The Earth Story

What’s under your feet? Have you ever dug into the ground and noticed that, even in the midst of a drought, the soil is still damp? If you’re reading this and you don’t own your own lawn, please do not attempt to find out for sure. Also please do not test this on the neighbor’s lawn. (Just making sure).

Graphic texture - intergrowth of quartz and feldspar. The planes of atoms inside the different minerals control the growth directions and make it start looking like writing.

Graphic granite.

Looking like an ancient Sumerian clay tablet engraved with cuneiform writing, this rock formed during the crystallisation of a magma. As the temperature dropped below a tipping point, the pink feldspar and grey quartz separated out from each other in a process called exsolution, and grew simultaneously at a constant gentle rate to form this texture. Graphic texture is not limited to the magmas of silica rich granites (known as leucogranites, the prefix leuco indicating siliceous richness), but is most commonly found in them, particularly in pegmatites, the last silica and rare element rich remnants of already crystallised granites, accounting for the usual absence of any dark minerals such as mica or hornblende.

Loz

Sample from Knickel's feldspar quarry at Bedford, NY, originally in the USGS Petroographic Collection, now in the Smithsonian's.

Image credit: Ken Larsen/Smithsonian Institution.

A paper on its origin: http://www.minsocam.org/ammin/AM71/AM71_325.pdf

http://www.mindat.org/min-39400.html

Reconstructing a cross section This gif was created by a geologist to show how she can take maps and cross sections of deformed, folded, and faulted layers of sedimentary rocks and undeform them to recognize the original stratigraphy and to calculate how much deformation has happened.

The lower pink layer serves as a breakaway fault, something we call a “decollement”, a weak layer that allows the rocks above it to break loose. As the rocks are squeezed from the side, that layer forms a flat fault that breaks upward repeatedly, stacking sedimentary layers into a pile called a Duplex Structure and folding the rocks above.

When folded and faulted layers are mapped in the field, cross sections can then be “undeformed” by pulling apart the fault deformation. A key check in these exercises is to make sure the length of each bed stays constant across every step. A setup where this works is called a balanced cross section and it indicates that the layers broke and bent along the faults in a normal pattern, as seen here - if the lengths of the beds are changing, then either you’ve misunderstood the tectonics or something has happened internally to disrupt one of the sedimentary layers. 

If a geoscientist can complete the calculation, as done in this animation, she can figure out how much the area has been shortened. In this case, the total shortening shown in this gif is about 40% (the folded package is about 60% as wide as the original package).

Image credit: aace “the period” geo “at” insert term for google’s standard mail system then a normal dot and then follow that up with the com, and I’m writing all this deliberately to avoid automated email finding programs.

Read more: http://pubs.usgs.gov/of/1998/ofr-98-0034/BC.pdf http://bit.ly/1VCL0mS http://www.neckers.siu.edu/pinter/pdf/suppeexercise.pdf

Meet a Passive Margin

Have you ever wondered what is buried beneath a seashore? This schematic cross section diagram of a passive margin shows exactly that for many common shorelines, the type found away from active plate boundaries.

Passive margins form when continents rift apart and the remnants are illustrated in this cross section. The portion labeled part 1 is the stable part of the continent deep inland; unfaulted and away from the rift. The parts labeled 7 are normal faults; these form when the continent is pulled apart and the land begins sinking down to fill the gap.

All these segments are floating on part labeled 4, the mantle that is still solid but is hot enough for minerals to flow. Label 2 shows oceanic crust, formed when the continents on both sides of the rift have pulled apart so much that the mantle in-between melts and fills the space. Label 5 is, of course, the ocean.

Label 6 is a common feature that shows up in seismic reflections. At the boundary where sediments begin to drape on the chunks of continent below, there will be layers that dip towards the continent as their surface has rotated while moving on the faults. Finally, label 3 shows the sediments that erode from the continent and are deposited near the ocean’s edge.

