quartz sandstone, is a good choice. Quartz is a chemically very stable mineral with an extremely low coefficient of thermal expansion. Pure quartzite is usually white to grey, though quartzites often occur in various shades of pink and red due to varying amounts of hematite. Other colors, such as yellow, green, blue and orange, are due to other minerals.
Graveyard geology doesn’t only include gravestones. The types of rocks found in the underground of a graveyard play an important role in the decay of corpses. Curiously enough, the name sarcophagus for box-like funeral receptacles for corpses derives from the Greek meaning “place where flesh is eaten.” In ancient times, sarcophagi were made of a particular type of sandstone. Being sealed airtight and reacting with fluids from the corpse, the sandstone formed a caustic environment inside the sarcophagus, where the decay of flesh was accelerated.