Aaron Zitner, age 11, of Pittsburgh, Pennsylvania, for his question:
How is marble formed?
Today is April Fool's Day, when schemes to fool friends are permitted and even expected. However, according to tradition, the jolly game ends at noon. So don't expect forgiveness for any merry nonsense that occurs after lunch. Meantime, there is no trickery in today's Andy question. The formation of marble is a long patient business, with no shortcuts or fooling around.
Marble is just about the loveliest mineral found in the earth's crust. It comes in a wide range of blended flowery colors that remind you of waxy white lilies, rose petals and smiling pansies, primroses and glossy black tulips.
Many of the earth's rocks were compressed from inorganic minerals that never were associated with living things. Marble is an organic mineral formed with the cooperation of the earth and the living world of nature. What's more, the patient recipe was performed in two distinct stages, which is why geologists rate it as a metamorphic or altered rock.
As a rule, a thick deposit of useable marble is found in a mountainous region. But the recipe started eons ago in some ancient sea that teemed with tiny shell builders. Through the ages, these living creatures extracted dissolved limy chemicals from the salty sea and used them to create their crusty shells. As countless generations lived and died, their shells sifted down and spread massive layers of calcium type minerals on the floor of the sea.
Eventually, the restless old earth set about remodeling this part of her crust. The salty sea receded and its old floor was lifted high and dry. Its massive carpet of limy shells became a massive layer of limestone rock. There is nothing special about this pasty pale chalky material, though we need it to manufacture iron and steel. However, here and there a limestone deposit was destined to become marble.
This reprocessing occurred when a deposit of limestone was involved in a mountain making project. Here, with stupendous drama, massive crustal layers were heaved and hoisted like piles of toy building blocks. Great slabs were bent, cracked and stacked in rocky sandwiches. During these super upheavals, shuddering earthquakes shook the ground and fiery volcanoes erupted rivers of lava.
Naturally, this mountainous activity created enormous heat and pressure in the crustal slabs. Our deposit of old limestone found itself in a sort of planetary pressure cooker. During the process, its mineral ingredients were changed, rearranged and completely remodeled. The chalky old limestone became a beauteous layer of waxy marble.
When the original limestone was clean and no impurities seeped into the later recipe, the finished marble is lily white. Specks of carbon and graphite added handsome streaks of charcoal black. In some cases, iron impurities seeped into the recipe tinting the marble with reds and petal pinks. Most leafy green marbles are colored by traces of chlorite. All true marbles are made of fine, compacted crystals. This mineral structure enables the stone to be cut in all directions and the surface can be polished to give a waxy luster.