@earthstory / earthstory.tumblr.com
Ancient Hills These peaks slipping above the clouds are the Nandi Hills, found about 40 kilometers west of the city of Bengaluru (Bangalore) in southern India. The view here is snapped on a cloudy day from the Western Ghat Mountains that run down India’s western coastline.
Stressed out trilobite This trilobite probably feels like an awful lot of us these days. It started off in its normal shape, discarded onto the ocean floor as the organism shed it. That little trilobite shell found its way into sediment and eventually became part of a rock. But then, something happened to that rock – it was put under stress, from the old sedimentary layer being pulled into a mountain building event. The stress was gentle enough that the rock didn’t break, but it was strong enough and lasted long enough that the rock started to gradually shift its shape. Our once normal looking trilobite began shearing and twisting under the strain, bending to the side and eventually taking the shape seen here.
Rock and roll When I look at a rock like this I find that I almost can feel the motion within. It’s like a snapshot, an action photograph that catches exactly what was happening to this rock. Can you feel the movement, the rolling of this grain? This is a classic texture found in metamorphic rocks – this one from the Ailao-shan Red River shear zone on the edge of Tibet.
Anticyclone Tornadoes This photo is showing more than just an impressive supercell thunderstorm; look underneath the clouds and you will see two tornadoes. The one on the right is a typical tornado, but the one on the left is a rare anticyclone tornado. This 2015 storm over Colorado spawned at least six tornadoes including three anticyclones – twisters that spin in the opposite direction of what we expect.
Underneath a fault
Here’s the kind of question you might not have thought about if you’re not a geologist; what does the bottom of a fault look like?
Small faults can actually just end, but huge, continent-scale faults like the San Andreas Fault in California or the Anatolian Fault in Turkey have hundreds of kilometers of movement between the rocks on either side. What does the bottom of a faultlike that look like? Strike-slip faults tend to only go down about 10 or 15 kilometers deep. Megathrust faults at subduction zones can go deeper, but for now let's focus on a strike-slip fault. In that type of fault, the rocks are sheared, sliding past each other. Shallow, close to the surface, the rocks are actually able to break leading to earthquakes, but at depths of over 10 kilometers, it gets so hot that the rocks don’t break, they flow.
Rocks at the bottom of a fault are still sheared, but they are ductile. The minerals are able to change shape through one of several mechanisms depending on the temperature. The stresses of the fault reorient the minerals into new patterns, creating a rock like this called a mylonite with long strings of minerals created by the shear.
This rock specifically is an S-C mylonite. See how there are light and dark mineral bands with an angle between them? Those bands form as the shear stress causes the minerals to twist and regrow. Minerals with planar shapes like micas form bands in one direction in-between the white minerals that are rolled or otherwise twisted into the other bands. The angle between these two directions is defined by how the minerals respond to the stress and how much deformation has occurred after the bands formed. As strain continues to increase, the minerals rotate even more and new bands form, creating a process of shear band growth and twisting that can be used to reconstruct which direction a fault was moving.
This rock formed more than 10 kilometers deep in Earth’s crust, buried deep beneath a fault. Ever wanted to know what the bottom of a fault looks like? This is the bottom of an ancient fault, now exposed at the surface.
-JBB
S Wave Shadow Zone
From and early age, we are all taught that the interior of the Earth can be divided into four distinct layers; a solid crust, a plastic mantle, liquid outer core, and solid inner core. But have you ever wondered how we know these layers exist? Obviously, nobody has every managed to drill down to the center of the Earth to check. The deepest hole drilled so far, the Kola Superdeep Borehole in Russia, is just over 12 km deep, far shy of the 6, 371 km that would be required to reach the center of the Earth. Most of what we know about the interior structure of the Earth comes from studying how seismic waves generated by large earthquakes move through the planet. By using networks of sophisticated seismometers installed in bedrock around the world, geophysicists can record even the smallest vibrations. Seismic waves that travel through the Earth are generally divided into two types; compressional (P waves) and shear (S waves). P waves compress and expand material in the direction the wave is propagating. S waves shear the material perpendicular to the direction the wave is propagating. P waves travel much faster than S waves, and can be differentiated on a seismograph on the basis of their arrival times. A great way to visualize how these waves move is with a slinky, here’s an example: http://bit.ly/2xXRXgq
Liquids cannot sustain shear forces, meaning S waves can only propagate through solid material. When an earthquake occurs, seismic waves are generated at the hypocenter and propagate in all directions through the Earth. S waves are able to pass through the solid crust and mantle and can detected by seismometers on the other side. When an S wave crosses the boundary between the mantle and the outer core, some of the energy contained in the wave gets reflected back toward the surface, and some is transmitted through the core as a P wave. In the 1930’s, Beno Gutenberg found a lack of certain previously predicted S wave recordings at seismometers located more than 103 degrees of latitude from where the earthquake took place. This is known as the ‘S wave shadow zone’ for a given earthquake, and is directly caused by the inability of S waves to pass through the outer core.
http://bit.ly/2xySNPt http://bit.ly/1m0UbMt http://bit.ly/2wMJT2c Fowler, C.M.R. 2005. The Solid Earth: An Introduction to Global Geophysics (2nd Edition). Cambridge University Press, Print.
