Igneous rock
In 1546, the Swiss physician and mineralogist Georgius Agricola published a work titled De Natura Fossilium. This text contained one of the earliest descriptions of basalt as a specific type of lava-derived rock. The word igneous itself comes from Latin roots meaning fire and composed. It describes rocks formed through the cooling and solidification of magma or lava. Magma originates from partial melts within a planet's mantle or crust. Three primary processes trigger this melting: an increase in temperature, a decrease in pressure, or a change in composition. Water plays a critical role here. At a depth of about 100 kilometres, peridotite begins to melt near 800 degrees Celsius when excess water is present. Without that water, the same rock requires temperatures near or above 1,500 degrees Celsius to melt. Decompression melting occurs when rock rises during convection in the solid mantle. As it expands adiabatically, it cools only slightly, yet the drop in pressure allows it to cross its solidus temperature line. This process creates ocean crust at mid-ocean ridges and drives volcanism in intraplate regions like Europe, Africa, and the Pacific sea floor.
Intrusive igneous rocks form when magma cools slowly within the Earth's crust. These bodies are known as intrusions and are surrounded by pre-existing country rock. That surrounding rock acts as an excellent thermal insulator, allowing mineral grains to grow large enough to be identified with the naked eye. Common intrusive examples include granite, gabbro, and diorite. Batholiths represent the central cores of major mountain ranges and can occupy huge areas once exposed by erosion. Rocks forming near the surface are termed subvolcanic or hypabyssal. They cool faster than plutonic rocks and often resemble volcanic varieties. Extrusive igneous rock forms on the Earth's surface through fissures or eruptions. Basalt is the most common extrusive type and creates lava flows, sheets, and plateaus. Some basalt solidifies into long polygonal columns, such as those found at the Giant's Causeway in Antrim, Northern Ireland. Volcanoes erupting into air are called subaerial, while those occurring under the ocean are submarine. Black smokers and mid-ocean ridge basalt serve as examples of this underwater activity. The volume of extrusive rock erupted annually varies significantly depending on tectonic settings.
In 1902, American petrologists Charles Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry Stephens Washington proposed a radical shift in how geologists classify rocks. They argued that all existing classifications should be discarded for a quantitative system based on chemical analysis. This new scheme created a sensation but was eventually abandoned by the 1960s due to lack of field utility. By 1958, there were some 12 separate classification schemes and at least 1637 rock type names in use. That year, Albert Streckeisen wrote a review article leading to the formation of the IUGG Subcommission of the Systematics of Igneous Rocks. By 1989, a single system had been agreed upon and revised again in 2005. The number of recommended rock names was reduced to 316. The International Union of Geological Sciences now recommends classifying igneous rocks by mineral composition whenever possible. For coarse-grained intrusive rocks, this is straightforward. Fine-grained volcanic rock may require examination of thin sections under a microscope. Glassy volcanic rock might make mineralogical classification impossible, forcing chemists to rely on chemical data instead.
Silicon dioxide serves as the single most important component in grouping these rocks. Felsic rocks contain the highest silica content, often exceeding 63 percent. They are predominantly composed of quartz and feldspar and usually appear light colored with relatively low density. Intermediate rocks hold between 52 and 63 percent silica and are typically darker than felsic varieties. Mafic rocks have lower silica levels ranging from 45 to 52 percent. These dark-colored rocks possess higher density and consist mostly of pyroxenes, olivines, and calcic plagioclase. Ultramafic rocks contain less than 45 percent silica and more than 90 percent mafic minerals. The percentage of alkali metal oxides follows silica in importance for classifying volcanic rock. Scientists use the TAS diagram to place volcanic rock based on silica and alkali percentages. Rocks in certain fields like trachyandesite are further classified by the ratio of potassium to sodium. Some mafic fields are subdivided by normative mineralogy calculated from chemical composition. Basanite is distinguished from tephrite by having a high normative olivine content. Magmas divide into three series: tholeiitic, calc-alkaline, and alkaline.
The term volcanic rock derives from Vulcan, the Roman god of fire. Plutonic rock comes from Pluto, the Roman god of the underworld. Granite traces its name back to at least the 1640s via French granit or Italian granito meaning simply granulate rock. Ferdinand von Richthofen introduced the term rhyolite in 1860. Naming new rock types accelerated during the 19th century and peaked in the early 20th century. Early classification relied heavily on geological age and occurrence rather than chemistry. Modern terminology reflects both physical properties and historical context. Words like igneous mean composed of fire while others describe specific textures or origins. The evolution of these terms mirrors the scientific understanding of how rocks form and behave over time.
Tholeiitic magma series rocks appear at mid-ocean ridges, back-arc basins, and oceanic islands formed by hotspots. They also occur within continental large igneous provinces. Calc-alkaline and alkaline series are seen in mature subduction zones related to deeper magma sources. Island arcs typically feature andesite and basaltic andesite as the most abundant volcanic rocks. The Japanese island arc system demonstrates a clear progression from tholeiite to calc-alkaline to alkaline with increasing distance from the trench. Decompression melting creates ocean crust at mid-ocean ridges and causes volcanism in intraplate regions. Hydrous magmas produced during subduction build up island arcs such as those in the Pacific Ring of Fire. Carbon dioxide can lower solidus temperatures significantly at depths greater than about 70 kilometres. Magmas generated following carbon dioxide influx include nephelinite, carbonatite, and kimberlite. These processes map the distribution of igneous rocks across plate boundaries, hotspots, and oceanic ridges.
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Common questions
When did Georgius Agricola publish the earliest description of basalt?
Georgius Agricola published a work titled De Natura Fossilium in 1546 containing one of the earliest descriptions of basalt as a specific type of lava-derived rock.
What temperature does peridotite melt at when excess water is present at 100 kilometres depth?
Peridotite begins to melt near 800 degrees Celsius at a depth of about 100 kilometres when excess water is present without that water requiring temperatures near or above 1,500 degrees Celsius to melt.
How many recommended rock names were reduced to by 1989 after the IUGG Subcommission agreement?
By 1989 a single system had been agreed upon and revised again in 2005 reducing the number of recommended rock names to 316 for igneous rocks.
What percentage of silica do felsic rocks contain compared to mafic rocks?
Felsic rocks contain the highest silica content often exceeding 63 percent while mafic rocks have lower silica levels ranging from 45 to 52 percent.
Who introduced the term rhyolite and in what year did Ferdinand von Richthofen introduce it?
Ferdinand von Richthofen introduced the term rhyolite in 1860 during an acceleration of naming new rock types that peaked in the early 20th century.