igneous structures

Igneous rocks comprise 95% of the crust and resulted from the cooling ± crystallization of rock that melted when Earth's radioactivity heated rocks buried deep within the Earth. Intrusive, plutonic rocks comprise the majority of igneous rocks, while extrusive, volcanic rocks are more accessible rocks that initially cooled at the surface. Volcanoes and earthquakes occur at tectonic subduction zones around the "Ring of Fire" or sit above mantle plumes.

Magma is molten rock formed and cooled at depth. Under the pressure of overburden, magma can rise toward the surface through cracks in the overlying rock strata. However, magma may never reach the surface, and if magma cools and solidifies in rock cracks within these strata, then it forms dikes, sills, diapirs, laccoliths, or huge solidified magma chambers called batholiths, any of which may later become exposed at the surface by erosion. Classification as a dike, sill, igneous diapir, laccolith (2, 3, 4, 5, laccolith (l) and set of sills (r), Mt Hillers), bysmalith, or batholith (1, 2, 3, 4, 5, 6, 7) depends upon orientation of the igneous intrusion within the country rock (cold, intruded rock) and size of the intrusion (diagram, diagram 2).

Dikes are tabular or sheet-like bodies of magma that cut vertically or almost vertically through and across strata (discordant plutonic bodies). Hundreds of dikes can invade the cone and inner core of a volcano. If the dike cooled very slowly at great depth, the large crystals of pegmatite dikes have time to form. Sills are tabular slabs or concordant intrusive sheets of igneous rock that have intruded horizontally or nearly horizontally. Laccoliths are moderately large concordant intrusions that cause uplift and folding of the preexisting rocks above the intrusion. Bysmoliths are more or less vertical and cylindrical bodies that crosscut (discordant) adjacent sediments. Batholiths are complex intrusive bodies, that are typically so large that their bases are rarely exposed. The exposed surface area is more than 100 square kilometers. Sometimes batholiths comprise several smaller intrusions. Batholith are usually of granitic composition with minor intermediate varieties. Stocks are smaller structures (less than 100 sq.m) that have probably been fed by deeper level batholiths. Stocks may have been feeders for volcanic eruptions, but because considerable erosion is necessary to expose a stock or batholith, the associated volcanic rocks rarely remain. Xenolithic phenomena produce fragments of metamorphically altered country rock (xenolith) that fell into the melt and became enveloped by the intruding magmatic rock. Xenoliths can be quite large and xenoliths can occur in clusters within the plutonic rock or demonstrate dikelet intrusions by the magma. (excellent, though large bandwidth, illustrations of a xenolith in the Halifax Pluton.)

When melted rock flows at the surface it is called lava – Hawaii's and Iceland's basalt lavas fountain or flow freely, while other lavas are sticky and explosive, producing deadly pyroclastic flows (Mount St. Helens, Vesuvius, Pinatubo). See volcanism or volcanoes for more detail.

Large igneous provinces (LIPs) are igneous extrusive structures created by flood basalt vulcanism that emplaced at least 1 million cubic kilometers of lava over periods of a million years. In the distant past, these flows of basalt lava were much more extensive than current flows and not only formed extensive geological provinces, but have been implicated in extinction events.  Flood Basalts and Stratigraphic Boundaries

Geologists debate the postulated origin of LIPs as resulting from newly formed mantle plumes or from continental rifts, though it is possible that both mechanisms have operated. Well known flood basalts include the Columbia River Basalts formed a mere 16 million years ago and are now exposed in the Snake River Gorge in Idaho, the Siberian Flood Basalts that are believed responsible for the Earth's greatest extinction event 249 ± 1 years ago at the Permian-Triassic boundary, and the Deccan Flood Basalts in India, which formed 66 million years ago, give or take a million.)

Interestingly, the Deccan Flood Basalts were formed at roughly the time of the dinosaur extinction at the boundary between the Cretaceous and Tertiary, though this extinction is widely believed (by scientists not creationists!) to have resulted from the Chicxulub meteor impact.

◙ subduction zone magmas ◙

volcanoes

Molten material, steam, and gases rise to the surface at volcanoes. The activity of volcanoes depends upon their source of heat, which is dependent upon their location and is ultimately derived from Earth's radioactivity, and upon the nature of their lava.

