Friday, April 26, 2013

Cleveland covered in clouds


Cleveland is the most active volcano of the Aleutian island arc and appears as a typical stratovolcano with its nearly symmetrical shape (symmetry being quite typical for Alaskan volcanoes in general). The 1730 m high volcano is located on the western portion of the uninhabited Chuginadak Island (Aleut name: Tanax̂ Angunax̂) which is also the native Aleut name of the volcano. Chuginadak is the Aleut goddess of fire who is believed to reside in the volcano. Interestingly, the name refers to the volcano’s constant activity throughout time. The name Cleveland was given to the volcano in 1894 after the then-president of the United States Grover Cleveland.

Chuginadak is part of an island chain called Islands of Four Mountains. The name derived from the Russian Четырехсопочные Острова (Ostrava Chetyre Soposhnye) which was given to the chain by Russian explorers that noticed four prominent mountains in 1826. The Islands of Four Mountains also include Amukta, Chagulak (a 3km wide island with a steep 1142m high volcano), Yunaska (the largest island), Herbert (a circular island with a 2km wide caldera), Carlisle (containing a 1610m high stratovolcano), Uliaga (the northernmost and smallest which in the 80s was ravaged by a shipwreck leaking oil) and the Kagamil Islands. Thus, it appears there are more than four mountains, although Cleveland, Chagulak, Herbert and Carlisle were probably the most ‘prominent’. These volcanoes have an unknown eruptive history.

The eastern part of Chuginadak Island, which contains the heavily eroded Tana volcanic complex, is connected to Cleveland volcano by a narrow isthmus. Aleut oral history mentions that Cleveland and the eastern part of Chuginadak Island were separate islands in the past.

Since 1893, Cleveland had approximately 15 eruptions which are characterized by explosive ash emission (eruption columns can be up to 12 km high), lava flows and lava fountaining. VEI (Volcanic Explosivity Index: seehttp://volcanoes.usgs.gov/images/pglossary/vei.php) 3 eruptions occurred in 1944, 1987, 1994 and 2006. However, it is difficult to monitor the volcano because of its isolated location. Nowadays a webcam located on the closest inhabited island, Umnak Island keeps a close eye on the volcano. In addition, the volcano is monitored by satellite imagery. Unfortunately, due to the typical bad weather in the area the island is invisible half of the time. Another aspect is that the volcano is not seismically monitored. Thus, eruptions could still go unnoticed.

From the end of January until the beginning of February 2013 a lava dome in the crater grew to a diameter of 200m. Since then, the few clear days that have occurred did not show any changes of growth of the summit dome. Currently, Cleveland is under volcano alert level yellow (see more information on volcano alert color codes see: http://volcanoes.usgs.gov/activity/alertsystem/index.php#colorcode) implying that unrest is occurring but there is no eminent eruption, well at least not yet.


Image: Cyrus Read (USGS). The image shows Cleveland volcano in the back with Carlisle volcano on the foreground.

References and further reading

http://www.avo.alaska.edu/volcanoes/volcinfo.php?volcname=Cleveland

http://www.volcanodiscovery.com/volcanoes/aleutians/cleveland/

http://www.volcano.si.edu/world/volcano.cfm?vnum=1101-20-

http://www.volcano.si.edu/world/volcano.cfm?vnum=1101-25-

http://www.volcano.si.edu/world/volcano.cfm?vnum=1101-24-

http://geonames.usgs.gov/pls/gnispublic/f?p=gnispq%3A3%3A4498896543377514%3A%3ANO%3A%3AP3_FID%3A1403972

Webcam monitoring Cleveland from the Alaska Volcano Observatory: http://www.avo.alaska.edu/webcam/Cleveland.php

Halong Bay, Vietnam - One of the best examples of tower Karst in the world


Halong Bay was formed through the dissolution of limestone millions of years ago, forming a perfect geological example of karst topography.

As rainwater collected carbon dioxide from the atmosphere, slightly acidic rainwater containing small amounts of carbonic acid were capable of eating away at the limestone, forming these towers and islets located in Halong Bay. This is a classic example of Karst's dissolution mechanism. This rain water made its way into the natural cracks and crevasses of the rock, and eventually widened the rock over the course of millennia into caves, tunnels, and bays. This process was amplified in the Vietnamese tropical region. Due to the extra vegetation, the rainwater absorbed extra carbon in its seeping path through the rock, becoming more acidic and better developing this karst environment, making this one of the best visible karst environments in the world.

The visible limestone today has existed for nearly 500 million years. Its first 100 million years of life was spent deep under the ocean, but uplift moved it into a shallow sea environment for an additional 150 million years. Sea level fluctuations allowed for the surfacing of many of these subaqueous limestone formations, allowing for the Karst features seen here. It is for this reason that many of the developed karst regions are underwater, unable to be seen. Now smothered by water from the melting of the last ice age, many unseen valleys and drowned Karst formations remain hidden.

Over the years, more limestone has been eroded than exists today. Halong Bay faces environmental danger today, as mangroves and seagrass beds are being cleared for tourist boats. Fuel and oil have created pollution problems, and portable toilets created for tourists have polluted the surrounding soil and water. Many additional dangers pose a threat to the continued existence of this geological treasure. Hence, efforts are being made to preserve Vietnam's Halong Bay.


