Limestone

• 1. Introduction

  ›

  Drop a bead of dilute hydrochloric acid onto limestone and it fizzes, bubbling vigorously as it dissolves. That small chemical reaction is one of the field tests geologists use to identify a rock that makes up most of the world's carbonate sedimentary rock. Limestone is composed mostly of calcite and aragonite, two different crystal forms of calcium carbonate. It is the main source of lime, and it forms when these minerals precipitate out of water containing dissolved calcium. How does a rock so soft that you can scratch it with a coin end up holding pyramids, cathedrals, and a third of the world's petroleum? Why does it carry fossils in exquisite detail, and why does rainwater carve it into caves and gorges? The answers begin underwater, where most limestone is born.

• 2. Born From The Sea

  ›

  For the last 540 million years, biological processes have likely dominated the making of limestone. Living organisms build skeletons of aragonite or calcite and leave those structures behind when they die. The accumulation of corals and shells on the sea floor has been the engine of carbonate production through the Phanerozoic. Before that, in the Precambrian, inorganic processes were probably more important, working in an ocean more highly oversaturated in calcium carbonate than today's.

  Near-surface ocean water is oversaturated with calcium carbonate by a factor of more than six, yet it does not rapidly turn to stone. Dissolved magnesium ions interfere with the nucleation of calcite crystals, the necessary first step in precipitation. Naturally occurring organic phosphates may suppress the precipitation of aragonite. So the bulk of carbonate that does form is the result of biological activity, much of it on carbonate platforms.

  Warm, shallow water makes the best cradle. Continental shelves and platforms host most limestone, even though such environments form only about 5% of the ocean basins. These warm waters combine high organic productivity with increased saturation of calcium carbonate, the latter because warmer water holds less dissolved carbon dioxide. Limestone is produced almost entirely from sediment originating at or near where it is deposited, unlike clastic rock carried from afar.

• 3. Grains And Mud

  ›

  Most grains in limestone are skeletal fragments of marine organisms such as coral or foraminifera. Their composition records both the creature and its habitat. Low-magnesium calcite is typical of articulate brachiopods and coccoliths. High-magnesium calcite marks bottom-dwelling foraminifera, echinoderms, and coralline algae. Aragonite is typical of molluscs, calcareous green algae, corals, and tube worms. Coral grains gather in high-energy water full of currents and turbulence, while bryozoan grains prefer quiet water.

  Ooids are sand-sized grains under 2 millimeters across, built of layers of calcite or aragonite around a central quartz grain or carbonate fragment. They form in high-energy settings such as the Bahama platform, and limestone made mostly of them is called an oolite. Peloids are structureless grains of microcrystalline carbonate, many thought to be fecal pellets from marine organisms. Limeclasts are fragments of existing limestone, divided into intraclasts that form near their resting place and rarer extraclasts that arrive from outside.

  The grains sit embedded in a matrix of carbonate mud, typically the largest fraction of an ancient carbonate rock. Mud of crystals under 5 micrometers is called micrite, and in fresh form it is mostly small aragonite needles that convert to calcite within a few million years. Larger calcite crystals, from 20 to 100 micrometers, are called sparite, standing out as white or transparent crystals under a hand lens. Distinguishing sparite deposited as cement from sparite formed by recrystallization tells a geologist whether the water was high-energy enough to wash the mud away.

• 4. Reading The Stone

  ›

  Limestone is commonly white to gray, but the impurities tell stories. Stone unusually rich in organic matter can be almost black, while traces of iron or manganese push the color toward off-white, yellow, or red. Most limestone is otherwise chemically pure, with clastic sediments of fine quartz and clay making up less than 5% to 10%. Organic matter typically sits around 0.2% and rarely exceeds 1%.

  The rock is soft, with a Mohs hardness of 2 to 4, yet dense limestone can reach a crushing strength of up to 180 megapascals. By comparison, concrete typically manages about 40 megapascals. Density depends on porosity, which ranges from 0.1% in the densest limestone to 40% in chalk, giving densities from 1.5 to 2.7 grams per cubic centimeter.

  Two classification schemes carry the names of their creators. Robert L. Folk built a system around the detailed composition of grains, matrix, and cement, using two-part names so that a rock of ooids in a crystalline matrix becomes an oosparite. Robert J. Dunham published his system in 1962, focused on depositional fabric and the original porosity of the rock. Folk rewards a petrographic microscope, while Dunham works better on a hand sample because it rests on texture rather than grain content. A later revision by Wright in 1992 added some diagenetic patterns to the scheme.

• 5. Turning To Rock

  ›

  Diagenesis is the process by which loose sediment is compacted and turned into solid rock, and in carbonates it brings sweeping chemical change. Aragonite converts to low-magnesium calcite. Cementing happens fast, typically within less than a million years of deposition, and accelerates once the sea retreats and rainwater infiltrates the beds. Rainwater can soak more than 100 kilometers into sediments beneath the continental shelf, cementing thick deposits even before the sea withdraws.