(left - USGS schematic of a composite volcano - click to see larger image in vulcanism item on Terms) [gallery of lava flows]

Volcanoes are submarine or terrestrial and are located at:
● divergent plate boundaries – spreading centers where ocean is spreading, like the volcanoes of Iceland atop the mid-Atlantic Ridge, or where continents are spreading as in Africa's volcanic Rift Valley
● convergent plate boundaries – subduction zone island arc volcanoes and explosive stratovolcanoes around the Ring of Fire
● hotspots or mantle plumes – intraplate volcanoes like the Hawaiian island shield volcanoes

Volcanoes release/extrude
● gases – carbon dioxide, sulfur and chlorine compounds, minor amounts of carbon monoxide, fluorine and boron compounds, and ammonia
● steam
● ash and pyroclastic flows (ignimbrites), bomb, lapilli, pumice, reticulite, obsidian, scoria
● molten lava – felsic dacite and rhyolite, intermediate andesite, mafic basalt, and ultramafic magma (molten rock)
● unusual materials like the warm black lava carbonates (natrocarbonatite) of the intracontinental volcano Oldoinyo Lengai in Tanzania, which are white when cooled and solidified (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24)

Volcanoes are classified by lava composition, and/or by shape, or by type of eruption:
shape
_ ● composite or stratovolcanoes
_ ● supervolcanoes and calderas
_ ● cinder cones
_ ● shield volcanoes
_ ● lava domes or volcanic domes
___ ● maar
other
_ ● calderas within shield volcanoes
_ ● fissures
_ ● lava lake
_ ● lava tube and lava cave
_ ● hornito – a small rootless spatter cone that forms on the surface of a basaltic lava flow (usually pahoehoe).[Mauna Ulu hornito, Petunia Skylight hornito, another hornito above lava tube, and view through a puka]
_ ● spatter cone – spatter, spatter and cinder cone, spatter cone and rampart, spatter photo
_ ● mud volcano
_ ● submarine volcanoes [diagram, black smoker, hydrothermal vent, limu, pillow lava, Nikko submarine volcano]
_ ● subglacial volcanoes
_ ● vents

◙ subduction zone magmas ◙

type of eruption VEI
Table  Volcanoes  Flood Basalts and Stratigraphic Boundaries
● low gases, low silica – shield volcanoes like Kilauea
● low gases, high silica – lava domes such as have formed in the crater Mt. St. Helens since the explosive eruption
● high gases, low silica – cinder cone like Paricutin, which explodes with fountains of fluid lava
● high gases, high silica – composite or stratovolcanoes, like Mt. St. Helens before the explosive eruption

_ ● Strombolian eruptions produce intermittent explosion or fountaining of volcanic gases and clumps of basaltic lava, which becomes rounded in flight, from a single vent or crater. Volcanoes that often exhibit strombolian activity also include Etna (Italy), Pacaya (Guatemala), and Erebus (Antarctica). [Stromboli erupting, Stromboli volcano, Stromboli gallery]
_ ● Plinian eruptions produce tall columns of ash and tephra, eject large amounts of pumice, and are often characterized by powerful, continuous gas-blast eruptions. [eruption cloud]
_ ● Vesuvian
_ ● Peléan or Nuée Ardente eruptions produce pyroclastic flows, which are extremely hot, ground-hugging avalanches of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour or more. [Mayon Volcano, 2, Philippines]
_ ● Hawaiian or Effusive eruption
_ ● Vulcanian eruptions are explosive eruptions in which ejected lava fragments are too viscous to take on a rounded shape during their flight through the air (unlike strombolian fragments). This type of eruption is named for Vulcano, one of the Aeolian Islands north of Sicily. Other volcanoes that have recently shown vulcanian behavior include Sakurajima in Japan (ongoing) and Irazu in Costa Rica (1965). [Tavurvur Volcano in Rabaul Caldera, Papua New Guinea.]
_ ● Phreatic (ultravulcanian) eruptions occur when rising magma comes into contact with ground or surface water, causing near-instantaneous evaporation of the water and resulting in an explosion of steam, water, ash, rock, and volcanic bombs. [Mt. St. Helens]

◙ subduction zone magmas ◙

'Normal plumes look like smooth-domed mushroom clouds. But this one, photographed in Ecuador in 2002, has unusual scalloped edges. Volcanic clouds can collapse back to Earth in searing, high-speed events called pyroclastic flows. Unlike with normal plumes, the ash from the scalloped cloud must have been unusually heavy, causing it to collapse more rapidly.' [NG news]

Video The Ring of Fire.

NG interactive : gallery of images : Alaska's Augustine Volcano : Time Lapse Movies :
> Volcanoes > Bromo Merapi Etna Kilauea, Hawaii Lokon Piton de la Fournaise (La Réunion) Semeru Stromboli Welirang (East-java) Lava flows Strombolian eruptions Lava lakes lava entering sea, close-up of lava entering sea

basalt

Basalt is a hard gray or black, mafic igneous volcanic rock that is usually fine-grained due to rapid cooling of lava, though it contain larger crystals in a fine matrix (porphyritic), be vesicular, or be a frothy scoria.