Image Credit: Lonely Planet

References:

http://www.halongbayharbor.com/about-us/22-halong-bay-geology.html

http://whc.unesco.org/en/list/672

http://hsc.csu.edu.au/geography/ecosystems/case_studies/2475/halong_bay.html

The Downs and The Weald


The Downs of Kent and Sussex are a quintessential example of English landscape. The rolling Cretaceous chalk hills in the north and south come to an abrupt end at a pair of 200 metre high escarpments facing each other, gazing out over the older Jurassic rocks exposed below. Between them lies the dome of the Weald, filled with fields and meeting the sea in the recently emerged lands of the Romney Marshes. On a clear day (I admit that it can be a rare event) the other side is visible as a dim bluish line on the far side of the flat bottomed valley. This gash in the geology of South East England is not a fault bounded graben as the landscape might suggest, but England's own piece of the Alps.

This formation is an anticline, a type of fold with the oldest rocks in the centre. The area was recently uplifted as a distant consequence of compressive stress from the faraway collision of Africa and Europe that built the Alps and Pyrenees. The Weald is bordered by two ridges, a higher one in the chalk and a lesser one in the greensand formation below. The scarps have a shallower dip slope behind them. A geological cross section shows the different formations and how they relate to the structure at http://tinyurl.com/c7mh65a . The chalk escarpment goes from Surrey in the North (where it peaks at 269m) right the way down to the sea, by Folkestone and Eastbourne.

Famous places of geological interest include the white cliffs of Dover ( see our past story athttp://tinyurl.com/d9m83z6), Beachy Head (our past post is at http://tinyurl.com/cmdvty3) and the Seven Sisters. The landscapes offer stunning views of green fields, sometimes filled with wildflowers and a range of varied habitats determined by the underlying geology.

The Weald is drained by several rivers, cutting though the ridges. The Stour passes through Canterbury, one of England's oldest towns, and the religious centre of Christian England. Here St Augustine, who first converted us to Christianity, established its cathedral, which is made of sandstone from the greensand formation. The Medway flows past many monuments to our ancient naval glory such as Chatham Docks, where Blackbeard was hanged.

A potted geological history of the area would run something like this: An ancient landmass, called the London Platform was repeatedly uplifted and eroded in the Jurassic, depositing sand and clay cycles in a marshy plain, in which dinosaurs such as Iguanodon roamed. These are called the Wealden series and end with a muddy plain. A branch of the Tethys ocean then invaded in a marine transgression, depositing the two greensand formations (upper and lower), with the Gault clay sandwiched in between marking a period of deeper water.

As sea levels rose further, no land sediment could reach the area, and the chalk limestone was deposited (see our past post on flint and chalk formation athttp://tinyurl.com/d578uuz). At the end of the Cretaceous, uplift and erosion created a major unconformity (see past post athttp://tinyurl.com/ce7ceos), before some marine Tertiary rocks were laid on top. Erosion then created the current landscape. The highest parts of the Weald dome were removed revealing the cross section you can discover for yourself while driving around the area on a summer's day, stopping in an ancient pub out in the sticks for some liquid refreshment.

The final Alpine uplift and folding took place between 20 and 2 million years ago. The most intense burst was very recent, since 5 million year old marine sands were deposited in some places above the chalk. The ice ages then shaped the current landscape, when it was an open tundra. Permafrost related processes operated in a series of cycles as the ice waxed and waned, creating a series of v-shaped dry valleys with steep sides, also known as wind gaps and coombes. The best known one is Devil's Dyke, north of Brighton.

In glacial periods, impermeable permafrost caused runoff, which carved the valleys, and in interglacial thaws the chalk absorbed the rain and melt water and the valleys dried out. The sponge like chalk remains south east England's main aquifer. Where the chalk meets the clay a line of springs follows the whole length of the ridges. The recent burst of uplift also created a series of river terraces, which preserve records of England's earliest inhabitants and the Pleistocene paleoclimate. Several of the best preserved prehistoric flint mines occur in the Sussex downs.

The downs contributed to the development of Geology as a discipline. Being close to London, it was easy to reach and explore. The revealed strata were pieced together and seen to be similar to thos outcropping further west (In England and Wales, the further west you go, the older the exposed rocks). The anticline shows up on Strata Smith's first geological map. The first land dinosaur, iguanodon, was found in the Wealden series. They also were the dwelling place of Charles Darwin, who meditated and wrote 'on the origin of species' in his garden.


Image credit of the Wye downs: A.Welbourn.

http://www.kgg.org.uk/kentgeo.html

http://www.kentdowns.org.uk/uploads/documents/Mplan7.pdf

http://www.southdowns.gov.uk/looking-after/landscape/geology-of-the-south-downs

http://www.southdowns.gov.uk/learning/themes-to-study/landscape/geology/geology-through-time

Durdle Door, Dorset, England


"They persuaded me to keep on, and at last stranded me on the pebbles, exactly opposite the magnificent arch of Durdle-rock Door. Here I stood and contemplated with astonishment and pleasure this stupendous piece of Nature's work" John O'Keefe, 1792."

Located on the World famous Jurassic heritage coast, Durdle Door is a stunning, naturally formed limestone arch. The arch is on privately owned land, located near Lulworth, but it is open to the public, The name "durdle" comes from the old English "Thirl" which means bore or drill.

The geology making up the arch is composed of almost vertical bands of narrow limestone rock, which runs parallel to the chalk of the coast line. The arch formed as a result of the softer rock behind the limestone being eroded away (through joints in the limestone itself), and eventually the sea managed to punch through the limestone, leaving the archway. Eventually the arch will collapse, leaving sea stacks behind similar to those that can be seen all along the South West Coast.

The Bull, a rock sticking out from the sea, close to Durdle Door is a continuation of the rock strata found in the arch.


