Why do waterfalls move upstream? Waterfalls gradually move upstream because they erode the rock beneath them.

You might think a waterfall will always stay where it is, but waterfalls can gradually move upstream, sometimes to the point where they disappear forever. Take Niagara Falls, for example. Niagara Falls is probably the most famous waterfall in the world. The one most people think of when they imagine Niagara Falls is actually called Horseshoe Falls, because it is in the shape of a horseshoe, and it is on the Canadian side of the waterfall. The other main waterfall is smaller and is on the American side of the border.

Niagara Falls is on the Niagara River, which drains water out of Lake Erie, the fourth largest of the Great Lakes, into Lake Ontario, the smallest of the Great Lakes. The river flows over a rock cliff at the point of the waterfall and drops about 50 meters. A huge amount of water goes over the falls every minute, although the exact amount changes because some water is diverted for hydroelectric power. Niagara Falls has become an enormous tourist destination, and towns have grown on both sides of the border to support the industry.

However, the waterfall is still eroding. In the past, Niagara Falls eroded much more quickly than it does now. Today, because of water diversion and engineering work, the current rate is much slower and not completely clear. One common estimate is about 30 cm a year, and it may become even slower. That doesn’t sound like much, but over long periods of time, it adds up. In 100 years, the falls might only move about 30 meters, which would not change the tourist towns very much. However, over tens of thousands of years, the change could be enormous. If erosion keeps going, the falls could eventually work their way back toward Lake Erie and stop existing as the waterfall we know today.

This demonstrates why waterfalls move upstream. Niagara Falls formed after the end of the last ice age, about 10,000 to 12,000 years ago. The ice sheet over North America slowly receded, and a huge amount of meltwater became trapped in the hollows that became the five Great Lakes. This water overflowed from the lakes, forming rivers, and one of those rivers was the Niagara River. The river flowed over a hard layer of rock, but underneath that hard layer were softer rocks that could be eroded more easily. Given enough time, even strong rock can be worn away, and the falls have moved many kilometers from where they first started.

Why do waterfalls erode the rock under them? Waterfalls usually form when rivers flow over different types of rock. There might be a hard layer of rock on top and a softer layer underneath. The softer rock erodes more quickly, leaving a height gap between the two surfaces. The water keeps flowing over the edge, and a waterfall forms.

Rivers erode in several ways. The first is through all the particles that they carry. As a river flows from its source, pieces of rock break off and get carried along. These pieces hit other rocks, becoming smaller and smaller until they are sand and gravel. These particles scrape along the bedrock of the river, in the same way that sandpaper scrapes against wood. The second way is by dissolving some of the minerals in the rocks they are flowing past. The third way is through the sheer force of the water. Water can get into cracks, loosen rock, and split pieces away. More powerful rivers erode rock more quickly, but, given enough time, even the slowest river will erode the ground it flows over. This is one of the reasons the Grand Canyon has become so deep.

Waterfalls erode and move upstream in the same general way as rivers, but the main action happens at the bottom of the waterfall. When the water crashes into the plunge pool, it does not just fall quietly. It hits with enormous force. The water also carries sand, gravel, and broken stones, and these smash into the rocks at the bottom. Over time, this makes the plunge pool deeper and larger.

As the plunge pool grows, it starts to undercut the rock behind the waterfall. If there is a hard layer of rock on top and a softer layer below, the softer layer is worn away first. Eventually, the hard rock above is left hanging with nothing to support it. Then it cracks, breaks off, and falls into the plunge pool. The waterfall has not really walked upstream, but the edge has collapsed backward. Then the same process begins again. The water keeps falling, the plunge pool keeps growing, the softer rock keeps eroding, and the unsupported rock keeps breaking away.

This is called waterfall retreat, or headward erosion. It is why some waterfalls slowly move upstream over time. It is also why waterfalls are not permanent features in the landscape. They seem fixed because human lives are short, but on a geological timescale, they are moving, changing, and sometimes disappearing. Niagara Falls looks eternal, but it is really just one stage in a very long process of water cutting through rock. And this is what I learned today.

Sources

https://en.wikipedia.org/wiki/Niagara_Falls

https://en.wikipedia.org/wiki/Waterfall

https://education.nationalgeographic.org/resource/waterfall

https://www.niagaraparks.com/visit-niagara-parks/plan-your-visit/niagara-falls-geology-facts-figures

https://goniagaratours.com/blog/how-fast-is-niagara-falls-erodingthe-slow-but-powerful-retreat-of-a-natural-wonder

https://www.bbc.co.uk/bitesize/guides/z6jx382/revision/2

Photo by Kt Ktgbk: https://www.pexels.com/photo/stunning-view-of-seljalandsfoss-waterfall-in-iceland-30423523/

The post #1705 Why do waterfalls move upstream? appeared first on I Learned This Today.

