New research reports that at least one species of fish engages in similar behavior to sports fans — collective waves.
It’s not uncommon to see collective — also known as ‘Mexican’ — waves on arenas hosting football (soccer) matches around the world. These involve large groups of fans successively standing up in unison, as a display of solidarity between them and for their favorite teams.
Sulphur mollies (Poecilia sulphuraria), however, do it for a completely different purpose. A new paper describes this incredible collective behavior in the wild fish species, detailing how hundreds of thousands of individuals coordinate, likely to protect themselves from predatory birds.
“At first we didn’t quite understand what the fish were actually doing,” said David Bierbach, co-first author of the study. “Once we realized that these are waves, we were wondering what their function might be.”
The study showcases just how many of the fish partake in such behavior — there can be up to 4000 fish per square meter of ‘wave’, and each can include hundreds of thousands of individuals, according to the team.
Sulphur mollies are small animals, who stand out due to their preferred environment: sulphuric springs whose chemical make-ups make them toxic to most other species of fish.
The team explains that they likely use this living wave behavior as a way to confuse or maybe deter predators, especially birds. Mollies engage in this behavior when a person’s shadow falls on the water as well, further reinforcing this hypothesis. Individual waves last three to five seconds each, but the mollies have been recorded as repeating the behavior for up to two minutes.
The team first had to rule out the possibility that this behavior was random — their experiments showed that the fish would engage in ‘waves’ in a conspicuous, repetitive, and rhythmic fashion in response to stimuli associated with the presence of predators.
Then, they examined whether this behavior had any effect on the predators themselves: it does. The team reports that experimentally-induced fish waves dramatically reduced the frequency of attacks from birds of prey, and doubled the time these birds took between attacks. For one of their predator species (kiskadees, Pitangus sulphuratus), wave patterns also decreased capture probability.
Birds exposed to these wave patterns would switch perches more often than control individuals, suggesting that they may prefer to focus their attention on other prey when confronted with the mollies’ wave behavior.
According to the team, this is the first time a collective behavior has been shown to be directly responsible for reducing a species’ chances of being attacked and preyed upon. It is an important discovery for the study of collective behavior in animals more broadly, they add.
“So far scientists have primarily explained how collective patterns arise from the interactions of individuals but it was unclear why animals produce these patterns in the first place,” says co-author Jens Krause. “Our study shows that some collective behavior patterns can be very effective in providing anti-predator protection.”
Something that the team can’t yet explain is why such behavior helps protect the mollies from attacks. It’s possible that the motions confuse the birds, or perhaps they work as a signal to the bird that they have been spotted, making it consider another target altogether. The team plans to explore these questions in the future.
The paper “Fish waves as emergent collective antipredator behavior” has been published in the journal Current Biology.
Imagine yourself being able to surf at night with waves that aren’t only breathtaking but also have an amazing glow. Well, that’s possible every few years along the coast of southern California.
Images were recently captured at beaches in California, where the night-time waters can be seen glowing bright blue. While this has happened before, locals say this year’s phenomenon is special thanks to historic rains that have hit the region and created algal blooms.
The bright blue color of the waves is created by blooming microscopic plants called phytoplankton. The organisms collect on the water’s surface during the day to give the water a reddish-brown hue, known as the red tide. By night, the algae put on a light show, dazzling most brightly in turbulent waters.
The bioluminescence is a chemical reaction on a cellular level within the algae caused by the motion of the waves, according to Scripps Institution of Oceanography Professor Peter J. Franks, who calls the phytoplankton “my favorite dinoflagellate.”
“Why favorite?” Franks wrote in an email Q&A posted on the blog Deep-Sea News. “Because it’s intensely bioluminescent. When jostled, each organism will give off a flash of blue light created by a chemical reaction within the cell. When billions and billions of cells are jostled — say, by a breaking wave — you get a seriously spectacular flash of light.”
The algae blooms have been spotted this year at several beaches in the south of California, including Newport Beach, Hermosa Beach, and Dockweiler state beach. Surfers and many others intrigued by the phenomenon have approached the beaches in the last few weeks to see the glowing waves for themselves.
California has implemented social distancing measures due to the coronavirus epidemic, but people can still visit its beaches. However, they must maintain a 1.8-meter distance between themselves and others. Swimming, surfing, kayaking, and paddleboarding are still allowed.