Links;
http://www.worldheritagecoast.net/place.aspx?place=25
http://www.southampton.ac.uk/~imw/durdle.htm

Image; Saffron Blaze

Dartmoor Tors


The high ground of England's southwest peninsula, from Devon, through Cornwall down to the Scilly Isles, is dominated by a huge subterranean geological structure (a batholith) that outcrops as a series of moorlands. Dartmoor, Bodmin moor and the moors of West Penwith are all underlain by connecting granite, outcropping as "tors" at the crest of each boggy hill. The granite has sat here for 300 million years or more, formed during a great mountain-building event: the "Variscan" (or Hercynian/Amorican) orogeny during the Devonian and through the late Carboniferous. Similar mountain-building processes that had formed the Appalachians in eastern USA formed the granites here in Devon and Cornwall, the result of the collision of the ancient continents of Gondwana (to the south) with Euramerica/Laurussia to the north. These collisions resulted in a single supercontinent, Pangea, which spanned from pole to pole.

In their more recent geological history, a mere 50 million years ago or so, the southwest granites were weathered. Their large feldspar crystals weathered down to clays, and china clay pits are worked today in Cornwall and Devon, extracting kaolin and other clays from the rotted granite. Weathering removed rocks above, and decompression and cooling has resulted in a pattern of joints or cracks in the granite tors, that are exploited in freeze-thaw weathering today to produce a characteristic pattern of outcrops.

Dartmoor was home to humans through the late Neolithic and early Bronze ages, as evidenced by the ubiquitous stone circles, rows of standing stones ("menhirs"), and ancient hut circles that can be found in the remote stretches of this National Park today. It is a fantastic space for recreation and exploration. But be sure to take a map and compass whenever you tread out onto the moors, for fear of being "pixie led". It is said evil spirits bring down the mists on the moors and lead the unwary astray. It is certainly true that the weather can close in, and each tor looks much like any other with rolling slopes, which makes navigation a challenge without technical assistance.


Image: Staple Tor, Dartmoor, courtesy of Matt Whorlow © www.mattwhorlowphotography.com

Links:

http://www.geolsoc.org.uk/Plate-Tectonics/Chap4-Plate-Tectonics-of-the-UK/Variscan-Orogeny

http://www.dartmoor-npa.gov.uk/lookingafter/laf-naturalenv/laf-ecologywildlife/laf-geology

http://www.legendarydartmoor.co.uk/piskie_led.htm

Scafell: An old volcano's exposed guts


We all love the photos of perfect conical stratovolcanoes and eruptions that appear on TES, but geologists don't only study the Earth's surface, they need to know what's happening underneath. There are several ways to do this. Geophysics has technological and interpretative limitations, along with being expensive. Drilling cores is also pricey, and only limited sampling can be done, even if oil or gold are at stake. The best and cheapest is to use erosion, and go sniff around a place where weathering has exposed the type of structure you wish to study. This allows a thorough survey of a large area, easy rock sampling, and allows us to assemble high resolution reconstructions of past events and processes. One of the best locations to study the insides of a volcanic caldera is at Scafell, in England's scenic Lake District.

Calderas develop when large amounts of quartz rich felsic magma are erupted and the ground sinks into the newly emptied magma chamber below, creating a large bowl shaped crater. This process often repeats itself, as the magma keeps on rising after each collapse, resulting in a complex set of nested bowls. Famous examples include Santorini in Greece, Krakatoa in Indonesia and Mammoth Lake in the USA. Understanding calderas is important to geohazard researchers, who wish to learn how to predict these destructive eruptions in order to protect threatened populations. As Lyell said in a proverb that became a keystone of Earth science, the first step to predicting the future is always understanding the past.

Back in the Ordovician, the proto-Atlantic (called the Iapetus ocean) was closing, and was later to weld Scotland to the rest of Britain. As this slow motion continental crash approached, an arc of volcanic islands formed in Iapetus, close to the oceanic plate's subduction zone under Avalonia (the England of that epoch, Japan is a modern analogue). The process is well understood: Water is baked out of the subducting plate, rises, and makes the mantle wedge above the sinking slab melt.

Scafell started life as a group of nested calderas in this island arc, became a mountain range twice as America and Europe collided (called the Caledonian and Variscan orogenies) and was later carved open by Pleistocene glaciers. This last event uncovered some very interesting geology for us to enjoy. The mountain is England's highest peak at 964 metres. William Wordsworth loved it some centuries ago, and it is a staple with modern hikers. Cumbria's poet was born and lived in the area, and acquired his love of the natural world 'wandering lonely as a cloud' around the Lake District.

The caldera is also England's best known volcano, and thanks to its exposure and well layered deposits, the reconstruction of the sequence of events at its fiery birth is one of the most detailed in the world. Unlike most calderas, where the collapse occurs piston style within a ring fault, Scafell repeatedly sank in a very complex piecemeal manner down a network of faults. Since geology's early days it has been the type locality for the study of this rare kind of subsidence.

The Borrowdale volcanics that compose the peak show us the succession of events as an arc forms and welds itself to a continent as an ocean closes. Several kilometres of mixed volcanics boiled out of the Earth, starting with repeated andesitic lava flows (the usual arc rock), with the odd ash spewing explosion. Then came enormous explosive rhyolitic eruptions whose pyroclastic flows deposited a couple of kilometres of welded ignimbrites. An wide variety of deposits has been recognised over decades of study, giving us a high resolution dataset to add to in the future. These rocks repeatedly sank into the crust accompanied by massive earthquakes, forming the structural complexity of the nested calderas.