Basalt magmas form by decompression melting of peridotite in the mantle. The crustal portions of oceanic tectonic plates comprised predominantly basalt, derived from upwelling peridotite in the mantle below ocean ridges. The basalt shield volcanoes of the Hawaiian island chain sit above a mantle plume, or 'hot spot'. (left - click to enlarge - aa flows over ropey pahoehoe in Hawaii - image courtesy of USGS.)

Basalt is TAS classified according to the relationships between the combined alkali content and the silica content. Basalt typically containts a preponderance of calcic plagioclase feldspar and pyroxene; olivine can also be a significant constituent. Accessory minerals include iron oxides and iron-titanium oxides, providing basalt with a paleomagnetic signature.

Phaneritic, shallow intrusive igneous rocks with a basaltic composition are generally referred to as dolerite (also called diabase) or gabbro.

(image left - click to enlarge - courtesy USGS - top-down: basaltic lava; lava field; flow-lines in basalt formation; close-up of vesicular basalt with olivine crystals; surface of basalt hand specimen; basalt columns.)

[images - roll-over link for preview (where available); large images of hand-speciments (well worth a visit) show only as a corner on preview : water-sculpted basalt at Fossil Falls in Yosemite : Basalt Fall unterhalb des Hengifoss, basalt columns, Dverghamrar basaltic columns in Iceland, 2 : cliff of basalt columns : Columbia River basalts, Catherine Creek arch in Miocene columnar basalts : flowing curves of basalt entablature in Yellowstone : basalt columns Armenia : basalt field : basalt and sandstone : 3.7 Ga moon-rock basalt : hand-specimen : hand-specimen vesicular basalt, vesicular basalt with olivine phenocrysts, 2 : hand-specimen diabase : hand-specimen diabase porphyry : hand-specimen diorite : hand-specimen gabbro : hand-specimen scoria : thin-section basalt, 2, 3; thin-sections moon basalts ]

igneous rocks

Igneous rocks result when molten rock cools (± crystallization) from plutonic (intruded) magma or from volcanic (extruded) lava.

Left - click to enlarge image - texture and mineral composition of common igneous rocks:

Increasing to the left: SiO2 content, viscosity; Increasing to the right: darkness, mafic : felsic composition, (Fe, Mg, Ca) : (K, Na) ratio, temperature of melting:

◘ a/p - aphanitic or porphyritic texture, derived from extruded magma (lava),
◘ 1. rhyolite ◘ 3. dacite ◘ 5. andesite ◘ 7. basalt
phaneritic texture, emplaced as magma (plutonic)
◘ 2. granite ◘ 4. granodiorite ◘ 6. diorite ◘ 8. gabbro to peridotite
0-100: percentage mineral content:
◊ a. quartz ◊ b. K-feldspar ◊ c. Na-feldspar ◊ pl. plagioclase to Ca-rich plagioclase ◊ d. muscovite
◊ e. biotite ◊ f. amphiboles ◊ g. pyroxenes ◊ h. olivine

Igneous rocks, predominantly plutonic, comprise about 95% of the Earth's crust, though their burial by sedimentary rocks and association with metamorphic rocks disguises their true extent.
The crystalline basement rock (shield) at the core of most continents is ancient, having arisen predominantly during a period from 3.0 to 2.5 billion years ago, which was the period of maximum continent formation. Most of the earliest rocks have been greatly altered through regional metamorphic processes, but later rocks (3.2-2.5 Ga) are mostly pillow-basalts that formed beneath the vast oceans. Archean sedimentary rocks are mostly coarse and poorly sorted sandstones and conglomerates.

| 0 Guide-Glossary

lava

The silica content of magma, along with amount of dissolved gas, affects the nature of volcanic eruptions as wells as the amount and composition of lava extruded during an eruption.

Volcanic rocks ◙ andesites ◙ basalts ◙ dacites ◙ rhyolites

Lava composition

_ ● felsic > 63% silica = viscous
___ ● dacitic
___ ● rhyolitic
– Rhyolite is named from the Greek for 'steam', and rhyolitic magma often erupts explosively because its high silica content results in extremely high viscosity, which hinders degassing. Effusive eruptions of rhyolite often produce obsidian, which is bubble-free and black. When bubbles form, they can cause the magma to explode, fragmenting the rock into pumice and tiny particles of volcanic ash. Some of the United States' largest and most active calderas formed during eruption of rhyolitic magmas (for example, Yellowstone in Wyoming, Long Valley in California and Valles in New Mexico). Rhyolite is the volcanic equivalent of plutonic granite.
– Even though dacite contains less silica than rhyolite, dacite can be even more viscous, and so just as dangerous as rhyolites. Dacite was erupted from Mount St. Helens 1980-86, Mount Pinatubo in 1991, and Mount Unzen 1991-1996.