Dale Huntington, a 37-year-old pastor, got up at 3am after beaches reopened to surf the waves. “I’ve been surfing for 20 years now, and I’ve never seen anything like it”, he told The Guardian. “My board left a bioluminescent wake. There were a few of us out there and we were giggling, grown men shouting and splashing around like kids.”
Researchers at the Scripps Institution of Oceanography at UC San Diego, who study the phenomenon, said the glow shows are most lively at least two hours after sunset. They don’t know exactly how long the phenomenon will last this year. Red tides have been observed since the early 1900s and can last from a few days to a couple of months.
There might be a silver lining to sea level rise — emphasis on ‘might’.
Coral reef rim islands, Huvadhoo Atoll, Republic of Maldives. Image credits Prof. Paul Kench.
New research proposes that rising sea levels may help the long-term formation of coral reef islands, such as the Maldives. However, all the other bits of climate change may destroy any benefits it brings.
Climate change, island change
“Coral reef islands are typically believed to be highly vulnerable to rising sea levels. This is a major concern for coral reef island nations, in which reef islands provide the only habitable land,” says lead author Dr. Holly East of the Department of Geography and Environmental Sciences at Northumbria University, Newcastle.
Coral reef islands aren’t very keen on altitude; typically, they’re less than three meters (about 10 feet) above the water’s surface. This obviously makes them very vulnerable to rising sea levels. However, the same high seas might’ve also created the islands, the team reports.
The researchers studied five islands in the southern Maldives. By drilling out core samples, they were able to reconstruct when and how the islands formed. They report that storms off the coast of South Africa created a series of large waves (‘high-energy wave events’) that led to the formation of the Maldives. These violent waves dislodged large chunks of pre-existent reefs and transported them onto reef platforms. This stacking of reef material created the foundations of the islands we see today.
“We have found evidence that the Maldivian rim reef islands actually formed under higher sea levels than we have at present,” Dr. East adds.
“This gives us some optimism that if climate change causes rising sea levels and increases in the magnitude of high-energy wave events in the region, it may actually create the perfect conditions to reactivate the processes that built the reef islands in the first place, rather than drowning them.”
The seas were around 0.5 meters (1.5 feet) higher than today during the islands’ formations — this allowed the waves to carry more energy. Both the higher sea level and large wave events were critical to the construction of the islands. Now, (man-made) climate change is also pushing up sea levels; the team says that projected increases in both sea level and the magnitude of large wave events could actually lead to the growth of reef islands.
For that to happen, however, you need living, healthy coral in the region’s reef communities, Dr. East stresses. And we’re murdering them pretty fast right now.
“As these islands are mostly made from coral, a healthy coral reef is vital to provide the materials for island building. However, this could be problematic as corals face a range of threats under climate change, including increasing sea surface temperatures and ocean acidity,” she says.
“If the reef is unhealthy, we could end up with the perfect building conditions but not the bricks.”
She also cautioned that “the large wave events required for reef island building may devastate island infrastructure, potentially compromising the habitability of reef islands in their current form.” Factoring in higher sea levels as well, she says that reef island nations need to “develop infrastructure with the capacity to withstand, or be adaptable to, large wave events” — a task she summarizes as being a “challenge”.
Their paper, “Coral Reef Island Initiation and Development Under Higher Than Present Sea Levels,” has been published in the journal Geophysical Research Letters.
With a bit of help from NASA, you can now hear the sun’s roar — and it’s glorious.
The Sun’s surface seen in ultraviolet light, colored by NASA. Image credits NASA Goddard.
Although you never hear it, the Sun is actually pretty loud. This massive body of superheated, fusing plasma, is rife with ripples and waves generated by the same processes that generate its light and heat — and where there’s motion, there’s sound. We never get to hear it, however, as the huge expanse of nothing between the Earth and the Sun acts as a perfect acoustic insulator.
With some of ESA’s (the European Space Agency) data and a sprinkling of NASA’s magic, however, you can now hear the Sun churn in all of its (surprisingly tranquil) glory.
Hear me roar (softly)
“Waves are traveling and bouncing around inside the Sun, and if your eyes were sensitive enough they could actually see this,” says Alex Young, associate director for science in the Heliophysics Science Division at NASA’s Goddard Space Flight Center.