The original pretty volcanoes have now been eroded into dust, making them impossible to find, but the underlying structures are exposed for our edification. The area has been repeatedly surveyed and sampled, and still provides new insights into how such complex calderas form.

For a geologist the most informative places aren't always the most spectacular sights, but the knowledge we acquire from places like Scafell sure inspires poetry in my soul...


Image credit: Sean McMahon of StridingEdge.net

http://www.rocksafoot.com/lake_district.htm

http://www.ipplepen.net/community/south-west_geology/walks/snowdon-scafell.pdf
Our story on Krakatoa: https://www.facebook.com/photo.php?fbid=497025593691823&set=a.352867368107647.80532.352857924775258&type=1

Our past story on ignimbrites:
https://www.facebook.com/photo.php?fbid=441339665927083&set=pb.352857924775258.-2207520000.1363889393&type=3&src=https%3A%2F%2Ffbcdn-sphotos-h-a.akamaihd.net%2Fhphotos-ak-ash3%2F704740_441339665927083_610876046_o.jpg&smallsrc=https%3A%2F%2Ffbcdn-sphotos-h-a.akamaihd.net%2Fhphotos-ak-frc1%2F604106_441339665927083_610876046_n.jpg&size=1417%2C1063

Moeraki Boulders, Otago, New Zealand


These boulders, located on the southeast shoreline of New Zealand's south island, are concretions composed of calcite. Think of a concretion like a geological rubber band ball. It is composed of sedimentary rock with certain minerals acting as cement between the layers of sediment. Concretions are known for the spherical shape in which they form.

The Moeraki boulders, measuring up to 2m in width, are composed of calcite, and their "veins" consist of rare, late-stage quartz and dolomite. They were coated in a mud rock layer from the Paleocene era, a geological epoch that ended approximately 50 million years ago. However, the boulders pictured have been exhumed from their mudstone enclosure. The mineral rich cracks, or septarian veins, described earlier, have been developed over a period of several million years.

The origin of these boulders is predicted to be similar to that of an oyster pearl. On the sea floor, layer upon layer of material gradually covered a central core, such as a fossil shell or piece of wood. Aqueous minerals accumulated, and eventually concretions were generated through the process of lithification.

Today, the Moeraki Boulders are protected in a scientific reserve. Local legends believe that the boulders are vegetables that washed ashore from a canoe that wrecked on the coast of New Zealand several hundred years ago. Scientists continue to study these fascinating geological formations today, continuously learning more about their formation.


Image Credit: James McDonald

References:

http://www.genkin.org/cgi-bin/browse.pl/landscapes/beach-ocean-seascapes/nz-moeraki-boulders

Journal of Sedimentary Research:
http://goo.gl/IUt9n
http://goo.gl/Ag60Q

Lake Nyos


In the Northwest Region of Cameroon is a crater lake called Lake Nyos. It may look like an ordinary lake but in August 1986 this lake released a massive cloud of carbon dioxide killing over 1700 people and about 3500 livestock in the surrounding areas. 

A pouch of magma exists below the lake. This magma converts the water into carbonic acid by depositing certain amounts of carbon dioxide into the water.

The 1986 disaster was a result of a limnic eruption (commonly referred to as lake overturn). This is an extremely rare type of natural disaster. Small amounts of carbon dioxide gather together to form a large pocket until it suddenly erupts, releasing this CO2 into the surrounding areas. These eruptions have been known to cause small tsunamis as the carbon dioxide that rises displaces the water. It is believed, by many scientists around the world, that landslides, volcanic activity or any kind of explosions can trigger a limnic eruption.

To prevent another disaster, in 2001 a tube was installed to draw water from the bottom of the lake to the top. This method allows the carbon dioxide to escape in small, safe amounts. In 2011, another two tubes were added to assist in the degassing of the lake.


Image Credit:

http://s281.photobucket.com/user/Lampedusa/media/Cameroon_Lake_Nyos_2009/IMG_6442.jpg.html

Further reading and references:

http://vulcan.wr.usgs.gov/Glossary/Lakes/description_volcanic_lakes_gas_release.html

http://news.bbc.co.uk/onthisday/hi/dates/stories/august/21/newsid_3380000/3380803.stm

http://www.iomcworld.com/ijcrimph/files/v01-n01-01.pdf

http://www.nytimes.com/2001/02/27/science/trying-to-tame-the-roar-of-deadly-lakes.html

Barchan dunes


Wind-blown sand in the desert can sometime build up into huge dunes. Quartz sand grains bump against each other and become rounded and well-sorted (typically medium-fine grained). The wind pushes them up the dune slope, a process called saltation, and when they reach the crest of the dune they fall down the steep front slope of the dune in the lee of the wind. This leads to a classic dune shape. In the presence of strong and persistent prevailing winds the dunes' shape reflects the wind direction. Individual dunes may develop "wings" pointing downwind. Looking like some alien space ship, they migrate across the desert on their way.

The ones shown here really are alien, they are isolated dunes seen on the surface of Mars, in the Noachis crater.

These crescent-shaped dunes are called barchans. Today they can also be seen on Earth, gliding across the arid wind-swept plains of desert environments.