Personal Photo.
I’m betting most of my followers like the view of this pool in the Cyclades islands, off Greece, but I’m gawking at the wall behind it. Look at all the sheared out and faulted layers!
Mica fish in a mylonitic rock. Excercise: take a look at the rock and figure out how the rock was strained!
Porphyroblasts and Rotated Porphyroblasts - These occur when a large mineral crystal in a metamorphic rock grows within the finer grained groundmass. These are commonly garnet as seen above, but other minerals can also grow as porphyroblasts. Rotated porphyroblasts are often important in analyzing strain and deformation planes that the mineral and outcrop were subjected to as they grew, leaving a rotated appearance.
Shearing Magma
This photo shows a chunk of an igneous rock (a tonalite) found in the Sierra Nevada, pencil for scale. Even if you’re not a geologist, just looking at this photo you can see this rock looks different from typical igneous rocks. Most igneous rocks don’t have the linear fabric you see in this rock, the minerals are somehow aligned from left to right and parallel with the pencil.
This fabric of oriented grains tells us about something that was happening as the rock was forming, and interestingly it is relevant to an earthquake that happened just a few months ago.
On April 15, 2016 a large earthquake struck the city of Kumamoto, Japan, on a strike-slip fault that runs through the city. Kumamoto sits just to the southwest of a large volcano known as Mount Aso, and according to a new study during the earthquake that volcano actually controlled how the crust responded.
The earthquake rupture started on a strike-slip fault that runs along the island of Kyushu and moved up to the area near the volcano, but then the rupture stopped within a few kilometers of the Aso caldera. Large earthquakes also generate smaller earthquakes called aftershocks that show where stress was redistributed during the larger quake and it’s been long enough for scientists to detect thousands of aftershocks from the Kumamoto quake. Not only do the aftershocks stop just like the fault rupture did, they even start again on the other side of the caldera after twisting onto a fault at a different angle!
Mount Aso hosts molten rock at depth beneath its caldera and it has heated the surrounding crust so that it is ductile and able to flow. When the earthquake happened on the brittle fault near Kumamoto, the rocks near Aso felt the stress change, but because they’re hot and able to flow they transmitted the stress all the way to a fault on the opposite side.
This rock from the Sierra Nevada experienced a similar history. It sits in a large shear zone and the grains in the rock were actually aligned by that shear stress. This rock was partially molten at the same time that the stresses of a fault were moving through it and the stress of that fault caused the grains to align as they were growing. Today erosion has preserved the hot remnants of that fault in the Sierras; if we could see the end result of the rocks beneath Mount Aso they might well look just like this one millions of years from now.
Both Japan and the Sierra Nevada formed above subduction zones and they both formed large strike slip faults. This setup is no coincidence. When oceanic plates subduct beneath a continent, they usually don’t move straight at each other. Usually there is some angle between the paths taken by the two plates. When this happens, the strain from the plate motion partitions. Part of the motion is taken up by the subduction zone – that’s the downgoing motion, but the side-to-side motion has to go somewhere. It has to break a new fault somewhere and where is the weakest place on the overlying rocks? The rocks are weak where they’re hot, so areas with crystals and molten rock like those beneath Mount Aso commonly have strike-slip faults running through them. A similar setup is found in Sumatra today as well.
-JBB
Image credit: My own! (I wonder when I lost that pencil)
Original paper (open access) http://bit.ly/29CnW6H
The Creeping Imperial Fault
Southeast of the Los Angeles basin and due East from San Diego lies California’s Imperial Valley. It is heavily irrigated, as you see from the small irrigation ditch featured in this photo. The land is quite flat and has been filled in with sediment from the Colorado River. Thanks to irrigation channels leading from the Colorado, the area has become a rich area for growing plants and vegetables.
But, many unseen geological features lurk beneath the surface of the Imperial Valley. The San Andreas Fault enters the northwestern tip of this Valley, but then breaks off into a couple different segments along the coastline of the Salton Sea. South of the Salton Sea, the fault reorganizes into a system that travels into Mexico known as the Imperial Fault.
In this photo of what appears to be a simple Imperial Valley irrigation ditch, you’ll notice that the concrete has cracked and there are plants growing through the concrete along these fractures.
This irrigation ditch sits directly atop of the Imperial Fault. This picture was taken from the “North American” plate side of the fault, looking across at the Pacific plate, so the land to the top of this image is moving to the right relative to the viewer (the San Andreas system being a right-lateral fault).