What Young is referring to are seismic waves, a type of acoustic waves — the same kind of motion that causes earthquakes in rocky planets — that form and propagate inside the Sun. Hypothetically, if you were to look at the star with the naked eye, you could actually see these waves rippling through its body and surface. Stars are formed of a much more fluid material than most planets, and so their bulk flows more readily under the sway of seismic waves — wiggling just like a poked block of Jell-O.
As most of us learned in early childhood, however, one cannot look directly into the Sun for long. Luckily for us, ESA recently embarked on a one-of-a-kind mission: they sent the Parker Solar Probe hurtling towards our star. Using its SOHO Michelson Dopler Imager (MDI) instrument, the probe recorded these motions inside the Sun. Researchers at NASA and the Stanford Experimental Physics Lab later processed into a soundtrack.
It’s not half-bad, as far as tunes go. I actually find it quite relaxing. Check it out:
[panel style=”panel-info” title=”Hear me roar” footer=””]
The sounds you hear in NASA’s clip are generated by the motions of plasma inside the Sun. These are the same processes that generate local magnetic fields inside the star and push matter towards the surface, causing sunspots, solar flares, or coronal mass ejections — the birthplace of space weather.
Space weather phenomena are associated with intense bursts of radiation, to which complex technological systems are susceptible. So most of our infrastructure, from satellites — and with them, cell phone networks, GPS, and other types of communication — transportation, and power grids.[/panel]
It took a great deal of work to turn the readings from Parker into something usable. Alexander Kosovichev, a physicist at the Stanford University lab, processed the raw SOHO MDI data by averaging Doppler velocity data over the solar disc and then only keeping low degree modes. These low degree modes are the only type of seismic waves whose behavior inside stars is known and accessible to helioseismologists. Afterward, he cut out any interference, such as sounds generating by whizzing of instruments inside the craft. He then filtered the data to end up with uninterrupted sound waves.
While scientists probably enjoy a groovy track just as much as the rest of us, the soundtrack actually has practical applications. By analyzing the sounds, researchers can get a very accurate picture of the churnings inside of our Sun — much more accurate than previous observations could provide.
“We don’t have straightforward ways to look inside the Sun,” Young explains. “We don’t have a microscope to zoom inside the Sun. So using a star or the Sun’s vibrations allows us to see inside of it.”
A more comprehensive understanding of the motions inside the Sun could allow researchers to better predict space weather events.
Sound is all around us and comes in a myriad of flavors. Some are nice, like music or wind blowing through leaves. Others, like the beep when your card gets rescinded, not so much. We know our ears pick up on sounds, but then, why do we also feel the hammering of a song in our chests when the bass is loud enough? And what’s the link between instruments playing by themselves and a bridge collapsing in 1850’s France?
Physically speaking, what we perceive as sound is a vibration produced by motion.
Imagine the world as a huge bathtub on which you, a yellow rubber duck, merrily float around. At various points along this tubby world, there are faucets pouring water. Some are bigger and pour a lot of water, while others are tiny and only give off occasional drips. Some are closer to you, while others are really far away. These are the sources of sound.
This is surprisingly similar to how sound would look like if we’d be able to see it. Image credits Arek Socha.
Regardless of their position or size, each faucet creates vibrations in the form of ripples on the water’s surface — which is the medium. Most of these will never make it all the way to you. For the ones that do, you’ll ‘hear’ the source faucet. How much you bob up and down on the wave it generated represents the sound’s amplitude — roughly equivalent to what we perceive as loudness. How frequently each sloshes you around, based on how close packed the ripples are, is the sound’s frequency — what we perceive as pitch. The way ripples push you is the direction of propagation — i.e. where we hear the sound coming from.
It’s not a perfect analogy because, as you may already suspect, the world is not a bathtub and we’re not rubber duckies. But it simplifies the conditions enough to understand the basics. For you to hear something, a few things have to happen: First, you need a source of motion to get it started. Secondly, sound travels as a wave, so there has to be a medium to carry the vibration between you and this source. You need to be close enough to the source to register the vibration before it attenuates or dies off. Lastly, the sound has to be in the right frequency interval — if the wave is too lazy or too steep, you won’t pick it up.
In real life, the medium can be any fluid (gas, liquid, plasma) or solid. Even you are one. The medium’s properties determine how sound propagates — fluids carry sound only as compression waves, which are alternating bands of low and high pressure, while it can propagate both as compression and transverse (shear) waves through solids. The medium’s density determines the speed of a sound, while its viscosity (how strongly particles stick to each other and resist motion) dictates how far it can travel before it runs out of energy/attenuates.