What is the watershed? A watershed is a turning point or a dividing point between two times or events, although the word originally comes from geography. In geography, a watershed is an area of land that drains water to a specific point.

Another word for a watershed is a drainage basin. It is basically the area of land where all of the water drains to the same place. If you are from the UK, you probably know the word more as the time when family TV shifts into adult TV. In the UK, the watershed is at 21:00. Family programming slowly transitions into adult programming after that time. Almost every country has some kind of system like this, but they don’t usually call it the watershed. The US generally sets it at 22:00, but it doesn’t have the same common word for it. The system in the UK first started with a 1964 Act, but the word watershed was used more as a nickname than an official name.

So, what really is a watershed? A watershed is a land area that channels rainfall and snowmelt into a common body of water. That body of water might be a river, a lake, or the ocean. Because of gravity, water always flows downhill. Rivers are fed by rainwater and snowmelt. When rain falls on mountains, or when snow melts on mountains, the water runs down and forms channels, which carry it to the base of the mountain. From the mountains or the high ground, the land slopes downward, and this is where the water flows.

However, the land is not completely flat. There are ridges and areas of high ground that separate lower areas into large basins. These ridges form the boundaries of each watershed. Rainwater or snowmelt running down a mountain could hit one side of a ridge and go in one direction, while water on the other side could go in a completely different direction. Two drops of rain could fall only a few meters apart and end up in different rivers, or even different oceans.

The rainwater and snowmelt in the basin begin to form streams. These streams flow into larger streams, then rivers, then lakes or the sea. The system gets bigger and bigger as more water joins it. The literal meaning of the word is an area where water is shed. Watersheds can range from fairly small areas to huge systems that cover a large percentage of a country. The Mississippi River watershed covers almost 3 million square kilometers, which makes it bigger than many countries. Water from 31 US states and two Canadian provinces drains into it. The final destination for all of this water is the ocean.

Because watersheds are networks of connected waterways, pollution can wreak havoc in one area. Pollution can get into a watershed from many different sources. It might be washed out of the sky by rain. It might have been frozen in snow or glaciers that have started to melt. It could be a byproduct of a factory that is released into the wild. One of the most common types of pollution in waterways is runoff from farms. Fertilizers and pesticides that are spread on fields can very easily get washed out of the soil and carried into streams. All of these pollutants can spread through a watershed, and the problem can build as smaller streams join larger rivers. People and animals living farther down the watershed may have to deal with pollution that started a long way away.

Damage higher up in the watershed can also lead to problems such as flooding farther down. In a healthy watershed, forests, wetlands, and soil all slow the movement of water. Trees catch some of the rain. Soil absorbs water. Wetlands act like sponges and release water slowly. However, if forests are cut down, wetlands are filled in, or large areas are covered with concrete, the water cannot be held back in the same way. Instead, it rushes into streams and rivers all at once. That can cause flooding and damage farther down the watershed. The silt and soil carried by the excess water can also become a problem because it can block rivers, damage habitats, and make the water harder to clean.

And this is why the metaphor of a watershed moment makes sense. You can’t always see the barrier between two watersheds, but it is there. One drop of rain falls on one side and begins one journey. Another drop falls on the other side and begins a completely different journey. With us, a watershed moment separates the flow of things that came before from the flow of things that will come after. And, just like a real watershed, what happens near the beginning can have huge and lasting impacts downstream. And this is what I learned today.

Sources

https://ecology.wa.gov/ecologys-work-near-you/river-basins-groundwater/what-is-a-watershed-and-why-does-it-matter

https://oceanservice.noaa.gov/facts/watershed.html

https://education.nationalgeographic.org/resource/watershed

https://en.wikipedia.org/wiki/Watershed_(broadcasting)

https://www.etymonline.com/word/watershed

Photo by Tom Fisk: https://www.pexels.com/photo/scenic-wetlands-landscape-in-alma-wisconsin-34305559/

The post #1682 What is the watershed? appeared first on I Learned This Today.

Why are some places windier than others? There are many reasons why some places might be windier than others. Some of those reasons are temperature differences, topography, the amount of surface friction, and wind funneling.

