earth-of-fire

The subject of this blog is volcanism and its various manifestations.

The regions affected by magmatism see it expressed by extrusive igneous structures : stratovolcanoes, shield volcanoes, cinder cones, maars and domes, etc., their type of activity and their products.

The expression in surface is minimal in volume compared to the underlying magma that gives birth and form large intrusive bodies.

Various intrusive bodies can be uncovered by erosion, igneous rocks beeing more resistant to erosion than the host rock, composed of sedimentary rocks will form positive relief.

                                                             doc. Ulaval.ca / Bourque.

Among the intrusive igneous structures, there are
- non-tabular object  : batholith, stock, protrusion, lopolithe, laccolith, neck,
- tabular objects  : sill and dyke.

Schéma summary of the various structures igneous extrusive and intrusive - doc. Structure of the igneous bodies / ULB - Universite Libre de Bruxelles.

Batholith and stock :

Etymologically, the term batholith derives from the Greek βάθος - bathos - deep and λίθος - Lithos - stone.
A batholith means a mass of granite or granodiorite at least 100 km ², whose contour is irregular in outcrop and whose roots are lost in the depths of the earth's crust, usually in the discordant rock.
A stock means the uppermost extension of a batholith and an area of ​​less than 100 km ².

Formation of a batholith:
Although they seem at first sight quite homogeneous, batholiths are in fact structures whose history and composition are relatively complex. They consist of multiple masses, or plutons (magma originally from an area of ​​partial melting at the base of the crust) that are closer to the surface. While still moving, we call these plutons light enough for magma plutonic diapirs. Thanks to their high temperature and viscous consistency, these diapirs will rise by forcing a way through the surrounding rock contribute to melt in the process. The majority of diapirs do not reach the surface as a volcano, but are slow to rise gradually as the temperature drops and they become solid, generally at a depth of 5 to 30 km, to form plutons (referring to Pluto, Roman god of the underworld underground).

 We'll talk about batholith when a number of plutons will eventually merge to form a sufficiently large mass of plutonic rock.

"Half Dome" in Yosemite National Park : This monolith of granite is a part of the Sierra Nevada batholith. - Photo Jon Sullivan.

Some batholiths are mammoth in size, extending along subduction zones,  past or present, or other heat sources of the continental crust over hundreds of kilometers.

For example in North America, the Sierra Nevada batholith: it is a continuous granitic formation that constitutes the major part of this chain of mountains in California. 

Un even more gigantic batholith, which forms most of the mountains of Canada's west coast, extends to the southeast of the Alaska 1800 km.

          Trenemene - Scilly Isles - granite rocks - photo Richard Knights / www.geograph.org.uk

In Europe, the Cornubian batholith consists of a group of associated granitic intrusions, some of which masses are visible in the Isles of Scilly, Dartmoor, Bodmin Moor, Carnmenellis ...

Cornubian batholith - doc. Mikenorton.

Another example in Africa: the granitic batholith in the Libyan Jebel Uweinat.

        Libyan - Jebel Uweinat - doc. NASA / The astronaut to gataway photograhy of earth.

This list of batholith is not exhaustive. 

Sources:
- Structure of igneous bodies - ULB - link
 - Igneous rocks - Bourque - link
- Reocities - College Park - Study of igneous rocks - link

Among the "minor" plutons, there are formations that result from filling of fractures that are younger than the host rock: dykes and sills.

A dike (or dyke), word of Celtic origin meaning "wall", after his release takes the form of small walls, mimicking a horizontal prismatic construction. They result from the lava filling of vertical, radial or concentric fractures to the building.
Unlike the sill, a dike intersects the layers present previously.

   Etna - in the Valle del Bove, some 363 dikes have been updated by its collapse - doc. photovolcanica.

The thickness of a dike can vary from a few centimeters to tens of meters while its horizontal extension at the outcrop, can reach several kilometers. The thickness of the vein is generally smaller than the other two dimensions.

The dike can appear in swarm up to several hundred, set up almost simultaneously at the same event intrusive. Often sources of fissure eruptions because building networks through which magma moves over long distances. The speed of development can reach one meter per second. This allows rapid movement of the lava does solidify too quickly in contact with cooler rocks through which it passes. In a dike 2 m wide, with a magma moving at 1m / s, the temperature drops by only 20 ° C in 10 km.

