Sometimes, anywhere, on a holiday or visiting friends, I look at a steep mountain that doesn’t quite seem to belong in that landscape and I think, wonder if that hasn’t been a volcano in the distant past? Well, in many cases that could be right, even in familiar places where you never knew they once had volcanic activity, even in the most sediment-laden plains…
Magma that solidifies in volcanic ducts or in sediments forms a duct filling (vent-, chimney filling), also called a volcanic plug, or a pipe. In an explosive eruption process numerous clasts from the chimney walls (xenoliths) can comprise a significant proportion of the filling. The igneous material in a plug may have a range of composition similar to that of associated lavas or ash, but may also include fragments and blocks of denser, coarser grained rocks (higher in iron and magnesium, lower in silicon) thought to be samples of the Earth’s deep crust or upper mantle, plucked and transported by the ascending magma. Plugs that bear a particularly strong imprint of explosive eruption of highly gas-charged magma are called diatremes or tuff-breccia. Volcanic plugs are believed to overlie a body of magma which could be either still largely liquid or completely solid depending on the state of activity of the volcano.
Plugs are known, or postulated, to be commonly funnel-shaped and to taper downward. Typically, volcanic plugs and necks tend to be more resistant to erosion than their enclosing rock formations. Thus, after the volcano becomes inactive and deeply eroded, the exhumed plug may stand up in bold relief as an irregular, columnar structure. They may be visualized as the fossil remains of the innards of a volcano (the so-called “volcanic plumbing system”). – Because of their loose fill, volcanic vents often serve as easy migration pathways for mineral solutions (fluids) from the depths. Many known deposits of copper and other metals are found in such structures
DEVILS TOWER, U.S.A.
I will start with Devils Tower in the Black Hills, in northeastern Wyoming, US. It is a good sample for all those rocky plugs in the world where scientists are not quite sure of whether or not their magmas have ever seen the surface of the surrounding landscape. All too often descriptions state that the steeple in question is an extinct volcano but, digging for more information it turns out that either there is disagreement over the question or that there has never been a vent to the intrusion, i.e. magma stopped progressing upwards and cooled before reaching the surface.
Devils Tower is one impressive sight of what is commonly called basalt columns. It rises steeply 386 m (1,267 ft.) above the surrounding terrain and the summit is 1559m (5,114 ft.) a.s.l. This igneous rock – columnar phonolite porphyry – intruded into the sediments of an ancient maritime environment about 40 Ma ago. It is a light to dark-gray or greenish-gray rock with conspicuous crystals of white feldspar. As hot molten magma is less dense and occupies more volume than cool hardened rock, it contracted in cooling, forming hexagonal (and sometime 4-, 5- and 7-sided) columns separated by vertical cracks.
Three theories as to the Origin: Although Geologists since the late 1800s were sure that Devils Tower was formed by intrusion of igneous material there is still disagreement on how that process took place and whether or not the magma reached the land surface. Several scientists believe the molten rock comprising the Tower might not have surfaced; others are convinced the tower is all that remains of what once was a large explosive volcano.
1) An idea quite popular in the early 1900s was that Devils Tower must be the remnant of a laccolith (a large, mushroom–shaped mass of igneous rock which intrudes between the layers of sedimentary rocks but does not reach the surface). This would produce a bulge in the sedimentary layers above the intrusion which would then have weathered away to reveal the igneous rock.
2) Another theory suggested that Devils Tower is the neck of an extinct volcano. Although there is no evidence of volcanic activity – volcanic ash, lava flows, or volcanic debris – anywhere in the surrounding countryside, it is possible that this material may simply have eroded away. Some pyroclastic material of the same age as Devils Tower has been identified elsewhere in Wyoming.
3) The simplest explanation is that Devils Tower is a stock – slowly upsurging magma spent its force, cooling and becoming solid before reaching the surface. During this process probably a very large mass of it, many miles across, moved within a few thousand feet of the surface. Before it cooled, fingers or branches of pasty-textured material moved upward along lines of weakness in the rock layers near the surface.
In 1905 Devils Tower was designated a National Monument. The site is also considered Sacred to the Lakota and other Northern Plains Tribes who lived in the area, they called it Matȟó Thípila, Grizzly Bear Lodge. The present name resulted from a misinterpretation from the native language – there have been efforts to rename the monument to its original but never succeeded so far, probably due to economic reasons.
TROSKY, Czech Republic
Trosky Castle is the most famous castle of the Bohemian Paradise, its silhouette has become the symbol of the Geopark Český Ráj. These impressive basaltic double-towers formed in the Tertiary, when two vents remained filled with magma. The 47m high Baba (Czech for granny) and 57m high Panna (Czech for virgin) protrude from the hill, carrying the medieval castle at an altitude of 514 m a.s.l. The higher ridge between them was originally built up with residential buildings – imagine to live in such a place! Geologically, the whole ensemble is a Tertiary volcano, today representing an erosional relic of a scoria cone.
The surrounding landscape consists of an extended plateau of sandstone rock, deposited hundreds of meters thick on top of limestone layers by the Mesozoic Seas. As volcanism occurred at a later time, magma had to work its way through the sediments, warping and breaking them up and causing chemical changes. The result is a geologically varied wonderful landscape, modeled by rivers and weathering into rock cities, karst areas and volcanic sites. Actually, my visit there as a fourteen-year-old was my first encounter with geology which interest has never left me all my life.
GROS PITON and PETIT PITON, Saint Lucia
Another pair of – supposedly – volcanic plugs are the The Pitons on the southwestern coast of Saint Lucia in the Lesser Antilles island arc – dramatic landmarks of the island. Gros Piton and the even steeper Petit Piton are pre-caldera lava domes west and SW of the late-Pleistocene Qualibou caldera. They were constructed along a NE-trending fault.