Wind is caused when air moves from an area of high pressure to an area of low pressure. Air behaves like a fluid, and a fluid will always move in a way that tries to balance differences in pressure and energy. The atmosphere is constantly trying to even itself out. If one place has warmer air, cooler air, denser air, or thinner air than another, the imbalance creates movement. This is a little like the reason hot coffee cools down or a chilled drink warms up. Energy tends to spread out until things are more balanced. In the atmosphere, that balancing process often shows up as wind.

One of the biggest reasons why some areas might be windier than others is because of temperature differences. When land is heated strongly by the sun, the air above it warms up, expands, and rises. This can help create lower pressure at the surface. If there is cooler air nearby, that cooler air is denser and can move in to replace the rising warm air. The greater the temperature difference between two nearby areas, the stronger the movement of air can be. This is one reason coastal places often get strong breezes. The land heats up and cools down faster than the sea, so air is always shifting between the two.

The topography of an area matters greatly in how windy it is. Mountains can act as huge obstacles to the movement of air. The mountain does not stop the wind, but it forces it upward and around the slopes. This can create very strong winds on ridges and summits. Once air has crossed a mountain barrier, it can descend on the other side and sometimes speed up as it goes. As air rises up one side of a mountain, it cools, and moisture in it may condense and fall as rain or snow. By the time the air comes down the other side, it is often much drier. This is one reason why deserts sometimes form in the lee of mountains. The area behind the mountain can end up both drier and affected by distinctive wind patterns.

Valleys can also cause strong winds because they can act like funnels, concentrating the movement of air. When a fluid moves through a constricted space, it often accelerates. This is related to what is called the Venturi effect, named after the Italian physicist Giovanni Venturi. In a valley or mountain pass, the shape of the land can channel the air into a narrower route. As the air is squeezed into that smaller space, it can move faster. This is why some valleys and gaps are famous for strong winds. The land does not create the wind from nothing, but it can make an existing flow much stronger and more focused.

The amount of surface friction is important as well. Wind carries energy, and anything that takes some of that energy away will slow it down. Friction with the Earth removes energy from the moving air. Objects on the ground, such as trees, hills, and buildings, also disrupt the flow and weaken the wind near the surface. The smoother the ground is, the less energy is taken away. Open areas such as lakes, deserts, grasslands, or broad plains have less surface roughness, so winds can often get much stronger there than they can in forests or crowded urban neighborhoods.

Built-up areas can experience all of these effects in a small space. Cities with parks can have significant temperature differences over short distances. Asphalt and concrete heat up more than green spaces do, and that can help create local air movement. Cities also have tall buildings that block and redirect the wind, pushing it upward, around corners, and down into the streets. Streets lined with high buildings can funnel the wind and make it feel much stronger at ground level. At the same time, the roughness of a city can slow the wind in some places while accelerating it in others. This is why cities often have odd patches of calm air and sudden gusty corners only a block apart.

Some cities are windier because of where they are located as well as how they are built. Chicago is a good example. It sits next to Lake Michigan, which is a large open body of water with very little surface roughness compared with the land. Air moving across the lake can pick up speed because there are few obstacles to slow it down. Then, when that moving air reaches the city, the buildings can redirect and funnel it, creating strong gusts. So when a place feels especially windy, it is usually not because of one single cause. It is often the result of temperature differences, the shape of the land, surface friction, and the way the wind is directed by the environment around it.

Sources

https://www.berito.nl/blog/why-are-some-places-more-windy-than-others

https://education.nationalgeographic.org/resource/wind

https://weatherology.com/trending/articles/Professor-Paul-Venturi-Effect.html

Photo by Jan  Zakelj: https://www.pexels.com/photo/a-big-tree-swaying-by-the-wind-under-blue-sky-9059257/

The post #1640 Why are some places windier than others? appeared first on I Learned This Today.

Why does China only have one time zone? China only has one official time zone because that is what Chairman Mao decreed in 1949 to promote unity in the newly formed People’s Republic of China.

The world is divided into time zones because of the speed at which the Earth rotates and the amount of time it takes for the sun to cross a certain area of land. The Earth rotates once every 24 hours, which means the sun crosses 15 degrees of the Earth per hour. That means that if it is noon in one location, the sun is one hour past noon 15 degrees to the east and one hour before noon 15 degrees to the west. This is why we need time zones. Theoretically, there should be 24 time zones, one for every hour or 15 degrees, but in fact there are 38. Some of them are only 30 or 45 minutes rather than a full hour.