Can be distinguished according to their geometry, radial dykes, conical, annular, oblique, coat.

Madeira - a volcanic island as evidenced by the numerous dikes of the north-west - © Bernard Duyck

Germany - Eifel - Wingertsberg's quarry - portions of ring dike (bluish gray), sadly missing today following the operating career ... the man between the two portions gives the scale - © Bernard Duyck

United States - Grand Teton National Park - a vertical dyke eroded at the top of Mount Moran, formed lower but rebounded following the uprising in the mountains of Grand Teton. - © Bernard Duyck

  Alaska - Baranof Island - dykes of different thicknesses and well contrasted - doc. Enc. Britannica.

A sill is an intrusion between older sedimentary layers or lava flows, tuff's already in place, respecting the general stratigraphy. A sill do not intersect the pre-existing rocks.

They usually have a horizontal orientation, but can be adjusted to vertical by tectonic processes. For differentiation, there will be evidence of overheating on the upper and lower surfaces in contact with the surrounding rocks.

  Scotland - Edinburgh Region - Salisbury Crags from Calton Hill Seen - photo Klaus with K.

England - Northumberland - the Great Whin sill, overtoped by the wall of Hadrian - photo Michael Hanselmann.

The great basaltic Whin Sill basalt / Northumberland has a thickness of 70 meters and lies gentle slopes towards the south, under the layer of overlying sedimentary rocks. This sill is overcome by the wall of Hadrian, built by the Romans around the year 120.

The Geological Society scheme.

Northumberland - the surface of "The Great Whin Sill" lightly prismatic (see diagram above)
© Copyright Antonia and licensed for reuse Under this Creative Commons License.

Sources:
- Volcanology - by J-M. Bardintzeff - Ed. Dunod
- Guide volcanoes - by Mr. Rosi et al. - Ed. Delachaux & Niestlé.
- Observations on the geology of Northumberland Durham nd - NJWinch

The laccolith - from the Greek Lakos, cavity and lithos, rock - is a  intrusive rocks mass, rounded at the top, which insinuates itself into a sedimentary series. The melting mass can not reach the surface or stops in the lithosphere and extends laterally between the parallel layers.


 A lopolite - from the Greek lopas, dish and lithos, rock - is a plutonic massive, much larger than the laccolith, several kilometers in diameter, which forms a basin and is consistent with the host rock.

An example in France, with the laccolith of Drammont, near St. Raphël in the Var.

The estérellite is intrusive in the arkoses dated from Permian and forms a laccolith NW-SE. It surfaces in the region of Agay and Cap Drammont.

It has been dated to be -31.9 ± 0.7 Ma, in the Oligocene period (Ivaldi et al., 2003). The intrusion of igneous rocks has resulted in the appearance of a contact metamorphism of the Permian rocks.

Geological Map of Drammont - BRGM - the estérellite (Tertiary) is light blue, the rhyolite (Permian) in red.

From left to right, the pebble beach and the quarry of Drammont, views from the semaphore of Cape Drammont - The Drammont beach with its blue-gray boulders was in fact a waste storage area of ​​the quarry of the same name. The sea with its ebb and flow, and storms have shaped the stones to make them smooth and soft, giving a special touch to this beach lined with pine trees .- © Bernard Duyck

         Estérellite's quarry of Drammont, converted into water sports - © Bernard Duyck

The estérellite is a microdiorite quarztique, containing zoned plagioclase phenocrysts of amphibole and biotite, sometimes found phenocrysts of quartz and feldspar. Accessory minerals are magnetite, apatite and zircon.

        Estérellite under microscope - doc. BRGM / Tertiary magmatism of the Esterel / M. Corsini

         Sample of estérellite from the quarry of Dramont, pyroxene and amphibole facies - © B. Duyck

It is characteristic of subduction contexts. The genesis of estérellite is placed in the context of the Alpine orogeny.

On the beach of Aiguebonne, the contact between the estérellite and rocks (sandstones and mudstones of the Permian age) can be observed. The sandstones exhibit oblique stratifications while a prismatic, more or less well developed affects the igneous rock (magma cooling).

Estérellite on the beach of Aiguebonne - photo Romeoeuf N / BRGM / Tertiary magmatism of the Esterel.

Another example is the National Park Torres del Paine in the Chilean Patagonian Andes, with its towering vertical walls created by glacial erosion, we can see that such a laccolith is thick. This granitic intrusion is about 20 km from east to west, 10 km from north to south for 1 to 2 km thick only.