Geologic settings like this one, formed near the ocean edge, are some of the most heavily populated in the world. Cities are commonly built on those sediments when they’re on shore and the offshore portion commonly holds oil and natural gas reservoirs. The legacy of that rifting can be found in the behavior of those cities; wide-scale subsidence or sinking of cities can happen as the sediments and faults deep below continue moving and settling downwards.

-JBB

Image credit: https://wikimediafoundation.org/wiki/File:Passive_margin.svg Read more: http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry26.html http://www.hs.umt.edu/geosciences/faculty/hendrix/g100/L19.html

Snowflake morphology as a function of crystallization conditions, via here.

What you are seeing here is an example of solargraphy; a pinhole photographic technique using long exposures and photosensitive paper. This technique is commonly used to track the movement of the Sun across as landscape, as is the case in these 4 images. Here we have solargraphs taken from different latitudes across Europe, for a period of around 120 days in late autumn and winter. These images demonstrate effectively that the Sun traverses the sky at different elevations dependent on latitude. We can note that the elevation of the Sun pathways in Bodo, Norway (approx. 67 degrees north) and Oslo, Norway (approx. 60 degrees north) are quite close to the horizon. Contrastingly, the solar paths for Ondrejov, Czech Republic (50 degrees north) and for Palma de Mallorca, Spain (approximately 39 degrees north) are much higher in the sky.  -Jean  Image courtesy of Maciej Zapior

Geologic Clock It really is hard to fathom the scale of geologic time. The recent TV series “Cosmos” used one favorite trick, compressing the history of the universe into a single calendar year. This graphic is a different setup, showing the age of the Earth on the face of a clock. If we tried to picture how long our lives are on a geologic timeline, the bit of the planet’s history that we occupy barely registers. The Earth is right around 4.56 billion years old. If that length of time were compressed into 12 hours, then each second on the clock face would represent over 100,000 years. 1 second ago on this clock face was during the start of the last glaciation. During that last second, 90,000 years of glaciation and the rise of the entire human civilization took place. Many of the major events in evolution are shown on this clock. The appearances of land plants, land animals, animals in the oceans with hard parts, and multicellular life all are shown. There Yesterday, we referred to an era of time in the Precambrian covering nearly a billion years; that is ¼ of this clock. Even geologists are used to thinking of time periods like the Cambrian as a long time ago, but the last 540 million years is but 10% of this clock, equivalent to an hour and 20 minutes. Some quibbles could be taken with exactly where the dates are hung on this clock, but the mental exercise of comparing all of human civilization taking place in a tenth of a second is hopefully an interesting way to think about geologic time. -JBB Image credit: (public domain label):http://en.wikipedia.org/?title=Talk%3AGeologic_Timescale#mediaviewer/File:Geologic_clock.jpg Yesterday’s post: https://www.facebook.com/photo.php?fbid=706357562758624