Barchan dunes can be preserved in Earth's rock record too. Red sandstones formed in ancient Palaeozoic continental interiors often show barchan features. A fine example are the massive dune-bedded New Red Sandstones from the Permian rocks of Arran, North West Scotland. The lee slopes of the dunes are preserved as fine bedding planes, curving asymptotically to the palaeo-horizon. When further dunes pass over the top of them, they create huge cross beds. Measurements of the orientations of sets of these dune beds can give an indication of the palaeo-wind direction. In places the dunes show evidence of lightning strikes, with fossilised "fulgurites" … centimetre-scale hollow tubes where a lightning strike has earthed to ground in the desert, instantly melting the sand and then quenching to silica glass. Fulgurites stand proud on the outcrops of the Corrie shore in Arran, where they are more resistant to present day erosion on the shoreline. Elsewhere, around the Arran coast similar Permian dunes show evidence of burrowing residents, precursors of today's scorpions.


Image: NASA/JPL/University of Arizona

http://www.arranmuseum.co.uk/Geology%20Pages/The%20Ages/the_permian_period.htm

http://jsedres.sepmonline.org/content/40/4/1226.abstract


METEORA – Halfway to Heaven


Possibly nowhere else on earth combines so rare a geological environment with a unique human settlement than the Meteora World Heritage site of Greece. The astounding rock pinnacles of Meteora rise ~500m above the Plain of Thessaly, an awe-inspiring view from afar. As early as the 9th century, hermitic monks climbed the rock spires, and began to live within erosional fissures in the formations. Between the 11th and 17th century, twenty-four monasteries were build atop the pinnacles, six remaining in use to this day.

The protection of the area as a sacred site has also provided centuries of environmental protection. Wandering among tens of rock spires, the sheltered areas below are micro-climates that harbor endangered species and endemic plant species that co-exist with traditional herding.

When we take friends and students to visit, we are always asked – how old are the spires of Meteora? This depends on what one interprets as their origin:
--The conglomerates that make up the pinnacles include cobbles that date to the oldest rocks found in Greece to date, 700 million years in age.
--The conglomerates themselves are Miocene in age, deposited about 21 million years ago.
--The terrain including the conglomerates was upraised, tilted about 12 degrees, and weathered to a level surface by about 700,000 years ago.
--Sometime since then, weathering of the conglomerates has preferentially eroded softer deposits and cracks, ultimately resulting in the formation of the spires. Probably the greatest erosion occurred with the aid of fluctuating weather conditions present during the ice ages, helped along by ice fracturing and interglacial rainy climates.
--Uplift and erosion continues to date, and the spires are still emerging from the landscape.

So, how old are the rock spires of Meteora? They are as old as the oldest rocks of Greece and as young as today.


Photo: Dina Ghikas

Refs: The Birth of Meteora, Rassios, Ghikas, Batsi @2013.
http://whc.unesco.org/en/list/455
http://meteora.com.au/about-the-association/about-meteora-greece/
http://www.greeklandscapes.com/greece/meteora/meteora.html

NATIONAL PARK WEEK IN THE US


National Park week begins Saturday, April 20 in the US and runs through April 28. All 401 national parks offer free admission this week. If you've wanted to check out one of the many preserved beautiful places, now would be the time to go. There is at least one national park in every state of the US. Several events are taking place at the parks throughout the week, including Volunteer Day on April 27.

Visit this link below to find a park near you:

http://www.nps.gov/findapark/index.htm


References:
http://www.nps.gov/npweek/
http://www.nationalparks.org/national-park-week

Image of Grand Canyon National Park, Imperial Point, North Rim, credit Amy McCullough

Saturday, April 13, 2013

A new story for the West Coast of North America


You know what? The West Coast of North America is a mess.

Well, maybe not from whatever perspective you’re thinking…my apologies to everyone I just (quasi-deliberately) offended, let’s be a little more specific. From a geologic perspective, the West Coast of North America is a mess.

If you look at the Andes Mountains, they are something of a clean story. For hundreds of millions of years, there has been subduction of an oceanic plate beneath South America, building the mountain range we see today. Oh, there are a few plateaus, there are a few volcanoes, but overall, it’s a thin mountain range that doesn’t intrude much into the continent.

Compared to that, the Rocky Mountains in North America are a nightmare. The Andes are about 500 kilometers wide, while the Rocky Mountains in places stretch nearly 2000 kilometers into the continent.

There are a number of causes for this huge damage zone in North America, but a big part of the puzzle is stuff that wasn’t always part of North America. Docked to the west coast of North America, there are a series of what are called “accreted terranes”. They have names like Stikina, Wrangellia, etc. These accreted terranes can be understood as ocean island chains that rode in on the subducting oceanic plate, slammed into the continent, but couldn’t make it down the subduction zone and wound up stuck where they arrived.

The image you see here is a beautiful shot from the National Park Service of a place in Wrangell-St. Elias national park (see the name?). The rocks that make up this shot are part of one of those accreted terranes. They rode in on a plate that was being subducted and stuck in place.

So, all of these terranes have been accreted in North America, but South America doesn’t have them. Obviously, that brings up the question “Why”?

New research published in the journal Nature tries to answer that question with a new model for subduction along the North American coast.

Once a plate is subducted, most of the evidence of that plate disappears. Piecing together a plate that has been subducted is an arduous challenge. Prior to this paper, the basic model for the Pacific Ocean involved 3 plates: the Pacific plate, the Kula plate, and the Farallon plate. The Kula plate is basically gone today, while there are a few fragments of the Farallon plate left, such as the Juan de Fuca plate currently subducting beneath Washington, Oregon, and British Columbia.

Although tracking down a subducted plate is difficult, there is one tool that might help called seismic tomography. Seismic tomography involves measuring the speed of earthquake waves very precisely as they travel through the Earth and using those speeds to understand the material the waves travel through. Subducted plates have a very distinct signature, so they can be fairly easily detected using this technique.