This is a clear granite laccolith from Miocene (12-13 Ma), intrusive in sediments from the Upper Cretaceous (Albian, 110 Ma, to Santonian, 85 Ma) much darker. Petrographically, the "granite" Torres del Paine is intermediate between a granodiorite and granite clear in the strict sense.

  Torres del Paine laccolith, light gray, exposed by erosion - Torres del Paine National Park, Chile - photo Miguel.v ..

"Devils Tower", located in the Black Hills in northeastern Wyoming, is regarded as a theory, as the remains of an eroded laccolith dating back 65 million years ... see on this blog in the "Myths and Legends"

Sources:
- BRGM - Lithothèque PACA - Tertiary magmatism of Esterel - link
- Planet Earth - the intrusion of granite Torres del Paine - link
- Structure of igneous bodies - ULB - link
- Igneous rocks - Bourque - link
- Reocities - College Park - Study of igneous rocks - link

The necks are magmatic intrusions characterized by magmatic intrusions radial prismation, resulting from ancient volcanic vents, filled, and exposed by erosion.
Erosion remove the brittle materials, surrounding the lava present in the pipes and dykes, much stronger. After a laps of time, the hard "plug" ends protrude the landscape to form a relief with steep slopes.

According to a human use mystifying incomprehensible things to the time, these necks are often linked to religious beliefs, this in various civilizations : in Burma, the Taung Kalat - in Sri Lanka, the Lion Rock of Sigiriya - in France, Church of St Michel l'Aiguille in Puy-en-Velay..

Others are related to legends, as Strombolicchio in the Aeolian Islands, inseparable from Greek mythology and its gods, the writings of Homer and the Odyssey of Ulysses.  

                   The scale of Strombolicchio is given by the boat, right - Photo Man77 / wiki.

New Mexico / United States, "Shiprock" is a neck, so named by English speakers for its resemblance to a clipper, circa 1870.
Shiprock is composed of fractured volcanic breccia and dykes of igneous rocks called "minette".
Minette : term for an igneous rock facies lamprophyres, essentially of orthoclase and biotite (M. Voltz, 1828).
The term "minette" is ambiguous and can also describe a sedimentary iron ore, Oolitic Middle Jurassic of Lorraine (E. de Beaumont, 1822)

                                   "Shiprock" - volcanic neck - photo blogspot.

It is the remnant of a volcanic vent that has blocked and solidified there are 27 million years ... this structure is part of the area northeast of the "Navajo volcanic field" . Millions of years of erosion have expose this vent and dykes surrounding it, removing the volcanic cone itself, more brittle.

Navajo Indians named him "Tse Bit'a'í ", "the winged rock" in reference to the legend of the great bird that brought from the north to this actual land. This peak is mentioned in many legends : in one of them, monstrous birds lived in the peak and ate human flesh ... twins warriors were mandated by the Navajos to kill these monstrous birds.

Shiprock / Navajo volcanic field - United States: the neck is surrounded by dykes, radiating in various directions - picture "the land and the mineral world."

 


 

 

 

Left:
neck and radial dykes
doc. from: ULB development of igneous bodies

Right: Dotted line, the volcanic structure AVAT its erosion - in gray, "Shiprock" and black, the outline of the various dykes ... All these structures are visible in the photo above. -

        The pattern was confirmed by an ASTER image / NASA Terra satellite taken on 02.06.2006.
NASA image created by Jesse Allen, Earth Observatory, using Data provided courtesy of NASA / GSFC / METI / ERSDAC / Jaros, and US / Japan ASTER Science Team. Thanks to Robert Johnson of the Navajo Nation Museum for information on the mythology of Tse Bit'a'í.

Agathla, also known as "El Capitan" is another volcanic neck of the Navajo volcanic field, there is formed 20-30 million years.

Section of Agathla volcanic neck. (Doc. Williams / Oregonstate one.)

The main body of the neck, consisting of tuff breccia is intersected by dykes of minette. Originally, the vent was that of a volcanic maar.

          The volcanic neck Agathla - Navajo volcanic field - 457 m. - photo Geographer.

Sources:
- Volcanology - by J-M. Bardintzeff - ed. Dunod.
- Navajo volcanic field - Oregonstate University - link