Pitons Management Area is a World Heritage site. “The Pitons are two steep forested cone-shaped mountains rising side by side from the sea on the southwest coast of Saint Lucia with spectacular abruptness. Gros Piton is three km wide at the base, Petit Piton is one km wide and is linked to it by the high Piton Mitan ridge.
The peaks are the degraded dacitic cores of two lava-dome volcanoes probably formed on the side of a collapsed andesitic strato-volcano. They rise on the edge of the geologically complex caldera-like gravity-slide formation known as the Soufriere Volcanic Center, also the Qualibou depression which surrounds the whole area, including the town of Soufriere. In the center of the depression are the Sulphur Springs, a geothermal field or solfatara with sulphurous fumaroles and hot springs surrounded by a variety of other volcanic features: explosion craters, lava flows and deposits of pumice and ash. The volcanic area has been dormant for at least 20,000 years. If the Pitons are remnants of cumulo-domes, and not volcanic necks or plugs which are common, they are geologically unique“. (M, U., 2008. Pitons Management Area, Saint Lucia)
– A cumulo-dome is a convex dome-like body formed by highly viscous lava that does not flow but stays in place, sagging and spreading into a lump of rock.
Some may know about the Pitons just because of “The refreshing mountain brew that satisfies and stimulates the passion for friendship” 🙂 , Piton Lager – a beer brewn by Windward & Leeward Brewery Ltd. in Saint Lucia.
ROQUE NUBLO, Canaries, Spain
Roque Nublo (Clouded Rock, Rock in the Clouds) is a volcanic rock on the island of Gran Canaria, Canary Islands, Spain. It is 80m (262 ft) tall, and its top is 1813m (5,948 ft) above sea level. The Roque Nublo is the remnant of a once much higher stratovolcano. The emergence of this volcano began about 4.2 to 4.6 mya with thick mafic lava flows which superimposed the eroded remnants of an older Miocene volcano and progressed up to 20km to the coast. The eruptions lasted for about 1.5 million years, the lavas changing to trachyte and phonolithe. The now more explosive eruptions in the summit area then produced breccia-like ignimbrites until the eruptions ended by the intrusion of phonolithic plug material about 3 mya . Since that time the volcano suffered heavy erosion, rock deposits of huge landslides can be found up to a distance of 25 km. Roque Nublo, once a plug in a volcano’s chimney, could resist better than the softer surrounding rock due to its tougher breccia. So remained as a “Härtling” – the quite appropriate German word (feel free to translate it as “a hardling”, I have not found a better translation).
MORRO ROCK, U.S.A.
Morro Rock is the best known of the Nine Sisters of San Luis Obispo County, a series of ancient volcanic plugs that line the Los Osos Valley between the cities of Morro Bay and San Luis Obispo in California, U.S.. It is a volcanic plug located offshore at the entrance of Morro Bay harbor, connected to the mainland by a causeway. It has been a landmark for countless sailors through the centuries and a truly spectacular sight from sea and from land.
It is one of 14 intrusive units that form volcanic plugs, lava domes and dike intrusions for 29 km from Morro Rock to the southeast. These are part of the Morro Rock-Islay Hill Complex (Oligocene, 27-23 Ma ago), along an almost straight line, strongly suggesting that intrusion has taken place along a pre-existing fault. This idea is supported by the San Luis Obispo Sheet of the California State Geologic Map which shows a steep fault directly in line with the volcanic features.
Theories suggest that 18-20 ka ago the sea level was 100m lower than today, and Morro Rock stood on dry land. After the last glacial retreat, the sea rose again and Morro Rock was completely surrounded by seawater and about 200 meters offshore until some time ago. Unfortunately, from the end of the 19th century until 1963, ca 1.2 million tons of rock were blasted out of the mountain for construction use.
The igneous rock is dacite; its composition is a ground mass of plagioclase with amphibole (hornblende), biotite, pyroxene (augite), quartz, and glass; phenocrysts of plagioclase, amphibole and often quartz are very obvious. It has a porphyritic texture common to igneous rocks that experienced two-stage cooling. White rectangular crystals are plagioclase phenocrysts set in a gray ground mass. The plagioclase formed first during a slow cooling phase. The ground mass was quenched when the magma rose quickly towards the surface. Potassium-argon dating yielded an age of 22.1 +/- 0.9 million years for the crystallization of this rock.
The generation of these igneous rocks was probably a thermal response of the western margin of the continental crust-capped American plate to the subducting Farallon plate, resulting in Morro Rock and Cambria units, followed by a complex encounter with the East Pacific Rise, resulting in the Obispo Formation.
Today, Morro Rock is a Natural Preserve and it is illegal for the general public to climb it. One reason are the peregrine falcons who have begun nesting again on the rock after they became nearly extinct by insecticides (DDT) dissolving their egg shells. Only the Native Indian tribes, who considered Morro Rock to be a sacred site, now have a right to climb Morro Rock for their annual ceremonies – which badly clashes with the objectives of the local group protecting the falcons. Read this thought-provoking article by A. H. Dillon.
Enjoy! – Granyia
SOURCES & FURTHER READING
– U.S. National Parks/Devils Tower Nat. Monument
– On first 50 years Devil’s Tower Nat. Monument; by Ray H. Mattison, Historian (1955, PDF)
– Geologic map of Morro Bay area
– Cuesta College – The Morros
– Supervolcano: A Geological History of the Jemez Area, Kent G. Budge. Great website!
– Geocities: Types of volcanic structures
– Wikipedia, Volcanic Plug