SUPERVOLCANO We’ve all heard about the destruction power of supervolcanoes and how they could cause years of volcanic winters. But what truly classifies a supervolcano? Volcanic eruptions are generally expressed in cubic kilometers. For an eruption to be considered a supervolcano the eruption must be greater than 1000 cubic km. This classifies the eruption as an 8 on the Volcano Explosivity Index (VEI), the scale that measures the explosivity of an eruption. The three most common criteria used to classify in this scale are the volume of ash produced, the lava ejected, and the height of the column. When trying to classify an ancient eruption that happened thousands to millions of years ago, scientists use the area that the volcanic tephra had fallen and calculate the volume of the ejected material from the deposits of that eruption. The most recent supervolcano eruptions were tens of thousands of years ago; specifically 27,000 years ago and 74,000 years with the Taupo supervolcano in New Zealand and the Toba Volcano eruption in Sumatra, Indonesia. For information about these previous supervolcano eruptions, see the links to our previous posts below. Even though a volcano can be considered a supervolcano, not all of its eruptions have to be an 8 on the VEI. For example: the last two eruptions of the Yellowstone supervolcano have been less than an 8 on the VEI, (less than 1000 cubic km of material ejected.) The regional effects created by a supervolcano can be devastating. Falling ash from the eruption can go on for years. If that were to happen today, very few planes would be flying during that time due to the ash in the atmosphere. There would also be years to decades long changes to the global climate. In geologic time however, there would be no long term effects, even though multiple decades might seem long to us. Areas surrounding the volcanic explosion are most intensely affected, but eruptions of this magnitude often affect the entire globe with ash circling the atmosphere for many years. Eruptions of this magnitude often form calderas. For more information on what is a caldera, see our earlier post, link below. Now that I have thoroughly scared you, your next question might be “What are the chances of a devastating eruption like this one?” According to the USGS: “Exceedingly small.” These eruptions happen thousands of years apart. While we can’t predict the eruptions, the volcanoes can show warnings years before they erupt. Over the past 25 years, volcanic prediction technology has advanced greatly. Scientists think that a supervolcano eruption would show signs of an impending eruption months to years in advance. Precursors to major eruptions would include numerous strong earthquakes and rapid ground deformation. These usually take place days to weeks before an eruption. So, should you worry? Probably not. Scientists around the world are currently monitoring the known supervolcanoes. This includes measuring the earthquakes in those areas, ground deformation, stream flow, and selected stream temperatures all of which can be affected by supervolcanoes. Now relax. There won’t be any supervolcano eruptions any time soon. Probably. (Knock on wood.) -Claire Our Taupo Supervolcano post: https://www.facebook.com/photo.php?fbid=375328312528219&set=a.352867368107647.80532.352857924775258&type=3&theater Our Toba Supervolcano Post: http://www.facebook.com/photo.php?fbid=358724264188624&set=a.352867368107647.80532.352857924775258&type=3&theater Caldera Info: http://www.facebook.com/photo.php?fbid=389267087801008&set=a.352867368107647.80532.352857924775258&type=3&theater More Supervolcano info: http://volcanoes.usgs.gov/volcanoes/yellowstone/yellowstone_sub_page_49.html

Orogeny We often talk about orogenies and orogenic events here at The Earth Story, but what is an orogeny? An orogeny describes a series of forces and events leading to the severe structural deformation of the Earth's crust and uppermost mantle (also known as the lithosphere). So in simple terms, an orogeny is a mountain building event. Occurring at the boundary where continental plates meet (though they can occur where a continental plate overrides an oceanic plate), the response to orogenic forces is basically a "crumpling" of the rock, leading to highly deformed and metamorphosed areas of rock, which extend far underneath the resulting mountain belt, and far beyond the front. The basic tectonics behind an orogeny is a subduction zone causing two continental plates to collide (or as discussed above it can occur at the meeting between an oceanic and a continental plate). The event can cause a number of tectonic features, including: volcanoes, mountain building, island arcs, back arc basins and of course earthquakes. A spectacular example of an orogeny is process is the Himalayas. This orogeny has been caused by the Indian plate and the Euro-Asian Plate, but all mountain belts have at some stage been part of an active orogeny. Once an orogenic event has completed the mountain building stage, the tectonics don't just stop! The mountain chain normally continues to uplift, and at the same time eroded; over millions of years this leads to spectacular views of metamorphic rocks and tectonics. -LL Links to some fantastic descriptions of orogenies and specific orogenic events: http://serc.carleton.edu/NAGTWorkshops/structure/visualizations/orogeny.html http://web.usal.es/~jrmc/MartinezCatalan/documents/AbatiEPSL99.pdf http://www.utdallas.edu/~rjstern/pdfs/PanAfricanOrogeny.pdf http://digital.library.adelaide.edu.au/dspace/bitstream/2440/23647/1/hdl_23647.pdf Image is a graphic depicting The Acadian Orogeny (although stages are relatively similar for most orogenic events). Thanks to the Northern Virginia Community College.