These properties aren’t constant through space or time. For example, heat dilation can cause a shift in parts of the medium’s properties, altering how fast sounds propagate at different points. Vibration can also transfer from one medium to another, each with different properties. If you’re dressed up in a sealed astronaut suit on Earth and talk loud enough, people will still be able to hear you. Take two astronauts into the void of space and they won’t hear each other talking because there are no particles to carry the vibration between them. But if they stand visor-to-visor they may faintly hear each other, as the suit and air inside it carry over part of the sound.
Perceiving is believing
From a subjective point of view, the answer to “what is sound” comes down to what you can hear. The human ear can typically pick up on frequencies between 20 Hz and 20 kHz (20,000 Hz), although age, personal traits, and the medium’s pressure shift these limits around. Everything below 20 Hz is called infrasound (under-sound), anything above 20 kHz is called ultrasound (over-sound).
All you have ever heard falls within this interval which, to be fair, is pretty limited. Cats and dogs can hear ultrasounds up to 45-65 kHz respectively, which is why they howl at a whistle you can’t even hear. Some whales and dolphins can even go as far as 100 kHz and over, an interval which they use to communicate. Still, they’re limited in what lower frequencies they can hear. An average cow, however, can probably perceive a wider range of sounds than you on both ends.
This cat has a whole playlist of awesome music you can’t even hear. Image from public domain.
Apart from those four physical properties of sound I’ve bolded earlier, the are also perceived qualities of a sound. Pitch and loudness are directly tied to physical properties for simple sounds but this relationship breaks down for complex sounds. There’s also a sound’s perceived duration (how long a sound is) which is mostly influenced by how clearly you can hear it, a sound’s timbre (the way a sound ‘behaves’ over time, making it distinct from other sounds), its texture (the interaction between different sources), and finally the spatial location (where the different sources are relative to one another).
Your perception can also further influence the sounds you’re hearing through the Doppler effect. A sound’s relative spatial location to you over time can lower or raise a sound’s perceived frequency. That’s why you can tell if a car is rushing toward, moving away from you, or just sitting in traffic, from the pitch in sound.
Some time ago, musicians found that playing particular notes could make chords vibrate on other instruments, even when no one was touching them. The phenomenon was dubbed after the Latin term for ‘echo’, since the chords seemed to pick up and repeat the sound played to them and Latin sounds cool. Unknowingly, they stumbled upon a phenomenon that would see today’s soldiers ordered to break stride when crossing bridges to prevent them from collapsing — resonance.
Possible side effects of resonance. Image credits Vladyslav Topyekha.
Ok, so the nerdy bit first. Every object has the capacity to oscillate, or shift, between several states of energy storage. If you fix one end of a spring, tie a weight to the other, pull down, and then release said weight, it will bob up and down like crazy then gradually settle down. That movement is caused by the system oscillating between different states of energy — kinetic energy while the weight is in motion, potential elastic energy while it’s down, and potential gravitational energy while it’s up. It eventually settles at a particular point because this shift is inefficient, and the system loses energy overall (called damping) when transitioning from one state to the other.
But objects also have something called a resonant frequency which works the other way. They can resonate with all kinds of waves, from mechanical/acoustic waves all the way to nuclear-magnetic or quantum resonance. Each object can have more than one such frequency for every kind of wave.
When vibrating at one of these frequencies, systems can undergo the shift with much greater efficiency, so tiny but sustained external vibrations can add up inside the system to build powerful oscillations. It can even lead to a system holding more energy than it can withstand, causing it to break apart. This phenomenon became tragically evident on the 16th of April 1850 at the Angers Bridge, France, when the marching cadence of a battalion of soldiers going over the bridge amplified wind-induced oscillations, matching the structure’s resonant frequency, leading to collapse and the death of some 200 troops.
Sound is basically a mechanical wave, so it can also induce these resonant oscillations in objects — called acoustic resonance. If you’ve ever seen someone sing a glass to the breaking point, this is the phenomenon at work. If not, you can watch Chase here be adorably excited when he manages it.
Other cool and not-cool things sound does
I’m gonna start the “cool-things” list with the sonic refrigerator because get it, cool? Refrigerators? I love my job.
Pun aside, about halfway through last year a team from the Department of Prime Mover Engineering at Tokai University in Japan developed a system that uses resonant frequencies to pump and compress coolant in a refrigerator in lieu of traditional systems. Their engine draws power from the fridge’s residual heat, making for a much more energy efficient system.
