Volcanoes/Volcanism

This view of the topography over Campi Flegrei shows the relative relief caused by volcanic remnants. Credit: MeteoWeb.

The image on the left shows relative relief around Campi Flegrei. This emphasizes that most or all of the relief is due to remnants of volcanic activity.

Bentonites

Main articles: Volcanoes/Volcanism/Bentonites and Bentonites

Bentonites are clay rocks consisting predominantly of smectite. Credit: George E. Christidis and Warren D. Huff.

Def.

1. any "of several impure clay minerals consisting mostly of montmorillonite" or
2. a "porous clay formed by the decomposition of volcanic ash that swells 5 to 6 times its original volume in the presence of water"

is called a bentonite.

Bentonites of definition 2 are volcanism, or remnants of volcanic activity, even though they are metamorphic sediments or rocks.

Def. a "clay of the illite group that contains potassium and is formed by alteration of volcanic ash" is called potassium bentonite, K bentonite, or potash bentonite.

"Bentonites [as shown in the image on the right] are clay rocks consisting predominantly of smectite. They form mainly from alteration of pyroclastic and/or volcaniclastic rocks. Extensive deposits, linked to large eruptions, have formed repeatedly in the past. Bentonite layers are useful for stratigraphic correlation and for interpreting the geodynamic evolution of our planet. Bentonites generally form by diagenetic or hydrothermal alteration, favoured by fluids that leach alkali elements and by high Mg content. Smectite composition is partly controlled by parent rock chemistry."

The layers shown in the image on the right were deposited on the bottom of a sea floor.

Theoretical volcanism

Def. any remnants "of the natural phenomena and processes associated with the action of volcanos, geysers and fumaroles", especially any remnants of past volcanic activity, are called volcanism.

Gneiss

Main articles: Rocks/Gneisses and Gneisses

At Morton Pass, where highway 34 crosses the crest of the Laramie Range, you can see a nice set of (younger) mafic dikes cutting (older) granite/gneiss basement complex. Credit: Callan Bentley.

"At Morton Pass, where highway 34 crosses the crest of the Laramie Range, you can see a nice set of (younger) mafic dikes cutting (older) granite/gneiss basement complex [in the image on the right]. The pink stuff is Archean; the black stuff is Paleoproterozoic; around 2 billion years old."

Greenstone belts

Main articles: Rocks/Greenstones and Greenstones

This image shows weathered Precambrian pillow lava in the Temagami greenstone belt of the Canadian Shield in Eastern Canada. Credit: Black Tusk.

In contrast to the Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments and banded iron formations. Greenstone belts are typical Archean formations, consisting of alternating high- and low-grade metamorphic rocks. The high-grade rocks were derived from volcanic island arcs, while the low-grade metamorphic rocks represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. In short, greenstone belts represent sutured protocontinents.

Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies.

The name comes from the green hue imparted by the color of the metamorphic minerals within the mafic rocks. Chlorite, actinolite and other green amphiboles are the typical green minerals.

A greenstone belt is typically several dozens to several thousand kilometres long and although composed of a great variety of individual rock units, is considered a 'stratigraphic grouping' in its own right, at least on continental scales.

"Greenstone belts" are distributed throughout geological history from the Phanerozoic Franciscan belts of California where blueschist, whiteschist and greenschist facies are recognised, through to the Palaeozoic greenstone belts of the Lachlan Fold Belt, Eastern Australia, and a multitude of Proterozoic and Archaean examples.

Archaean greenstones are found in the Slave craton, northern Canada, Pilbara craton and Yilgarn Craton, Western Australia, Gawler Craton in South Australia, and in the Wyoming Craton in the US. Examples are found in South and Eastern Africa, namely the Kaapvaal craton and also in the cratonic core of Madagascar, as well as West Africa and Brazil, northern Scandinavia and the Kola Peninsula (see Baltic Shield).

Phanerozoic ophiolite belts and greenstone belts occur in the Franciscan Complex of south-western North America, within the Lachlan Fold Belt, the Gympie Terrane of Eastern Australia, the ophiolite belts of Oman and around the Guiana Shield.

Mud volcanoes

Here, early Archean serpentine mud volcanoes in Isua, Greenland, are imaged. Credit: PNAS/Marie-Laure Pons, et al.

"A critical building block for creating the first life on Earth was found in 3.8-billion-year-old rocks from Isua, Greenland, [...] For the first time, rich concentrations of the element boron have been found in Isua's ancient marine rocks [...] The discovery signals that boron was circulating in seawater and was absorbed by marine clays, which eventually became tourmaline [...] Boron can stabilize ribose, one of three key components of RNA. Ribose, an organic sugar molecule, has a short half-life and naturally decomposes without a stabilizer. [...] Until now, theories for the origin of RNA life pointed to RNA-based chemicals arriving on Earth from Mars. That's because Earth's first rocks and oceans seemed devoid of boron, which takes the form of borate minerals on Earth. On Mars, clays with boron and another RNA stabilizer, molybdenum, are abundant."