  Deeper burial squeezes the rock. Pressure solution dissolves minerals at the contact points between grains and redeposits them in pore space, cutting porosity from an initial 40% to 80% down to less than 10%. This produces stylolites, irregular surfaces within the limestone where silica-rich sediment gathers. Below a depth of 1 kilometer, burial cementation finishes the job, though it leaves no stylolites behind.

  Magnesium-rich fluids can convert limestone into dolomite, a closely related rock, and the evidence includes sharp replacement boundaries that cut across bedding. Proposed mechanisms include evaporative reflux in hot environments and tidal pumping, where ordinary seawater is regularly flushed through the rock. Dorag dolomitization, once invoked for diluted seawater in deltas, has fallen out of favor, with a 2004 review describing it bluntly as a myth. Once dolomitization begins it proceeds rapidly, which is why carbonate rock tends to be almost all calcite or almost all dolomite, rarely a mix.

• 6. Builders Of Reefs

  ›

  Stromatolites, mound-shaped structures in ancient limestones, are interpreted as colonies of cyanobacteria that accumulated carbonate sediments, yet they grow rare in younger rock. Organisms make limestone both directly, as part of their skeletons, and indirectly, by removing carbon dioxide through photosynthesis and lowering the solubility of calcium carbonate. The creatures responsible for reefs have changed across geologic time.

  Archaeocyathids appeared in the early Cambrian, gave way to sponges by the late Cambrian, and were followed by stromatoporoids, corals, algae, bryozoa, and the bivalve molluscs called rudists. Organic reefs were likely most extensive in the middle Devonian, covering an estimated 5 million square kilometers, roughly ten times the extent of modern reefs. Those Devonian reefs, built largely by stromatoporoids and tabulate corals, were devastated by the late Devonian extinction.

  Before the early Ordovician, micritic mud mounds were the dominant reef type in both deep and shallow water, and they are likely microbial in origin. Modern examples reach several hundred meters thick and a kilometer across, with steep slopes near 50 degrees, sometimes stabilized by Thalassia grass or mangroves. Limestone also records the work of borers: the cyanobacterium Hyella balani, the green alga Eugamantia sacculata, and the fungus Ostracolaba implexa all drill through it.

• 7. Caves And Coastlines

  ›

  Rainwater is slightly acidic, and limestone is slightly soluble in it, so exposures of the rock erode into karst landscapes of pavements, pot holes, cenotes, caves, and gorges. Surface water drains downward through joints in the limestone, leaving karst regions with few visible ponds and streams. Over thousands or millions of years, water and organic acid widen these cracks, dissolving the calcium carbonate and carrying it away in solution. Most cave systems run through limestone bedrock.

  Limestone is less resistant to erosion than most igneous rock but more resistant than most other sedimentary rock, so it tends to form hills and downland among clays. Bands of it emerge in spectacular outcrops and islands, including the Rock of Gibraltar, the Burren in County Clare, Ireland, Malham Cove in North Yorkshire, the Great Orme in Wales, the Niagara Escarpment, Notch Peak in Utah, Ha Long Bay in Vietnam, and the hills around the Lijiang River and Guilin in China. The Florida Keys are built mainly from oolitic limestone in the Lower Keys and coral reef skeletons in the Upper Keys.

  Flat expanses of limestone with thin soil, called alvars, host unique habitats, and the largest in Europe is the Stora Alvaret on the island of Oland, Sweden. Coastal limestone is often eroded by organisms that bore into it, a process called bioerosion that is most common in the tropics. In northwestern Europe, huge quarries such as those of Mount Saint Peter on the Belgium and Netherlands border extend for more than a hundred kilometers.

• 8. A Material In Demand

  ›

  The Great Pyramid and its complex at Giza, Egypt, were made of limestone, an early sign of how readily the stone could be cut into blocks or carved into fine detail. By 200 to 100 BCE, in the Late Preclassic period, the Maya civilization of Ancient Mexico carved refined limestone panels and ceiling lintels covered with political and social stories that carried the king's messages to his people. The stone is long-lasting and stands up well to exposure, which is why so many limestone ruins survive. Its density of about 2.6 makes it heavy and impractical for tall buildings.

  Limestone was most popular in the late 19th and early 20th centuries, when railway stations, banks, and other structures rose from it. Indiana limestone, much of it from the Bloomington area, became a source of high-quality quarried stone in the United States, while many famous London buildings use Portland limestone. So many buildings in Kingston, Ontario, were and still are made of it that the city is nicknamed the Limestone City. In 19th-century Odesa, Ukraine, limestone houses left behind the mines now known as the Odesa Catacombs.

  Beyond architecture, limestone is the raw material for lime, used to treat soils, purify water, and smelt copper. It serves as aggregate for roads, a reagent in flue-gas desulfurization, a white pigment and filler in toothpaste and paint, rock dust to suppress methane explosions in coal mines, and a calcium source added to bread and cereals. Many porous limestone formations hold petroleum, with about 20% of North American hydrocarbon reserves found in carbonate rock and roughly a third of all petroleum reserves worldwide held in carbonate reservoirs, especially in the Middle East. The same chemical activity makes limestone a host for metal ores, such as the lead-zinc deposits of Missouri and the Northwest Territories.