These authors performed seismic tomography underneath the Western U.S. and North Pacific Ocean and found something that seems a bit startling; extra plates! Underneath the North Pacific there are what appear to these authors to be multiple old, subducted plates, and there are other fragments of plates deep beneath North America.

If the authors’ identification of these plates is correct…that means there was more subduction happening in what is today the Pacific Ocean than anyone thought. Subduction within oceanic plates tends to create island arcs; the same type of material seen off the coast of Asia today in the Philippines or Japan, and the same type of material that was repeatedly accreted to North America.

This work might give an answer for why there is so much stuff stuck to North America. If the simple “Pacific-Kula-Farallon” model was much more complicated, if the Farallon plate was broken into many pieces with subduction zones in-between, that could create the island arcs that slammed into North America and became the accreted terranes. The authors actually give names to some of these subducted plate fragments: the Angayucham and Mezcalera plates. They could have even played a very important tectonic role; if they were attached to North America while they were being subducted, they could have applied a force to the continent that helped open up the Atlantic Ocean 200+ million years ago.

It’s an interesting story and one that will be worked on, particularly now that things have names (that always helps!). If the story is right, it’s one that can only be told using information trapped a thousand kilometers deep in the Earth’s mantle. From the surface, all we can see is that North America is a mess, but maybe for once the mantle has made things simpler and told a story we’d otherwise never see.


Photo Source, public domain photos provided by the National Park Service:
http://www.nps.gov/common/uploads/photogallery/akr/park/wrst/3BF3A378-1DD8-B71C-07A66530C4E13429/3BF3A378-1DD8-B71C-07A66530C4E13429-large.jpg

Full paper:
http://www.nature.com/nature/journal/v496/n7443/full/nature12019.html

News & Views summary:
http://www.nature.com/nature/journal/v496/n7443/full/496035a.html

Sciencedaily article:
http://www.sciencedaily.com/releases/2013/04/130403141402.htm

Augustine volcano


The almost perfect symmetrical Augustine volcano in Alaska is part of the Aleutian Island Arc. It is one of the most active volcanic belts in the world with approximately 130 volcanoes, of which 90 have been active for the last 10,000 years. Between 1957 and 1965 three earthquakes of around 9.0 on the Richter scale (9.2 in 1964, 9.1 in 1957 and 8.7 in 1965) have occurred in the Aleutian Arc, indicating that the area is very active.

Discovered in 1778 by Captain James Cook, Augustine had 10 major historic eruptions. Now 1260 m high, the height of its summit has changed frequently in the past due to dome collapse. Actually, almost every eruption Augustine’s dome collapses and subsequently a new dome is created. In 1883 a VEI 4 eruption (Volcanic Explosivity Index: http://volcanoes.usgs.gov/images/pglossary/vei.php) triggered a landslide that generated a tsunami of a possible 19m high.

Due to location of the volcano near a subduction zone, Augustine’s magma is mostly andesitic and rhyolithic in nature although during the 2006 eruption basaltic magma was also produced. Eruptions at Augustine usually start with an explosive phase that lasts for days or weeks and is followed by an effusive (characterized by effusion of lava) phase which could go on for months.

The last major eruption occurred in 2006 and was very well monitored, which could help volcanologists in their future research. Aside from minor earthquake swarms in 2007 and slight degassing from 2008-2010 the volcano has been quit. Nonetheless, it was selected as Alaska’s potentially most hazardous volcano by the USGS.


Photo: Cyrus Read, Alaska Volcano Observatory. Augustine volcano during the 2006 eruption as viewed from the ship M/V Maritime Maid.

References and further reading:
http://www.volcano.si.edu/world/volcano.cfm?vnum=1103-01-
http://www.avo.alaska.edu/volcanoes/volcinfo.php?volcname=Augustine
http://www.gi.alaska.edu/ScienceForum/ASFO/008.html
http://www.gg.uwyo.edu/aleutians/



Lago di Bolsena - A sacred volcanic lake?


The oval Lago di Bolsena is a 16 kilometer wide lake-filled caldera. The caldera is the largest of the Volsinii volcanic complex which also contains the Latera, Vepe and Montefiascone calderas. Bolsena was created 370,000 years ago during a large eruption; its deepest point of 151m is right in the middle. The islands of Bisentina and Martana were formed during underwater eruptions shortly after this. Roman historic sources mention volcanic activity (flames shooting up) in 104 BC after which the volcano has been dormant.

It is evident that the lake was of importance in ancient times since many Etruscan and Roman sanctuaries surround the lake. However, in connection to the lake as a sacred place especially the Etruscans are of interest. The Etruscans lived in Italy from approximately 700 BC to the 1st century BC when the Roman Republic was assembled and the Etruscans mysteriously disappeared or perhaps blended in with the Romans. The area around Lago di Bolsena was also inhabited in proto-Etruscan times.

An important aspect of Etruscan religion was that signs of divine power could be manifested trough natural phenomena as mountaintops, riverbeds, ancient groves and lakes. Also, natural phenomena as lightning, thunder and earthquakes were seen as signs from the gods. Not only did the Etruscans build sanctuaries around Lago di Bolsena, they also constructed a remarkably large amount (over 20!) of necropoleis. Necropoleis were cities for the dead in which in some cases rows of tombs were arranged as streets and the tombs were decorated as houses.

The necropoleis, the material culture found at sites surrounding the lake as well as deities that were worshipped at sanctuaries suggest a connection to a cult of the dead. Thus, it is possible that the lake was seen as an opening to the underworld in Etruscan times. Underwater research and discovery of Etruscan offerings in the lake could provide more evidence for this hypothesis. On the other hand, other sanctuaries and deities connect the lake to water and fertility cults. Historical sources have suggested that the myth of the fire-god Volta was established at the lake. However, reliable evidence for this is difficult to find.

