All of the rocks that form on the Earth are either sedimentary, igneous, or metamorphic.
Sedimentary rocks form where sand, mud, or pebbles collect in layers, usually over long periods of time; ; where water evaporates and leaves its dissolved minerals behind; or where calcium carbonate collects in the ocean (coral reefs, the ocean floor).
| Sandstone at the Boston Athenaeum |  |
Sandstone forms when sand is buried deep enough to be cemented together under the pressure of overlying material. Sandstone deposits can form from desert dunes, beaches, riverbeds, and continental shelf deposits. It can be fine or coarse-grained. Many sandstones vary internally in texture and grain size. Most sandstone that is used for building stone is relatively fine-gained and evenly textured. Limestone is a sedimentary rock that consists mostly or entirely of the minerals calcium carbonate (CaCO3) or magnesium carbonate (MgCO3). Most of the limestone on the planet forms in the ocean. Tiny, floating plants and animals called plankton take dissolved calcium, carbon, and oxygen from seawater and use it to make shells. When the plankton die, their shells drift down to the seafloor and collect in a sediment called ooze. Over time, the ooze hardens to limestone. Shells, bones, and corals can also form part or all of a limestone. Limestones tend to form in warmer, shallower water and to dissolve in colder, deeper water. Limestone can be very hard and take carving and polishing well, but it is vulnerable to acid precipitation. Calcium carbonate dissolves easily in acid. In fact, if you drip a little dilute acid on a piece of limestone, it will fizz. Travertine is a type of limestone that forms at hot springs. It can be very hard and is a common decorative or cladding stone. In volcanically active areas, hot , acidic water can come up from underground and pass through limestone deposits, dissolving them. When the hot water reaches the surface, it evaporates and the calcium carbonate is left behind. Over time, this can accumulate into massive beds of travertine. |
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| Sandstone at Stanford University |  |
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| The sandstone at a San Francisco mansion is a long way from home |  |
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| A bank tower in Providence, Rhode Island and the Colosseum in Rome have something surprising in common | |||
Igneous rocks form when molten rock, called magma, solidifies within the earth's crust or erupts to the surface from a volcano.
| Granite
at the Shell Building, San Francisco |
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Pumice is a light, foamy rock that forms when a volcano throws blobs of hot lava into the air. The rock is full of hot gases that escape quickly into the air, leaving lots of bubbles in the lava, which then cools very fast. Pumice is often so full of air that it can float in water. It is too light and brittle for most construction, but it is often used as a decorative trim on the outside of concrete buildings. Granite is one of the most common building stones. It is strong and solid and takes a polish. It forms when high-silica magma cools underground. The longer the rock takes to cool, the larger the crystals in the granite are. The most common minerals in granite are quartz, feldspar, and mica. Whether a granite is grey or pink depends on the chemical composition of its feldspar. Granite tends to occur in large bodies called batholiths that cool underground. The White Mountains in New Hampshire and California's Sierra Nevada are immense granite batholiths that have been exposed by erosion. |
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| Granite
at the Pacific Coast Stock Exchange, San Francisco |
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| The Providence Arcade has pillars carved from granite that formed more than 500 million years ago |  |
Metamorphic rocks form when rocks are heated, squeezed, and chemically altered. This usually happens when they are buried deep in the Earth's crust.
| Green marble at a San Francisco bank building |  |
Marble forms when limestone is buried and heated. It is hard and shiny, and comes in many colors, depending on its chemical impurities. Marble is used extensively as trim and in statues. Unfortunately, since it is made of calcium carbonate, it is vulnerable to acid precipitation. Air pollution is slowly dissolving the marble fountains and monuments of Rome. Marble gravestones become unreadable long before slate or granite stones do. In the building stone industry, some hard, high-quality limestones are called marble, even if they haven't actually metamorphosed to marble. Slate is a dark, hard rock that is used for roof tiles, stone walls, and flagstones. It forms when shale, a sedimentary rock, is heated and compressed. Shale forms from fine sediments, such as silt and clay, which settle in quiet environments such as swamps, deep lakes, and lagoons. Slate is harder than shale, but retains its flat, layered structure. |
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| The lions at the New York Public Library started life in the ocean | |||