The fault hasn’t had a major earthquake since a magnitude 6.4 event in 1979, which happened before this channel was built, but it’s still there and still slowly creeping. Concrete doesn’t handle shear stress very well, so here it’s cracking due to that slow motion. The Fault hasn’t had large motions since 1979, but it still creeps along, allowing very subtle motions to take place while the stress builds up that will eventually drive a larger earthquake.
This fault also offsets a nearby rail line and telephone poles, some of which were built before the 1979 quake and are offset by a meter or more. There is also, for some reason, a buried high pressure gas line at this location.
-JBB
Image credit: Me, image owned by the author of this post.
1979 Earthquake details: http://www.data.scec.org/significant/imperial.html
OH MY GOODNESS IT’S A SUNSET AND KELVIN HELMHOLTZ CLOUDS. IN LOOPING VIDEO FORM. THIS IS AWESOME.
The folly of long-term hurricane predictions and the hurricane drought
This ISS photo captured Hurricane Danny, which intensified to a category 3 storm in the Atlantic Ocean last week but has dissipated and The United States is in a record-breaking period for hurricanes right now, but you may not have noticed. The record right now being extended every day is “longest gap between major hurricane strikes in the United States”.
The 2005 Hurricane season was a super-season, breaking records for most named storms and for monetary damage, but since a Category 3 storm named Hurricane Wilma hit Florida late in 2005 there has not been a single major hurricane strike on the U.S. (major hurricanes defined as category 3 or stronger).
There are some basic patterns that contribute to hurricane formation patterns. At the top of the list is the El Niño oscillation pattern in the Pacific Ocean; warm waters off the South American coast leads to strong winds over the Atlantic that blow circulating storms apart. This pattern lets us confidently make some predictions about low hurricane intensity years; 2015 has a strong El Niño present in the Pacific so it’s fair to expect a year with few Atlantic hurricanes. In fact, Hurricane Danny met just this fate: weakened and blown apart. On the other hand, during El Niño years, the warm water in the Pacific can feed major storms in that basin as we’re seeing this year.
La Niña years, the opposite of El Niño years, have abnormally cold water in the Eastern Pacific. These years tend to be associated with weak Atlantic shearing winds and can be prime years for hurricane formation; 2005 was a weak La Niña year and it was the strongest Hurricane season on record. On the other hand, there have been several La Niña years since 2005 and no major hurricane strikes on the U.S. What’s the deal with that?
The most recent weak La Niña years were 2013 and 2014, however, especially in 2014, systems that could have developed into hurricanes off of Africa were disrupted by abnormally large plumes of dust coming from the Sahara. 2010 was a La Niña year with an active hurricane season, but none of the major storms hit the U.S. – literally just a roll of the dice.
That example shows to me how hard it is to predict total numbers of hurricanes or their impacts even a few months in advance. Hurricane Katrina, which provoked this series, formed when two distinct low pressure systems from Africa combined off the Cuban coast. In other words, Katrina was a very unlikely situation and an illustration of the complexity involved in hurricane generation.
Hurricanes need stable conditions, lack of wind shear, warm water, and nothing in the atmosphere that disrupts them; getting the mixture just right for a strong hurricane is always going to be a roll of the dice.
I write this post to illustrate some of the follies of hurricane prediction. Every year, NOAA will release an official forecast that is based mostly on historical patterns and the presence or absence of La Niña/El Niño, but even NOAA’s best estimates can be disrupted by something as unpredictable as Saharan dust storms. Other estimates based on “private prediction methods” will get publicity every year, but they’re even less accurate than NOAA’s predictions.
After Katrina, there was a worry that the world was entering an era of stronger hurricanes due to warming ocean surface waters. Although the past 10 years hasn’t produced major storm impacts on the U.S., this prediction still is generally possible; warmer waters means more energy that can feed storm growth, but until we understand and can simulate hurricane formation in extreme detail it’s going to be difficult to understand how all the variables work together. Warmer waters can lead to more hurricanes but they can also lead to stronger weather systems and shearing winds that tear apart hurricanes. What the long-term trend will be is a question scientists are still working on. There's a new tropical storm, Erika, in the Atlantic right now that has plenty of warm water to feed on and could threaten the U.S. coast, but even that would be about a week away and subject to the vagaries of the winds.
Furthermore, let’s also highlight one last storm in this “major hurricane drought”; the second most destructive (monetary) hurricane in U.S. history struck in 2012, a year with a weak positive El Niño signal. That storm was hurricane Sandy. Although The U.S. has been in a drought of strong hurricanes, it hasn’t been in a drought for hurricane damage.
Finally…if I’m going to write a whole piece on hurricane development and long-term predictions I might as well make one, right? Typically after strong El Niño years, the planet tends to slip into a La Niña phase with extra heat floating around the tropics from the El Niño; prime conditions for hurricane formation.