Image: Copyright Michele Ricci. Aerial view of Lago di Bolsena with the islands Bisentina en Martana.

References:
http://www.socgeol.info/altro/documents/guide_books/P09.pdf
http://www.volcano.si.edu/world/volcano.cfm?vnum=0101-003
Edlund I.E.M., The gods and the place. Location and function of sanctuaries in the countryside of Etruria and Magna Graecia. Stockholm, Svenska Institutet I Rom, 1987.

The formation of Earth’s atmosphere


Almost all of life on Earth is sustained by oxygen. Comprised of two oxygen atoms, the gaseous form of oxygen is held together by a strong double bond (O2). Oxygen is crucial for respiration, a metabolic process whereby organisms convert carbohydrates into energy, with carbon dioxide being a by-product. 

4.6 billion years ago, when Earth was formed, it consisted of nothing more than a rocky sphere, surrounded by an atmosphere of hydrogen and helium. Due to the lack of a magnetic field it is believed that this early atmosphere was subjected to harsh interstellar winds and radiation and it dissipated into space. The formation of many volcanoes, which accompanied the establishment of Earth’s crust, caused large amounts of gaseous compounds to be ejected into the atmosphere, such as ammonia, carbon dioxide and water vapour. The introduction of these new gases, however, would not support the modern diversity of life that we know.

3.3 billion years ago, bacteria arose, and cyanobacteria, a specialised type of bacterium began to dominate earth’s surface. Cyanobacteria are special in that they contain the same green, light-trapping pigment as plants: chlorophyll. The success of cyanobacteria set the stage for the evolution of life and the development of the atmosphere we know today. Chlorophyll is a pigment that allows an organism to harness the energy from the light of the sun by converting carbon dioxide into usable carbohydrates, a food source. Although requiring carbon dioxide, the process produced oxygen. Cyanobacteria became so prolific that they changed the entire composition of the earth’s atmosphere to an oxygen-rich one which would later be the foothold for life. Currently earth’s main atmospheric constituents are 21% oxygen and 78% nitrogen as well as smaller amounts of other gases such as water, argon and carbon dioxide.

Will life be responsible for the next major atmospheric transformation?


Acknowledgements:

Kashmira Raghu

Image credit
http://www.geograph.org.uk/photo/3002724

References and Further Reading:

hyperphysics.phy-astr.gsu.edu/hbase/biology/celres.html
www.universetoday.com/26659/earths-early-atmosphere/



VASQUEZ ROCKS: The World’s Poster Child for Tectonics


Located about 40 km north of Hollywood, California and 15 km southwest of the San Andreas Fault are some the world’s most recognizable rock formations; these are the rugged San Vasquez Rocks. 

The stones themselves are rather unremarkable on first sight: basically, they’re sandstones of about 25 million years of age with a fairly strong tilt, dipping away from a larger anticline. Okay, there are many Oligocene rock formations scattered around the earth with similar tilts and folds. The Vasquez Rocks contain clues to their depositional environment with ample mudcracks, ripple marks, crossbeds and graded beds – always geological fun to find in the field, but by no means rare phenomena. Their rugged topography is due to differential erosion; okay, so what else is so outlandish that these rocks deserve such special tectonic recognition?

The North American Plate was once separated from the Pacific Plate by another plate named the Farallon. About 25 million years ago (yes, the same age as the Vasquez sediments), the Farallon plate was over-ridden by the North American Plate and subducted to great depths (you can see a series of cross-sections of these tectonic motions on our recent Earth Story post:http://tinyurl.com/c6jq4yj ). Below and within the sedimentary rock formations of Vasquez are basalts; these basalts intruded into the broken cracks and fractures forming above this subducting plate. The relentless movement of the North American Plate continued, and continues still, with the San Andreas fault zone now taking the place of a plate contact between the North American Plate and Pacific plate. The early days of the Vasquez formation records this phase of geologic history, and its sedimentary record includes several mega-cycles of uplift, erosion, and deposition as the new plate margin evolved. A Google Earth view of the Aqua Dulce area shows the anticlinal form of the outcropping Vasquez Rocks as a product of motion between the North American Plate and Pacific Plate along the San Andreas fault.

Perhaps you’re scratching your head at this point, and wondering where on earth you've seen these rocks before? In addition to seeing them “on earth,” they’re also highly visible amongst alien planets. Being conveniently accessible to movie and TV studios, as well as being fairly easy for actors and camera crews to scramble over, the Vasquez Rocks are filmed so frequently as to have become an icon for several Star Trek episodes, Bonanza, and have been used as stunt doubles for Tibet, Egypt and other desert countries.

So, the next time you’re watching the TV and the Vasquez Rocks are featured, remember that they’re not famous only for the battle between Captain Kirk and the Gorn, but also as a geologic landmark known the world over.