The sound generated by individual atoms’ vibrations can be used to identify their chemical species, one team from Georgia Tech reported last year. These vibrations can even tell researchers what substances, and in which particular states, multiple atoms bunch together to form. It’s so accurate that CERN is already using the method to identify individual subatomic particles.
Sound may also help us stop tsunamis before they reach the shore according to Dr Usama Kadri from Cardiff University’s School of Mathematics. The math shows it’s a viable method, although we don’t yet have the technical capabilities to implement it.
Researchers at the Max Planck Institute for Intelligent Systems in Germany have also figured out a way to use sound in an acoustic tractor beam. I don’t even need to explain why that’s awesome.
Sound can also be very pleasant, in the form of music — for humans and cats alike.
Certain sounds can make your food taste sweeter or sourer, others can help you diet — but these are more tied to perception than physics.
On the “not-cool, dude” list we have sound-based weapons. From ancient weapons that used perception to shake the enemy’s morale, the infamous Jericho sirens on Nazi StuKas used as psychological weapons during WW2 (quite effective at first, then withdrawn from service since it ruined the plane’s aerodynamics and soldiers got used to them), to modern crowd-control acoustic cannons employed by police and armed forces — used with varying degrees of ethical success — sound has always played a part in warfare.
There are also some more exotic items on the list, such as the much-searched-for-but-still-undiscovered brown note. This was believed to match the human bowel’s resonance frequency and make soldiers inadvertently soil themselves in combat. Though I’d say it would only make their camouflage more effective.
Blasting high-powered acoustic waves at tsunamis could break their advance before reaching the shoreline, a new theoretical study has shown.
Tsunamis are one of the most dramatic natural phenomena we know of, and they’re equally destructive. These great onslaughts of water are powered by huge amounts of energy — on a level that only major landslides, volcanoes, earthquakes, nukes, or meteorite impacts can release. And when they reach a coastline, all that water in motion wipes infrastructure and buildings clean off.
Traditionally, there are two elements coastal communities have relied on against tsunamis: seawalls and natural barriers. Seawalls are man-made structures that work on the principle of an unmoving object, resisting the wave’s kinetic energy through sheer mass. Natural barriers are coastal ecosystems, typically mangrove forests or coral reefs, that dissipate this energy over a wider area and prevent subsequent floods. Each approach has its own shortcomings however, such as high production and maintenance cost or the risk of being overwhelmed by a big enough tsunami.
Dr Usama Kadri from Cardiff University’s School of Mathematics thinks that the best defense is offence — as such, she proposes the use of acoustic-gravity waves (AGWs) against tsunamis before they reach coastlines. Dr Kadri proposes that AWGs can be fired at incoming tsunamis to reduce their amplitude and disperse energy over a larger area. Ok that’s cool, but how does it work?
The tsunami whisperer
Waves are a product of the interaction between two fluids (air-water) and gravity. Friction between wind and the sea’s surface causes water molecules to move sideways and on top of one another, while gravity pulls them back down. Physically speaking, ‘waves’ are periodic wavetrains — and as such, they can be described by their period (length between two wave crests), amplitude (height), and frequency (speed).
One thing you can do with periodic waves is make them interfere constructively or destructively — you can ‘sum up’ two small waves to make a bigger one, or make them cancel out. Apart from a different source of energy, tsunamis are largely similar to regular waves, so they also interfere with other waves. Here’s where AGWs come in.
Think of AGWs as massive, sound-driven shock-waves. They occur naturally, move through water or rocks at the speed of sound, and can stretch for thousand of kilometers. Dr Kadri shows that they can be used to destructively interfere with tsunamis and reduce their amplitude before reaching the coast. Which would prevent a lot of deaths and property damages.
“Within the last two decades, tsunamis have been responsible for the loss of almost half a million lives, widespread long-lasting destruction, profound environmental effects and global financial crisis,” Dr Kadri writes in her paper. “Up until now, little attention has been paid to trying to mitigate tsunamis and the potential of acoustic-gravity waves remains largely unexplored.”
“The main tsunami properties that determine the size of impact at the shoreline are its wavelength and amplitude in the ocean. Here, we show that it is in principle possible to reduce the amplitude of a tsunami, and redistribute its energy over a larger space, through forcing it to interact with resonating acoustic–gravity waves.”