If you wanted a long-term prediction, it’s entirely possible that 2016 could be a good year for hurricane formation and could produce storms that break the drought of major impacts on the U.S., but it also might not. They might just go the wrong way like 2010, they might be broken up by dust like in 2014, or we might see the pendulum swing back like we saw in 2005. Beyond that general description of what the planet tends to do, we just have to sit and watch and prepare for the future.
-JBB
Image credit: Scott Kelly/NASA https://twitter.com/StationCDRKelly/status/634386097301118976/photo/1
References: http://www.livescience.com/50704-hurricane-drought.html http://slate.me/1qLakcc http://www.hurricanescience.org/science/science/activity/ http://1.usa.gov/1Efr1qT http://ggweather.com/enso/oni.htm http://bit.ly/1NPDYaD
Rock and roll
When I look at a rock like this I find that I almost can feel the motion within. It’s like a snapshot, an action photograph that catches exactly what was happening to this rock. Can you feel the movement, the rolling of this grain?
This is a classic texture found in metamorphic rocks – this one from the Ailao-shan Red River shear zone on the edge of Tibet. As the Himalayan Mountains grew, the rocks of eastern Asia were out of the way, forming large strike-slip faults. 20 million years ago these rocks were buried deep within one of those strike-slip faults where they were warm and ductile.
The rocks above were faulting and sliding past each other while the rocks at the base sort of oozed past each other. But, not every mineral behaves the same way under metamorphic conditions. Some minerals, like quartz or calcite, become weak and able to flow at temperatures hundreds of degrees Celsius lower than other minerals.
The pale layers in this rock are carbonate rocks, the remnant of limestones deposited along the edge of the Tethys seaway. They were buried tens of kilometers deep as the Himalayas grew and warmed by the heat of the earth until they were able to flow easily. The stress of associated with the fault far above stretched those layers, pulling them like taffy. But, within those layers also sat stronger minerals, like the black amphibole.
That amphibole grain did not get hot enough to become ductile, instead it remained rigid and the calcite flowed around it. As the calcite flowed, the amphibole grain rolled and occasionally fractured. Pieces of it were pulled off and moved away, creating tiny little faults within the grain. When you find a grain like this out in the field, it’s not only spectacular to see and worthy of sitting on a shelf; it also tells exactly how the rocks were moving. Can you figure out which way the rocks were moving, were the rocks on top moving to the right or to the left?
Finding a rock like this preserved from an ancient shear zone gives geologists the ability to interpret the motion on faults that have been inactive for millions of years by answering the same question you just did.
-JBB
Image credit: Credit: Philippe Leloup via http://imaggeo.egu.eu/view/2362/
Read more: http://www.sciencedirect.com/science/article/pii/S1342937X10001863 https://books.google.com/books?isbn=3642684327 http://www.science.marshall.edu/elshazly/Igmet/texture.doc
Sheared ruby
A rather nice little crystal of red corundum from the Mogok stone tract of Burma testifies to the geological forces accompanying its birth and subsequent life in the crust. The gems themselves were born in the fire and pressure of metamorphism during the Himalayan mountain building event, as marine limestones were baked and recrystallised into marbles. The aluminium liberated in the process mixed with oxygen to form the corundum since it doesn't fit into the calcite crystals in the forming marble, also sucking into its crystal lattice the chromium that gives it its colour. The forces are still present and the mountains are tectonically active, and at some later point the crystal was snapped by shear forces, part of the response of crust when it forcible meets other crust.
Loz
Image credit: Dick Hughes
Experimental geology can be quite neat. This is a box filled with gypsum powder under a shear stress. You can watch as first a series of fractures form at about 30 degrees to the strongest stress direction and then they connect, localizing into a strike slip fault.
GeoTrivia: Classical Shear Folding Finding geology in unexpected places, this classical era Greek column sitting along the beachfront at Achilleo, Greece (yes, named after the hero Achilles) exposes an amazing view of “similar folds.” These are folds that form in relatively hot rocks undergoing metamorphism by means of a mechanism which is, well, like poking your finger into a deck of cards, and “shearing” the cards along their surfaces. The resulting pattern includes layers that are thinner along fold limbs (where more shearing occurs) and thicker at fold noses (places where less movement is facilitated). In the region where this column is found, there are abundant supplies of plain old white marble in the hillsides. Why the ancient column maker used this spectacular rock in construction is unknown: possibly he thought it was pretty? Or – could he have been an incipient structural geologist? Annie R. My photo: column ~60 cm in diameter Reading to get started on folds: http://www.glossary.oilfield.slb.com/Display.cfm?Term=similar+fold http://www.geo.wvu.edu/~jtoro/structure/ppt342/16folds-342.pdf http://www.jstor.org/discover/10.2307/30062467?uid=4&sid=21101508915407
