The Earth Story

Aurorae shimmering over the steaming Earth Geysers and hot springs occur when water gets superheated by underlying magma chambers filled with molten rock. A convection cell of sinking rainwater and rising superheated H2O establishes itself, dissolving and redepositing minerals as it cycles up and down through the energy gradient. Like the glowing plasma above which is fuelled by charged particles fired off from the sun interacting with our magnetic field and the particles at the tenuous edge of the atmosphere, the energy source is removed from the visible effect, but both reveal the essential interpenetration of all things and energies that make up the beautiful universe (or multiverse) that we live in. Loz Image credit: Stéphane Vetter https://apod.nasa.gov/apod/ap131031.html

What is happening to the Earth’s Magnetic field? Is it the next disaster? Over recent weeks, you may have seen articles about the Earth’s Magnetic Field, specifically focused on the “South Atlantic Anomaly” – a zone in the Atlantic Ocean where the magnetic field is notably weaker than it is elsewhere on the Earth. This zone has been growing larger and the intensity overall weakening over the past few years. Is this something we should be concerned about? The best answer to that comes from geophysics and geology, which allow us to know how the magnetic field is generated and how it has behaved in the past.

What is a plate? Often on this page you’ll hear me talking about plate motion, plate tectonics, or plate collisions as processes that form mountains and reshape the Earth’s surface. But, what exactly is a plate, to a geologist?

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The morning glory over Cape St Vincent Also known as roll clouds, no one knows exactly how they form, but these tubes can be thousands of kilometres long and (often but not always) appear in the early morning along coastlines. It is thought to be due to a circular air current pattern creating tube like zones of condensation when strong breezes blow inland off the sea. They are accompanied by turbulent winds and surface jumps in atmospheric pressure. We get them every now and again in my home city of Montevideo, forming along the length of the Rio de la Plata just offshore, sometimes in long perfect tubes, sometimes amazing ragged wind sheared formations. This one was shot off the coast of Portugal. Loz Image credit: Vanda Rita via APOD

Swim with a Viking

The ocean is in constant motion. Forget about waves and tides here - in the grand scheme of things, they don't move that much water. We're talking about the 'global ocean conveyor belt'. A vast, global circulation system driven by the Sun and the spin of the Earth, which moves water from surface to the deep and back, and between every ocean basin on the planet.

This cycle is big and slow. Water that sinks into the depths in the North Atlantic won't come back to the surface for 1300-1500 years. This means that if you were to go swimming in the North-East Pacific, there's a fairly good chance that the last person in that water was a Viking. The image above shows us how old the deep water (2500 m) is in different parts of the world.

This system is more correctly called 'thermohaline circulation'. It is mainly driven by changes in temperature ('thermo' = heat) and salinity ('haline' = salt), which determine where water sinks, and where it rises to the surface. Once water sinks or rises, the path of the currents is determined by the rotation of the earth, and the shape of the continents. This global cycling of the oceans determines long-term climate, fuels the productivity of the ocean, and maintains global chemical cycles that keep the planet in balance.

This circulation process is... complex, but here are the basics: the density of seawater is determined by its salinity and temperature. Hot, fresh water is less dense, and cold, salty water is more dense. Imagine a bucket of seawater at the equator - the hot sun will heat it up, and cause a lot of evaporation, which will make the water more salty, and more dense. However, if you poured your bucket into the ocean in the Carribean, it wouldn't sink because it's warmer, and therefore less dense, than the water underneath. Now take that bucket on a long journey North, let's say Iceland. It's much cooler up there, and your warm, salty bucket of water gets cold. If you poured this cold, salty water into the sea off Iceland, it would sink. This cold, salty liquid is about as dense as seawater can get.

Once it gets moving, the water has momentum. Dense, sinking water pushes the water underneath out of the way, and shunts it along the ocean floor, directed by the shape of the ocean floor, and the spin of the Earth. As the Earth spins, there's a tendency for water to be 'left behind' - think about what happens if you suddenly move a glass of water (don't try this near your computer!) - the water wants to stay still, and sloshes up the side of the glass. This is basically what happens to an ocean current: as a current moves South in the Northern Hemisphere, it will gradually bend to the right, as the movement of the Earth 'leaves it behind'. This is known as the Coriolis Effect, [and is the reason that water going down a plug-hole spins clockwise in the Northern Hemisphere, and anti-clockwise in the Southern Hemisphere - we originally put this in here, but it's NOT TRUE! The Coriolis effect is far too weak at this small sale to have an effect - the design of the basin is more important in which way your pughole-water spins. Thanks to our readers for correcting this.

An ocean current will flow along, affected by Coriolis, until it hits a barrier (i.e. a continent) and is deflected. This sets the path of global thermohaline circulation.

Our bucket analogy, while ridiculous, is pretty much what happens. Ocean circulation is mainly driven by warm salty water zooming North up the Gulf Stream, cooling and eventually sinking in the North Atlantic. There is a constant 'underwater waterfall', as this dense water sinks and begins to flow South. This deep current hugs the East coast of America (Coriolis, remember?), and eventually joins the Antarctic Circumpolar Current, which flows round and round the South Pole. Tongues of this circular current lick up into the Indian ocean, and into the South Pacific. The South Pacific current heads North East until it hits North America, where the water has nowhere else to go, and is forced to the surface. This is why the Pacific Coast of the USA has such delightfully cold water, and it's somewhere around here that you might be able to swim with the Vikings. Once on the surface, The water starts its slow journey back to the North Atlantic, via the Indian Ocean, round the Horn of Africa, in the ferocious Agulhas current, and back up the Western Atlantic to form the Gulf Stream, which keeps Western Europe warm. These currents are unimaginably vast. The largest one, the Antarctic Circumpolar Current, flows at 125 Sverdrups. That's 125,000,000 cubic meters per second. Or 50,000 Olympic swimming pools per second. Or 600 times the flow of the Amazon (largest river on Earth, by discharge). And yet, despite the size of these currents, one complete circulation can take up to 3000 years.