"I want to challenge this idea that the early ocean was borate free. The early ocean already contained borate, and therefore, early Earth — not Mars — could provide environments to stabilize ribose."

"The Isua rocks are among the oldest pieces of crust still around from Earth's earliest eons. The layers were deposited under a liquid water ocean, perhaps when life was first emerging. After billion of years of continental smashups, the rocks have been heated, faulted and folded, [...] Some of the rocks were seafloor sediments, such as mud and chert, and others were lavas erupted from underwater volcanic vents, such as pillow basalts. [The] boron in tiny tourmaline crystals trapped inside garnets in the ancient seafloor sediments. The garnets and tourmalines formed after the sediments were deposited, when the rocks were metamorphosed. Boron is one of the major elements of tourmaline. Isua's volcanic rocks also carry boron-rich tourmalines [...] Hydrothermal fluids circulating in the rocks are the likely source of the boron [...] Boron has two isotopes (elements with different numbers of neutrons in their nuclei). The boron isotope ratio in Isua's volcanic rocks also suggests early oceans carried enough boron to support RNA-based life".

"There could have been a role for boron in the stabilizing of ribose in the RNA origin of life. [...] boron-rich seawater [has been] cycling through the Isua volcanic rocks, despite a lack of continental crust. The tourmaline formed in an environment resembling today's deep-sea hydrothermal vents, where superheated seawater and other fluids spew from volcanic fractures. The abundant tourmalines indicate the fluids circulating through the ancient rocks were rich in boron. There is no convincing evidence of seawater boron concentrations being lower at 3.8 billion years ago than at the present."

Cones

Main articles: Volcanoes/Volcanism/Cones and Cones

The tuff cone on the right last erupted 4120 b2k. Credit: National Museum of Natural History, Smithsonian Institution.

This is the central portion of the Hopi Buttes Volcanic Field in north east Arizona. Credit: Mallory Zelawski.

Fort Portal is a tuff cone in Uganda at 0.7°N 30.25°E. Its summit is at 1615 m. It is volcano number 223001, with a last known eruption dated to 4120 b2k.

"These grass-covered tuff cones are among the many of the Fort Portal volcanic field in Uganda. The carbonatite lavas and tuffs of the Fort Portal volcanic field were erupted from about 50 volcanic tuff cones and maars, some of which now contain crater lakes."

"The Hopi Buttes volcanic field [shown on the left] consists of ~300 late Miocene volcanic centers within ~ 1800 km² of the field, located in northeastern Arizona [...]. Excellent cross-sectional exposures of well-preserved diatremes, vents, and related maar-crater deposits are exposed in the cliffs of the lava-capped mesas and buttes of the area [...]. Vents in the eastern part of the field preserve surficial maar deposits while vents in the western part preserve the sub-volcanic “plumbing” system or diatreme. The maars formed through explosive interaction of groundwater, liquefied lower Bidahochi sediments, and/or lake water with monchiquitic and nephelinitic magmas. The ratio of water to magma during the eruptions may have controlled the type of landform produced, which includes maars (including tuff rings and exposed diatremes), scoria cones, and lava flows. The style of eruption ranged from phreatomagmatic to magmatic and the morphologic characteristics of some vents, particularly maars and tuff rings are typical of eruptions that occur in wet, low-lying areas. Clay-rich tephra of the maars indicate explosive interactions of magma with a clay-water slurry, inferring that molten fuel-coolant interactions involving magma and wet-sediment are important in maar and diatreme emplacement."

Skarns

Main articles: Volcanoes/Volcanism/Skarns and Skarns

This skarn is from the Monzoni ridge, Northern Italy. Credit: Siim Sepp.

Spiz del Malinvern is the highest summit on the Monzoni Ridge in the Marmolada Group of the Dolomites. Credit: Gangolf Haub.

Def. the process by which the bulk chemical composition of a rock is changed by the introduction of components from an external source, especially by a hydrothermal fluid is called metasomatism.

Def. any of various metamorphic rocks formed by metasomatism> is called a skarn.

The skarn in the image on the right is from the Monzoni Ridge. It is "composed of calcite (blue), pyroxene augite (green), and garnet grossular (orange)."

A "geological survey of the Fassa district in the Dolomites [comprises] the Bufaure and Monzoni mountain groups, where the porphyritic and monzonitic rocks are widely exposed. [An objective is to] study in detail the tectonic relations between the igneous and stratified rocks".

"Spiz del Malinvern is the highest summit on the Monzoni Ridge in the Marmolada Group of the Dolomites. When seen from the north it catches your eyes - a north face of dark volcanic rock set between the porphyry summits of the Rizzoni Crest and the white limestone-like Dolomite of Vallaccia."

Hypotheses

Main article: Hypotheses

1. Metamorphic rocks contain information about early volcanism.

See also

• Eruptions
• Volcanic minerals
• Volcanic rocks
• Volcanicity
• Volcanology