HOW DO CURVY MOUNTAIN BELTS FORM? Mountain belts form most commonly from the collision of one or more tectonic plates. Mountain belts are typically straight but there are many examples of arcuate (curved) mountain ranges. The Appalachian range in Pennsylvania, the Rocky Mountains in central Montana, the Blue Mountains in Oregon, the Bolivian Andes of South America, and the Cantabrian Arc in Spain and northern Africa are curved mountain belts. There is still debate over what causes these belts to be curved; whether they were originally straight and then bent or whether they were in their present state when uplifted. The thicknesses of the rock units involved are also debated. Some researchers suggest that the ranges are made of thin (several kilometres) slices of crustal rocks, while others have argued that some of the curvy ranges involve the entire thickness of the lithospheric plates (30 to 100 km thick). Read more about the bending of lithospheric plates here: http://www.geo.cornell.edu/geology/classes/geol388/pdf_files/platbend.pdf The methods scientists use to answer these questions include comparing the orientation of fault planes and joints in rocks, records of ancient magnetic field directions in rocks and the timing of deformation and uplift of mountain belts. By using these methods, Dr. Gabriel Gutiérrez-Alonso and fellow researchers from Spain, Canada, and the United States have found that the curved pattern of the Cantabrian Arc in Spain was produced by the bending of an originally straight mountain range. The Cantabrian Arc formed from the collision of Gondwanaland and Laurentia to produce Pangea. The age of the bending of the Cantabrian Arc is confined to between 315 and 300 million years ago. This age range was compared with ages of igneous activity and uplift in the region; it seems changes in the deeper mantle portion of the lithospheric plate in the area are coeval and probably linked to the rotation of the Cambrian Arc. -TEL http://www.sciguru.com/newsitem/14325/curvy-mountain-belts;http://sciencedaily.com/releases/2012/06/120629211929.htm;http://www.geosociety.org/gsatoday/archive/22/7/article/i1052-5173-22-7-4.htm Diagram from G. Gutiérrez-Alonso et al. Diagram caption: (A) Block diagram depicting the effect of lithospheric bending around a vertical axis and the resultant strain field (modified tangential longitudinal strain). Strain ellipses depict arc-parallel shortening in the inner arc and arc-parallel stretching in the outer arc. Note the different behavior of the mantle lithosphere in the inner and outer arcs and the increase in thickness of mantle lithosphere below the inner arc and thinning below the outer arc. (B) Snapshot illustration of arc development starting with a linear belt resulting from a Gondwana–Laurentia collision. (C) Second snapshot illustrating oroclinal bending, which causes lithospheric stretching in the outer arc and thickening beneath the inner arc (Gutiérrez-Alonso et al., 2004). (D) The final stage of oroclinal bending, depicting delamination and collapse of thickened lithospheric root beneath the inner arc, replacement of sinking lithosphere by upwelling asthenospheric mantle, and associated magmatism in the inner and outer arc regions. (E) Two tomographic views of the analogue modeled mantle lithosphere geometry after buckling around a vertical axis where the lithospheric root is developed under the inner arc (top—frontal view from the concave part of the model; bottom—view from below); 3-D coordinate axes given. (F) Tomographic 3-D image of the delaminated lithospheric root obtained with analogue modeling; 3-D coordinate axes given.

