The Life Cycle of Stone
The earth's crust is constantly evolving and the same mineral may melt deep inside the earth and then freeze when it reaches the surface (igneous rocks). In this process, it may change stones that come into contact with it (metamorphic rocks). Wind and weather will erode any mineral, but over millennia its particles may recombine until they form a new rock (sedimentary rocks). Thus, the same mineral, quartz, for example, may give rise to stones that are as different as granite and sandstone.
The following illustrations help you to understand these processes since they determine what sculptors and quarrymen call law, a concept that defines the order in which stone grows, which should not be contradicted when carving.
Schematic drawing showing the location of the different types of stone in an imaginary landscape. After a magma eruption, pressure and heat intensively matamorphose the closest sedimentary rocks, but have a lesser effect on rock that is farther away, sometimes making it difficult to distinguish bewteen marble and limestone, for example.
Alabaster and travertine are created by the precipitation of salts dissolved in water. This is similar to the way stalagmites are formed. The difference is that travertine solidifies only in sulfurous water.
Evolution of the same landscape after millions of years of erosion. Igneous rocks are more resistant and give rise, for example, to basaltic needles. The accumulation of sand or clay, rock waste, and organic detritus, particularly of the sea, will create new sandstone and limestone.
What happens most often is we find landscapes where the rocks do not appear in their original location. The emergence of new mountains and continental drift lead to pressures that move or fracture the rocks, particularly soft rocks, forming folds or faults.
(Text and Illustrations from 'Sculpture in Stone' by Josepmaria Teixido i Cami & Jacinto Chicharro Santamera, Barrons, 2001)
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