There was no real need for standardized time until the invention and spread of the railways, and there was even less reason for ordinary people to worry about international time differences before the modern age. This is because most people did not own clocks, very few were concerned with exact time, and there was no way to get anywhere fast enough for the time difference to matter much. If someone went from London to New York, which has a five-hour time difference, in 1650, the journey would take roughly ten weeks. Nobody would be checking a wristwatch, and daily life would still be governed by the sun. By the time a traveler arrived, their habits would already have adjusted to local daylight.

When the train was invented, people could suddenly move across a country rapidly. England had no standardized time because noon was about 20 minutes later in the west than in the east. Before rail travel, that was not much of a problem because people lived by local time. Once trains began running on schedules, though, that changed. Timetables had to match from one town to another, so the whole of the UK gradually had to use London time. That was not a huge problem because the west was only about 20 minutes out. International travel still took so long that most people would not feel very concerned about time differences between countries. Later, with faster global travel, those differences became much more obvious and coordinated timekeeping became more important.

China is a vast country that stretches about 5,200 km from east to west. That is roughly 46 degrees on the Earth’s surface, meaning about three hours of difference in sunlight. When the sun is at noon in the center of China, the eastern side is about 90 minutes past noon, and the western side is about 90 minutes before noon. For a long time, official timekeeping in China centered on Beijing, but that was not much of a problem because very few people had clocks and many people who worked the land mainly cared about the position of the sun. As travel became faster and more people had watches, it became more of an issue. Five time zones were proposed in the 1930s, but different cities ended up using different local systems, which was confusing. The five time zones were then set to be used after the end of World War II.

Before the five time zones could really take hold, the government fell to Mao Zedong and the Chinese Communist Party. The Communist Party wanted to create a more unified China, and one way it tried to do that was through timekeeping. The government decreed that all of China would operate on Beijing time.

Through a large part of the 20th century, many people in China were poor and worked the land. They did not use watches much and did not care very much what official time zone they were in. They rose with the sun, worked the land, and slept when the sun went down. As China became more prosperous and urbanized, people began to suggest that a single time zone was not necessarily a good idea. However, the government generally prioritized unity and control over regional convenience. In politically sensitive areas, even small policy questions can become tied to larger fears about stability and separatism. For that reason, China has continued to use a single official time zone. Whether or not it will ever return to multiple time zones is anyone’s guess. And this is what I learned today.

Sources

https://www.aljazeera.com/news/2023/8/9/conflict-over-the-clock-china-among-countries-where-time-is-political

https://en.wikipedia.org/wiki/Time_in_China

https://velvetshark.com/how-many-time-zones-in-the-world

Photo by Ramaz Bluashvili: https://www.pexels.com/photo/chinese-flag-and-great-wall-of-china-in-beijing-31639349/

The post #1639 Why does China only have one time zone? appeared first on I Learned This Today.

Do we still need weather balloons? Yes, weather balloons are still needed. Satellites can provide a lot of data, but they still cannot sample the atmosphere directly from inside it, and they are far more expensive.

There are thousands of satellites in orbit around Earth at the moment, so it is easy to assume there is no longer any need for weather balloons. Weather balloons have been around since the end of the 19th century, although hot air balloons were invented almost a century earlier. One reason weather balloons were not used sooner is that automatic recording equipment had not yet been invented. Léon Teisserenc de Bort launched some of the first weather balloons in France in 1896, and he used hundreds of them. He made numerous discoveries about the upper air and helped identify the tropopause and the stratosphere. The tropopause is the boundary between the troposphere (the lowest layer of the atmosphere, where most weather happens) and the stratosphere. It occurs at roughly 9 km over the poles and about 17 km over the equator, although it varies. Despite these discoveries, early balloon work was limited by the instruments available at the time, and finding the balloons after they landed was often challenging.

What are modern weather balloons and what do they do? Weather balloons are made of strong, flexible latex. They are filled with hydrogen or helium, loaded with an instrument called a radiosonde, and released. Hydrogen is commonly used. Balloons ascend at variable rates depending on the amount of gas inside them; 300 meters per minute is a common speed. Some modern balloons use valves or vents so gas can be released if a slower ascent is needed. As the balloons get higher, the air pressure drops and they expand dramatically. They often reach about 30–40 km, because the balloon stretches so much that it ruptures. The basic idea is to let the balloon go as high as it can until it bursts.