LUSI NIGHT Earlier we posted about Cyclone Lusi and its path of destruction through the South Pacific:http://on.fb.me/OckVhV. Twitter user @bobsyauncle thought the way the winds were shown in the graphic by earth.nullschool.net was pretty enough to be combined with Vincent Van Gogh’s ‘Starry Night’. The result? Lusi Night! -TEL https://twitter.com/bobsyauncle/status/444267491179499520/photo/1 Original graphics were acquired from http://earth.nullschool.net/#current/wind/surface/level/orthographic=-181.69,-25.62,281

If the Moon were the Size of a Pixel… Oh my gods! This post is not for the faint-hearted. If however you really want to go out and explore space, to boldly go where no one has gone before… be forewarned that the main foe on your journey will not be alien creatures, solar flares, or even stray asteroids. You will yearn for the company of a stray asteroid, believe me. Your mission statement will be to boldly overcome pure complete total boredom. Josh Worth has created the universe’s most tedious tour of the solar system. It’s utterly brilliant. I dare you to take the trip from the Sun to Pluto athttp://joshworth.com/dev/pixelspace/pixelspace_solarsystem.html Everything on this trip is to absolute scale (though some planetary positions have been approximated, since they are variable). The scale is lunar: that is, the moon’s size is reduced to a single pixel to establish the scale. Everything else on this epic journey is at the same scale – distances, sizes of planetary bodies, and the blackness and emptiness of space. Start there at the Sun, it’s quite huge on this scale, and you feel wow, this should be fun! then scroll through the solar system, scroll, scroll, scroll, and eventually you’ll get to… Mercury. Only the truly brave boldly scroll further… I thought I had an understanding of how LARGE is space, and that it’s a pretty empty place. After all, I took astronomy and space physics. I watch all the Nova documentaries. But by the time I scrolled my way to the Earth, I was beginning to understand that the solar system is even LARGER than I’d estimated, and far more empty. I was beginning to comprehend the true scale of the size of space. The true insignificance of the dust mote we call the Earth. And this is the brilliance of this clip. I don’t know if all computers scroll at the same rate, but mine is about 5 million km/second. That is, on this scale I’m surpassing the speed of light (299 792 458 m / s, or ~300,000 km/sec). Even this is not fast enough. I tried; indeed I did. I considered it my scientific duty. Science is, after all, 99% tedium. But I did have to take a bathroom break somewhere around the asteroid belt, and somewhere past Neptune, someone in the house flicked on the microwave which caused the circuit breaker to trip, the computer to shut off, and… when I went back on, I found I cannot skip back to where I’d left my journey unexpectedly, but would have to start over again. Alas, I guess I am not made of the stern stuff needed to explore the universe. Are you?  How many of you can scroll scroll scroll all the way to Pluto?  Annie R With thanks to Josh for creating this tedious, boring, and utterly brilliant clip. PS – somewhere around Jupiter as I recall, he comments that were the Sun the proton of a hydrogen atom, then on this absolute scale it would take another five screens PAST the distance of Pluto to find its electron. Suddenly, not only is space huge and empty, but I’m feeling vastly vacant as well.

View the ocean's currents. Every five days the University of Seattle's OSCAR project uses NASA satellite data to produce a projection of global ocean currents onto a grid map of the Earth viewable online, with archives going back to 1992. The data is used to analyse changes in currents over time, assess some of the effects of climate change, and provide a growing database against which future changes can be compared. The diagrams show currents averaged for the top 30 metres of the oceanic surface. This photo shows the Gulf Stream bringing warmth to Northern Europe from the Caribbean sea. Loz Image credit: NASA OSCAR project: http://www.esr.org/oscar_index.html The world's currents: http://earth.nullschool.net/#current/ocean/surface/currents/orthographic=-195.00,0.00,303 http://podaac.jpl.nasa.gov/ www.oscar.noaa.gov

We talk a lot about thunderstorms while they are in their “prime”, but what happens to a thunderstorm when they die? A rare phenomenon known as a heat burst is associated with dying thunderstorms, heat bursts usually occur at night and are characterized by a rapid increase in temperature and gusty, sometimes even damaging winds. Temps during a heat burst are known to exceed 90 °F (32 °C) and some unofficial reports have recorded temps exceeding 120 °F (49 °C). -NF http://www.theweatherprediction.com/habyhints/341/