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Cemeteries not only provide a peaceful place to contemplate and commemorate the dead. They provide refuges for insects, wildlife, lichens, and plants, and they are open air museums 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 found on Earth is more than 4 billion years old , formed during the Hadean Eon —Earth's earliest geological period named after Hades, the Greek god of the dead and the king of the underworld. Gravestones can be made from plutonic rocks (after Pluto , the earlier name for Hades), like gabbro and granite ; metamorphic rocks, like slate and marble ; and sedimentary rocks, like 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. But even rocks age and eventually erode into dust. The minerals composing gravestones crystallize at high temperatures and pressures inside Earth.
Under atmospheric conditions, most minerals are unstable and very susceptible to physical and chemical weathering. Calcite and dolomite in sedimentary rocks, like limestone and dolostone, and metamorphic rocks, like marble, are soluble in water and tend to erode quickly. Feldspar and mica , the main minerals in plutonic rocks like granite and metamorphic rocks like gneiss and schist , react with water and oxygen, decaying to clay minerals.
Plutonic and metamorphic rocks are made up of relatively large crystals of different minerals with different physical properties. If the sun heats up a gravestone, the mineral grains will expand, causing strain to build up. During the night, the mineral grains will contract again with falling temperatures.
Repeated day by day, this cycle will break apart even the hardest gravestone. Air pollution can speed up weathering and decay . Sulfuric components or nitrogen from car exhaust fumes and industry form acid, dissolving mineral grains.
A long-lasting gravestone should be made from 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 that was originally pure quartz sand, 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 traces of iron-oxides in the rock. 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.
How fast a body decays within a grave is controlled by temperature, humidity, oxygen levels, water availability, the acidity of the surrounding substrate, the chemical composition of the corpse, and the presence or lack of certain microorganisms. The decay of soft tissue in an underground grave can take up to 15 to 20 years. The skeleton or bones can survive for some centuries or even longer.
Geological circumstances can slow down the decay or even stop it. Clay deposits, forming an impermeable layer, result in waterlogged soil. The water acts as a barrier for scavengers or decomposing organisms, and it is not rare to discover centuries-old wet mummies in some graveyards.
Maybe this observation contributed to the belief in haunted cemeteries. In sandy soils, the lack of water is a limiting factor for microbial activity. Maybe after discovering the natural mummification effects of dry desert sand, ancient Egyptians developed a religion involving the artificial mummification of their deceased relatives.
Geological factors still play a role in finding a suitable site for a cemetery. As the bodies should decay as fast as possible, the underground should be partially permeable to air and water, like in sand and chalk soils. The soil layer should be thick enough to filter contaminants formed by the decaying body.
So the next time you visit a graveyard, remember that you are dust, and to dust you will return, becoming part of geological history..