Her paper also shows that it’s possible to create advanced warning systems based on AGWs, which are generated with the tsunami and induce high pressures on the seabed. She also suggests harnessing these natural AGWs against tsunamis, essentially using nature’s own energy against itself.
The challenge now is to develop technology that can generate, modulate, and transmit AGWs with high enough accuracy to allow for interference with tsunamis. She admits that this won’t be easy to do, particularly because of the high energy required to put a dent in the waves.
The full paper “Tsunami mitigation by resonant triad interaction with acoustic–gravity waves” has been published in the journal Helyion.
Ahh, the cave, cradle of humanity since time immemorial. Early humans sought them for shelter, plastered their walls with paintings, made them into the first temples. And even after we’ve moved out, they still captivate and terrify us — unknown, but somehow familiar.
Without caves, our life might have been very different now. So how did they come about? How does a cave form? Well, in a lot of different ways, really. Caves come in different sizes and shapes, and the way they’re created depends on the type of cave. Most often, they form when water dissolves limestone, but they can also be shaped by waves, even lava.
So don your hardhats and pull your learning pants on, because I’m going to tell you all about:
The Types of Caves
Son Doong Cave in Vietnam, the largest cave ever found, is a solutional cave. It’s big enough to have its own ecosystem. Image credits Doug Knuth
These are the structures people most readily associate with the idea of a cave, and for good reason. They’re the most numerous, the largest and most often-encountered structures. If you’ve ever been spelunking or seen a cave in a movie chances are it was a solutional cave. The secret to their abundance is two-fold: for starters, the rocks that house them are found throughout the globe, and the chemical elements required to shape them are abundant. As Andrei wrote:
“Solutional caves are generally formed in limestone or other similar rock such as gypsum or dolomite. They form when acidic water dissolves the rock, seeping through the bedding planes.”
Let’s consider a geological environment of soil over a bedrock of limestone, as solutional caves are most frequently found in this type of rock. Limestone is a carbonatic rock, formed over millions of years from the remains of coral, zooplankton, shells or bones, all mashed up together. This material gets bunched up and subjected to huge pressure, fusing into solid rock.
The main mineral found in limestone is calcium carbonate, or CaCO3, a mixture of calcium and carbon trioxide, an unstable compound. While limestone is pretty resilient and nice to look at, it tends to be relatively brittle and fractures a lot due to tectonic stress. Its chemical makeup also makes it susceptible to attack by acids which break up the calcium carbonate into calcium compounds (Ca + the non-metal that forms the acid), carbon dioxide (CO2), and water (H2O).
Limestone cave in Australia. Image credits Andrew McMillan
[panel style=”panel-info” title=”Fun geology fact” footer=””]Rubbing a diluted solution of acid onto a geological sample is still the easiest way to determine if there are any carbonatic compounds in the rock. If so, the solution will bubble and foam quite vigorously.[/panel]
These conditions work together to make limestone an ideal place for cave formation. In nature water invariably becomes acidic by mixing with carbon dioxide molecules (H2O+CO2=H2CO3) forming a solution of carbonic acid. Part of this can happen in the atmosphere as rain pours down, but most of the mixing takes place in the soil which is rich in CO2 left over from decaying organic matter.
This solution trickles down through the soil and cracks in the limestone until it reaches the water table. Here it starts to eat through the rock, forming channels. In an almost cruel twist of geological fate, while limestone dissolves it releases the exact components needed to make more carbonatic acid. This chain reaction and the extra acids that seep in from the surface keep expanding the cavern until the water table level drops. If this happens, water with dissolved calcium compounds will trickle down to the new area of dissolution, forming stalactites and stalagmites.
And looking awesome. Image via pixabay
If on the other hand water remains mobile throughout dissolution, the caves take on the appearance of an underground drainage system, a landscape known as karst.
Something like this, but underground. Image credits Jonathan Wilkins
It takes a few million years for a solutional cave to form.
While dissolution caves are formed by hollowing out preexistent packets of rock, lava caves form at the same time as the geological environment around them — and so, they’re considered to be primary caves. They’re centered around areas of volcanic activity and resemble huge underground rock pipes.