Photo: from Wiki Commons by a photographer who identifies him/herself as hear2heal:http://en.wikipedia.org/wiki/File:SUNSET_ROCKS_San_Andreas_Fault.jpg
Thanks to the Tectonophysics Page for reminding me how special these rocks are:https://www.facebook.com/TectonophysicsOfficial?fref=ts


More information on the World’s Most Highly Recognizable Tectonic Locality:
http://geology.campus.ad.csulb.edu/VIRTUAL_FIELD/Vasquez/vasqmain.htm
http://www.edmar-co.com/adriano/field/VasquezRocks/vasquez.html
http://www.scvresources.com/geology/aguadulce/

Wednesday, April 10, 2013

FORESHADOWING EARTHQUAKES: FORESHOCKS AND EARTHQUAKE PREPARATION


Many major earthquakes are often preceded by foreshocks. Foreshocks are smaller vibrations that occur prior to some highly destructive earthquakes. Because not every earthquake has accompanying foreshocks, scientists have found it challenging to use foreshocks as an earthquake prediction tool. A new study published in Nature Geoscie...nce has revealed more information about these vibrations.

Earthquakes usually occur at plate boundaries. Some plate boundaries feature plates that slowly slide past each other over time. When the plates reach points along their boundaries where the rocks become stuck, pressure builds. According to the study, these areas eventually break under enough pressure from the slow movement and release foreshocks. Large, more violent earthquakes are produced when the break in the rock occurs more quickly.

North Pacific Ocean-based earthquakes larger than a magnitude of 6.5 that were generated at depths shallower than 50 kilometers, and occurring between 1999 and 2011, were analyzed. This region was chosen because Japan, Mexico, Taiwan and the United States have many instruments set up in this region designed for earthquake detection. Half of the 62 earthquakes studied occurred at plate boundaries with subduction zones (where one more dense plate slides beneath another less dense plate) and strike-slip faults (where plates slide past each other); the other half of the data came from intraplate earthquakes, which occur far away from plate boundaries.

About 20 days before a major earthquake, the researchers noted that the seismicity (earthquake distribution in a certain location over time) increased due to the presence of foreshocks. This increase in activity continued until a major earthquake occurred. There was no such predictable pattern of activity for intraplate earthquakes, since foreshocks here occurred much less frequently.

Foreshock events such as the ones examined in the Nature Geoscience study were noted prior to a major earthquake in Izmit, Turkey in 1999 that killed 17,000 people and left 500,000 homeless. According to a study published in Science, the magnitude 7.6 earthquake along the North Anatolian fault was preceded by bursts of seismic activity in the form of foreshocks that increased over time. The crust in the region had undergone a period of slow sliding for 44 minutes prior to the earthquake, generating the foreshocks. The sliding accelerated in the two minutes before the major earthquake occurred.

Foreshocks do not create larger earthquakes, but may help scientists to understand the process of earthquake generation. The prolonged activity generated by foreshocks preceding major earthquakes can help us to better prepare for potential earthquake events.


Photo courtesy of Kyodo News/Associated Press. The photo depicts some of the damage incurred from the magnitude 9.0 Tohoku Earthquake and subsequent tsunami near the east coast of Honshu, Japan. Studying seismic zones such as this one for foreshocks could help scientists become more aware of potentially hazardous future earthquakes.

References:
http://www.nature.com/ngeo/journal/v6/n4/full/ngeo1770.html#access

http://www.livescience.com/28134-earthquake-slow-slip-foreshocks-found.html

http://www.sciencemag.org/content/331/6019/877

http://www.ouramazingplanet.com/945-earthquakes-foreshocks-faults.html

http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/#summary


25 years under the volcano


The Alaska Volcano Observatory (AVO) celebrates its 25th anniversary this month, marking a quarter century of volcanological studies and work aimed to provide warnings and forecasts of volcanic hazards. "Since 1988, AVO has responded to over 70 eruptive events from Alaska’s 52 historically active volcanoes," said John Power, USGS geophysicist and scientist-in-charge of ...AVO. "Many of these eruptions affected local and international air traffic, oil production, the fishing industry, municipalities, businesses, and citizens."

As explained in a post here earlier today (http://tinyurl.com/cbrpsjt) volcanic ash is a significant hazard, with a record of affecting air transport across Alsaska on a number of instances. Perhaps the most dramatic incident was played out in December 1989, not long after the AVO was established. A KLM passenger flight passed through an ash cloud spewed 9000 m into the air by Mount Redoubt. All engines failed, and the plane dropped 3000 m in four minutes before they could be restarted, the flight landing safely at Anchorage. It is just this sort of event that AVO now seeks to plan against.

Seismic monitoring of the most potentially dangerous volcanoes is one of their principal tools. Often the first indication of an impending eruption is an increase in earthquake activity. Generally, a network of six to eight seismometers are positioned around a volcano. The AVO successfully made advance warnings a number of times during the 1989-90 eruptions of Redoubt Volcano, as well as for two of the three eruptions of Mount Spurr in 1992.

The volcanoes of Alaska are associated with the north-eastward subduction of the Pacific oceanic plate beneath the North American continental plate. As hydrous minerals within the subducted oceanic crust follow their path into the mantle beneath North America the increased temperature and pressure dehydrates them. The water and other volatiles released from the subducting slab percolates into the mantle wedge above, lowering the melting point of the rocks, resulting in partial melt and volcanism at the surface. Here it results in volcanoes that form the the Aleutian Island chain extending west-southwest from the Alaska mainland. These volcanoes are strung out on the continental plate side of the subduction trench, set back from it, a phenomenon reflected all around the Pacific ring of fire.

Image: Augustine during eruption, January 18th 2006. A vigorous steam plume travels many miles to the east at an approximate altitude of 8500 ft. The cloud deck is at approximately 4000 ft. (Credit: Jill Shipman, image courtesy of the AVO/UAF-GI)

Links:
http://news.discovery.com/earth/rocks-fossils/alaska-volcano-observatory-25-years-130403.htm

http://www.avo.alaska.edu/
There was an error in this gadget