The picture above is complex, but doesn't begin to scratch the surface of these global ocean processes. Oceanographers spend their entire lives trying to work out the peculiarities of ocean currents, and how they tie in to climate and our daily lives. A pressing question at the moment concerns the flow of the Gulf Stream: melting of ice sheets have the potential to disrupt a lot more than Polar Bears. The worry is that as the Greenland Ice Sheet melts, it will dump a huge amount of fresh water into the North Atlantic. As we know, fresh water is less dense, and could mix with North Atlantic water and stop it sinking, and take away the main driving force behind the 'conveyor belt' circulation. It's almost impossible to work out what this would do to ocean circulation. The world is as we know it because the ocean currents move the way they do.

Image Credit: http://goo.gl/FBMrdr

, via http://goo.gl/TRNhc

NASA

Further Information: - The ocean currents in action (video):http://goo.gl/UHsJ6m

Cloud Army The image below is a cloud battalion, organized into neat ranks advancing over the Southern Appalachians. This cloud formation reflects the topography below. When air is forced up over ridges, excess water vapor carried by the air, is cooled and clouds are created. The ridges in the image look to be running parallel to one another, giving us the perception of a cloud army on the move. KKS Source: http://bit.ly/1bQN1HN Convective Cloud Formation: http://bit.ly/1ze7xKZ Photo courtesy of Seth Adams

Churning clouds and weather systems across the United States viewed in timelapse.

Amazing view of turbulence inside a noctilucent (Night-shining) cloud

Orbicular granite

We recently shared a photo of a Rapakivi granite, which contains grains of pinkish potassium feldspar surrounded by pale or white rims of plagioclase feldspar (http://tmblr.co/Zyv2Js1Zz1JTG). This is a different kind of granite called orbicular granite but it shows a similar feature – minerals with rims.

Orbicular granites form when different minerals crystallize from a magma in alternating patterns. Here, the first mineral to form was probably plagioclase feldspar. A small grain of it formed, but then something about the magma changed, possibly a temperature increase, causing the edges of the grain to re-melt and rounding it into a spherical shape. Crystals don’t grow spherical on their own, so the shape must imply some sort of erosion of the grains. The magma then began crystallizing a different mineral, the dark layer, presumably in this case an amphibole mineral like hornblende, as is common in rocks that are crystallizing plagioclase. As the amphibole formed, the magma composition changed and the temperature decreased until again plagioclase was able to crystallize.

From there, the alternating pattern continued, possibly due to arrival of new, hotter magma, or possibly due to movement of the crystal up and down inside a churning, convecting magma chamber. Finally, after this grain formed, it was broken and cross-cut by a dike of a different, later magma.

The sample is likely a few centimeters across and is about 400 million years old. It comes from Sout Island on New Zealand’s west coast and is on display at the Auckland War Memorial Museum.

To be completely accurate, this probably isn’t a true “granite”. True granites have a lot of potassium feldspar in them and this one doesn’t seem to, probably making it closer to a diorite or a quartz diorite. Commonly, the mining industry will call almost any igneous, crystalline rock “granite” even though geologists break these rocks apart into different groups depending on composition.

-JBB

Image credit: https://flic.kr/p/iQyXa

Read more: http://www.aradon.com.au/orbicular_granite.html http://www.kristallin.de/orbiculite/orbicular_rocks1.htm

On top of a roll cloud Roll Clouds are a rare and fascinating phenomenon. They form extremely long, cylindrical bands that run long distances in one direction. We’ve covered them before (see here: http://tmblr.co/Zyv2Js17XFkUp) and their formation isn’t totally understood but probably relates to small-scale convection and alternating layers of rising and sinking air. Astronaut Reid Wisemann caught this photo of what certainly appears to be a roll cloud viewed from the top, from his vantage point on the International Space Station, during a pass over Africa. Can’t tell for certain if it has the perfect shape and the wind patterns may be controlled in this area by a set of mountains, but it probably fits that description of small scale wind currents shaping a single long cloud path. -JBB Image credit: Reid Wiseman https://twitter.com/astro_reid/status/522095164748488704

Circular cloud formation over the Pacific

NASA's TERRA satellite snapped an unusual formation over the Hawaiian archipelago that was probably caused by air being heated over the dark lava of an island or a patch of warm ocean. As it rose, it formed clouds and light rain that cooled the air beneath the clouds causing a downdraft of air that spread out from the original clouds. As this cooler air encountered warmer air further away, it pushed it up causing its moisture to condense in turn forming the cloudy fairy ring.

These typed of nebulosity are known as open cell convective clouds, and result from temperature differentials causing patches of air to rise or fall. They can be circular or hexagonal, formed from the patterns of Rayleigh Bernard cells that form when fluids are heated form below. Closed cell ones retain the honeycomb hexagonal shape of the initial cells and are found in their centres, while open cell ones line the borders of the cell. The convective movement is also inversed, with warm air rising and condensing in the centre while sinking at the edges for closed cell formations and sinking in the centre while rising at the edges for open cell ones.

These clouds provide another example of the endless interactions mediated by the laws of physics that tie the sea, air and land together in the new interdisciplinary paradigm of Earth System Science that is being celebrated as the theme of this year's Earth Science week.

Loz

Image credit: NASA

http://earthobservatory.nasa.gov/IOTD/view.php?id=84444 http://www.livescience.com/48057-ring-shaped-cloud-photo.html?cid=514636_20140930_32534206

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