_ ● intermediate 52-63% silica
___ ● andesitic
– Andesite magma commonly erupts from stratovolcanoes as thick lava flows, and can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Andesite, which is common in the Andes mountains, was the main rock type erupted during the great Krakatau eruption of 1883.

_ ● mafic 45 - 52%
___ ● basaltic
– Basalt's low silica content gives it a low viscosity (resistance to flow), so basaltic lava can flow quickly and can easily move >20 km from a vent. The low viscosity of basaltic magma usually allows volcanic gases to escape without generating enormous eruption columns. However, basaltic lava fountains and fissure eruptions still form explosive fountains hundreds of meters tall. Shield volcanoes, such as those in the Hawaiian islands (movies), are composed almost entirely of basalt, which is the commonest rock type in the Earth's crust; huge, ancient outpourings of lava called 'flood basalts' make up large igneous provinces on many continents; and, most of the ocean floor is basalt that has been extruded at spreading mid-ocean ridges. aa, aa active photo, hornito, lava tube, lava lake, pahoehoe, pahoehoe photo, pahoehoe active photo, Pele's hair, Pele's tears, tumulus

_ ● ultramafic ≤ 45%
___ ● komatiites

mafic

Mafic (magnesium-ferric) minerals and rocks are silicate minerals, magmas, volcanic, and intrusive igneous rocks with relatively high concentrations of heavier elements.

The term sima designates the lowest layer of the Earth's crust and the ocean floors (basal crust, basal layer, basalt layer). Sima comprises magnesium rich silicon minerals and when it rises to the surface as lava sima forms mafic rocks (usually basalts) or rocks with mafic minerals. The densest sima forms ultramafic rocks.

Constituent elements include Mg, Fe, Ca, and Na. Mafic minerals are usually dark in color, with a specific gravity greater than 3 (2800 to 3300 kg/m^3). Common rock-forming mafic minerals include amphibolte, biotite, olivine, pyroxene, and other micas, augite and the calcium-rich plagioclase feldspars. Common mafic rocks include basalt and gabbro.

Because of its lower silica content, molten mafic lava has a lower viscosity than felsic lava. Mafic volcanoes are less explosive than felsic lava eruptions because water and other volatiles more easily and gradually escape from mafic lava. Most mafic lava volcanoes are oceanic volcanoes, like Hawaii.

| 0 Guide-Glossary

magma

Magma is molten rock, which forms igneous rock when it crystallizes on cooling as igneous intrusions or extruded lava.

Most magma comprises solutions of silicates melted at temperatures from 700-1600 °C depending upon the environment in which the parent rock melted. Unusual black lava carbonates (natrocarbonatite) of the intracontinental volcano Oldoinyo Lengai are molten at temperatures as low as 600 °C.

The melting of rock is determined by temperature, pressure, and composition and occurs in association with mantle plumes or tectonic processes. The composition of magma can alter after melting of the parent rock by processed that include contamination, fractional crystallization, and mixing of magma with other molten rock.

Rocks melt at a range of temperatures, depending upon pressure and the presence of water and gases. Greater temperatures are required to melt a given dry rock at greater pressures, whereas wet rocks initially melt at decreasing temperatures with increasing pressure and then transition to requiring greater temperatures with further increase in pressure. (above left -dry rock with partial melts of liquid and crystals; above right - wet rock with partial melts comprising liquid, crystals, and vapor).

Burial of rock exposes the minerals to heating along the geothermal gradient, which is elevated by convection within the asthenosphere, bringing the rock to temperatures high enough for partial melting.

The geothermal gradient is defined as the rate of change of temperature (ΔT) with depth (ΔZ), in the Earth. At depths down to about 60 m, temperature is constant at about 11°C. Between 60 and 120 m, the geothermal gradient is variable because it is affected by atmospheric changes and circulating ground water. Below 120 m, temperature almost invariably increases with depth, though the rate of increase with depth varies with both tectonic setting and the thermal properties of the rock.

High gradients (up to 200°C/km) are observed along oceanic spreading centers and along island arcs due to magma rising to the surface. Low gradients are observed in tectonic subduction zones because of cold, water-filled sediments thrusting beneath the existing crust. Tectonically stable shield areas and sedimentary basins have average gradients that typically vary from 15–30°C/km.

◙ subduction zone magmas ◙