Lava River Cave in Arizona. Image credits Volkan Yuksel
And in a way, that’s just what they are. Molten rock that reaches the surface (called lava) can form sprawling cave networks while it flows down the path of least resistance. The material is very hot initially, but as the outer layer of lava starts to cool it solidifies into a shell of rock. This process insulates the lava within and starts at the base of the flow (because the rock it’s pouring over is a better thermal conductor than air,) forming a through-like structure through which the hot lava at the center keeps on flowing. Over time, material clings to the edges of this through and solidifies, eventually closing into a pipe-like structure.
Because this shell of rock is solidified from a flowing material its inner walls are neat, almost polished, with horizontal conduits on the inner side that channel the flow. Once the lava supply starts to dwindle the cave cools down and thermal constriction starts fragmenting the walls. The pressure of volcanic gasses in the cave, however, support the roof from collapsing. As these gasses mix with air from vents in the roof resulting oxidation processes sometimes generate enough heat to re-fuse the ceiling, solidifying it. Sometimes, this process can lead to the formation of stalactites as molten material drips from the ceiling.
Long exposure picture of a lava tube near Bend, Oregon. The lighting is artificial. Image courtesy of Michael Harms.
These structures are called lava tubes, and it’s important to note that they form on the surface and are later covered with sediments. They often have lava streams solidified along their floors. The most common access points into these caves are areas with collapsed ceiling.
Similar processes form inflationary caves or vertical conduits underground, which can be big enough to qualify as caves. The former are areas where lava pushed on neighboring rock then receded, leaving domes of solid rock behind. The latter are formed in areas where lava escaped to the surface.
Erosion is the process by which soil or rock is removed from their original structures by surface factors. Dissolution can be viewed as a particular case of erosion, but we’ve already talked about those.
Sea caves are also formed by water. But, while dissolution caves get hollowed out through chemical reactions, sea caves are constructed by wave-powered erosion, either above or below the waterline. They can be found on the shoreline, as the name implies, but also inland, in areas that were once close to the sea but have since dried up — in parts of Norway, for example. They can form in all types of rock: igneous, metamorphic or sedimentary.
This Minecrafty beauty is named Fingal’s Cave, Scotland. It is a sea cave formed through basalt pillars. Image via reddit user narwalmart
Waves form these structures by sheer attrition, throughout millions of years of battering with particle-rich water. As such, they tend to form in weaker areas of the rock, such as fault lines in igneous or metamorphic rocks or bedding plane contacts in sedimentary rocks. Once waves open a fissure through the rocks, the process becomes much faster — confined to a narrower space, the water and suspended particles exert more pressure on the walls and pressurize the gasses within, acting like a wedge.
Their walls are usually chunky and jagged, as erosion breaks off irregular slabs of rock from them. Some sea caves, however, have circular shapes with smooth walls and are filled with pebbles. This is caused by the waves taking on a circular motion inside the caves as they wash in and out, grinding the pebbles against the walls and smoothing them down.
Such as this beautiful cave in the Algarve region, Portugal. Image via Imgur
Because erosion is a continuous process, removing rock bit by bit, sea caves are prone to collapse, leaving behind a “littoral sinkhole.”
There are many other kinds of caves, each one with its own story to tell. Each one tells of how an area’s geology interacts with the world above it, being shaped by it over countless centuries. But, the paintings our ancestors adorned them with, the lines of sooth they burned into their walls stand testament to how they can, in turn, shape the world around them.
MIT researchers deployed intricate contraptions, including cables that run to the sea floor and an autonomous submarine, to measure internal weaves around the South China Sea. The researchers followed and measured these waves from their origin, until they dissipated, and in doing so have recorded the “largest waves documented in the global oceans.”
The water in the oceans and seas is laid down in layers, each of different density and other varying physical quantities like temperature, salinity and so forth. These interfaces extend horizontally and oscillations of the water across these layers are called internal waves, which in many respects aren’t that different from the surface waves that hit beaches and surfers use to ride them. Unlike surface waves, though, these are very large and with long periods.
In the South China Sea where the researchers concentrated their efforts, internal waves are due to tidal currents raking back and forth across sharp features of bathymetry. Ridges in the Luzan Strait are particularly effective at creating these sort of large amplitude waves. These waves can travel thousands of kilometres from their sources before breaking, making it very difficult to observe and measure.
To measure them, Thomas Peacock along with colleagues at MIT deployed cables that stretched more than 3000m until the rock bottom sea floor. These were attached to large buoys and along the cables various sensors were fixed at certain depths to measure physical properties. An autonomous underwater vehicles were also deployed to gather more data in the surrounding waters.
“Internal waves are the lumbering giants of the ocean,” Peacock says. “They move fairly slowly but they are very large in amplitude and carry a lot of energy.”
According to the paper published in Nature, the researchers found:
the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena
there are >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean. The biggest one was larger than 500m.
the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait
there’s a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region.
“Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions,” the researchers write.
Understanding how inner waves form and crash is important for a lot of reasons. For one, nowadays the oceans are littered with hundreds of thousands of underwater cables and understanding how these are affected by the ocean’s currents and waves is paramount for engineers seeking to design better solutions. Secondly, these are important mechanisms that need to be accounted for in global climate models; these blend the various ocean layers and mix matter.
Internal waves are also involved in making the moon move away from Earth. The moon recedes at a rate of around 1.78cm per year. That means it is now around 1.75m more distant than it was on the first moon landing. Maybe that’s why we’re not landing spacecraft that often, not to mention humans, on the moon anymore (/joke).
“To cut a long story short, it’s not unreasonable to say internal waves play a role in the moon moving away or receding from the Earth,” he says. “They are big enough that they affect large-scale celestial motions.”
BICEP2 (in the foreground) and the South Pole Telescope (in the background). Credit: Steffen Richter, Harvard University
Nobel prizes, international press coverage, awards – these were all promises and cheers thrown about all over the web after a team of physicists trumpeted during a conference at Harvard that they’ve made one of the biggest discoveries in science: gravity waves. Some theories claim that these waves were generated brief moments following the Big Bang, and a team of researchers based at the BICEP2 facility in Antarctica claimed during the aforementioned press conference in March that they’ve finally confirmed the models and have discovered evidence that support gravitational waves. A week ago, however, scientists from another team that works with the Planck satellite made its own measurements and came to an entirely difference conclusion. What BICEP2 detected weren’t gravity waves at all, but polarized light produced by specks of dust in the galaxy.
No problem, the BICEP2 was proven wrong. That’s how science works, fortunately – one scientist or group reports a result (revolutionary or not), then another group replicates the results to confirm or not the findings. Science prides itself on contradictions, because that’s how the veil of confusion is ultimately lift to reveal truth. The problem is that the BICEP2 team behaved unscientific by announcing their results before these were verified by anyone outside the group. I’m not out to criticize them personally, I’m just trying to signal that this very event is an example of what can go wrong (hint: everything) went scientists steer away from a basic pillar that’s been proven time and time again to work: peer review. It’s not like this is the first time something like this happens. Only a couple of years ago a team at CERN made a most audacious claim that neutrinos could travel faster than light. The claim was later refuted and the initial results were attributed to some bad wiring in the detection tech. Some scientists from the project resigned, lives may have been ruined. Once again, we return to the basics : extraordinary claims require extraordinary evidence.
Video showing one of BICEP2’s researchers at Andrei Linde’s doorstep to celebrate. Linde is one of the theorists whose work they claimed to have proved.
Both the Planck satellite and the South Polar BICEP2 telescope are designed to study what’s called the cosmic microwave background (CMB). First discovered in 1964 in a groundbreaking paper, CMB can be liken to fossil light waves that are rippling through space to this day after being emitted some 300,000 years following the Big Bang. Since their discovery 50 years ago, the CMB has been the go to source for studying cosmological phenomena. Like CMB, gravity waves are also relics from a time long past, only these are theorized to have been emitted only fractions of a second after the Big Bang. By probing gravity waves, physics would be able to tell a great deal about what was going on in those critical moments that spurred the cosmic inflation. Most notably, it would help scientists differentiate between the so many cosmic inflation theories proposed until today.
After carefully analyzing their own observations for the cosmic microwave background, the Planck team concluded that the supposed signal for gravity waves was, more likely, just emission from dust. To analyze cosmic background radiation, scientists need to first subtract the glow of our own galaxy in the same wavelengths as the CMB microwaves. Apparently, the BICEP2 experiment missed to include maps of the dust polarisation from the Milky Way in their calculations (they didn’t have them at their disposal, granted). In the end, the researchers underestimated the foreground emission from dust, and therefore over-estimated the significance of any claimed gravitational wave detection.
Some people said this event gives science a bad rep. We’re all flooded with misinterpreted, out of context news that herald all sorts of scientific developments that turn out to be otherwise in reality. This time, it wasn’t the press that blew it; it was the scientists.