A bursting balloon scatters pieces of latex that drift down and eventually land somewhere on Earth, sometimes ending up in the oceans. In some systems, gas can be vented for a more controlled descent, but many flights still end with a rupture. The radiosonde is attached below the balloon, usually with a parachute. The parachute is not there to save the radiosonde from damage so much as to reduce the risk of it injuring someone on the ground. Radiosondes are generally treated as disposable. If found, many have a return address printed on them, but the majority are never recovered. That is not a problem for the weather data, because the information has already been transmitted back to the weather center. Each unit is not that expensive (often quoted at around $25), but the larger concern is the environmental cost of dropping equipment across the landscape. For that reason, people are experimenting with biodegradable parts and better recovery systems.

As the weather balloon ascends, the radiosonde keeps a record of its altitude and position and takes measurements of temperature, humidity, atmospheric pressure, and wind speed and direction. This kind of vertical profile is often called an upper-air sounding. Wind is measured indirectly, because the balloon drifts with the air currents and its changing position reveals how the air is moving at different heights. In the early days of weather balloons, the operator had to find the balloon to recover the data, but these days the data is beamed back in real time. All of this information gives the weather center a snapshot of the column of air directly above the launch point, which can give a much better idea of what the weather is likely to do next. The data is also used to check and adjust computerized weather models. Weather balloons are extremely common, and many launch sites send up balloons at fixed times each day. The USA and Canada together launch roughly 100,000 of them a year.

So, wouldn’t it be possible to replace them with satellites? Satellites are an amazing way to gather enormous amounts of information about Earth, but they cannot do what weather balloons can do. Satellites can detect ground temperature, cloud heights, the amount of water in the atmosphere, and many other things, but they have difficulty producing the same kind of direct measurements through many altitudes, the way a weather balloon can. Also, even though there are huge numbers of satellites in orbit, they are extremely expensive and getting access to one is not easy. Repositioning satellites for specific weather needs is tricky as well. There are dedicated weather satellites in space, but the data they gather is used in conjunction with the data gathered by weather balloons. And this is what I learned today.

Sources

https://en.wikipedia.org/wiki/Weather_balloon

https://en.wikipedia.org/wiki/Radiosonde

Photo by ROMAN ODINTSOV: https://www.pexels.com/photo/white-hot-air-balloon-flying-in-the-cloudy-sky-7539903/

The post #1629 Do we still need weather balloons? appeared first on I Learned This Today.

Who were Currer, Ellis, and Acton Bell? They were the pseudonyms used by Charlotte, Emily, and Anne Brontë.

The Brontë sisters are known for some of the most famous novels in the English language. Charlotte Brontë wrote four novels, the most famous of which is Jane Eyre. Emily Brontë wrote Wuthering Heights. Anne Brontë wrote two novels, Agnes Grey and The Tenant of Wildfell Hall. The three sisters were unable to write more because they all died at very young ages. Anne was 29, Emily 30, and Charlotte 38. Anne and Emily both died of tuberculosis, while Charlotte is now often thought to have died from complications connected to pregnancy. Who knows how many novels they would have been able to write if they had lived longer?

The three sisters were born in the early 19th century, and they were prolific writers, sharing their stories with each other from a young age. Their father, Patrick Brontë, was an Anglican clergyman who was well educated and unusually supportive of his daughters’ learning. He was also an Irish immigrant who had changed his name from Brunty or Prunty to Brontë, although nobody knows for certain exactly why he did it. The three famous Brontë sisters were actually three of six children, five girls and one boy. Their two older sisters, Maria and Elizabeth, died young at the ages of 11 and 10. Tuberculosis was one of the great killers of the age. Patrick Brontë was not a wealthy man, but his job made him lower middle class and gave the family a house to live in. Unusually for the time, he believed in educating his daughters and sent them away to school. It was not common to educate girls in the same serious way as boys, and when girls were educated, the subjects were often limited to music, singing, French, drawing, and sewing. These were subjects they were expected to use later in life. Pretty much the only respectable jobs open to women at the time were teaching in schools, being a governess to children in a wealthy family, or doing domestic work of some kind.

The three Brontë sisters did end up teaching or working as governesses, and they did not like it. They wanted to write. They spent most of their spare time writing and sharing their stories with each other. However, when it came to publishing their work, they had another problem. Society at the time did not think women should be authors. It was thought that women were not suited to intellectual or academic pursuits. There were almost no women in academia and very few female authors who were taken seriously. Two famous examples were Jane Austen and Mary Shelley, who were not originally published under their own names. Jane Austen’s books appeared anonymously or as “By a Lady,” and the first edition of Frankenstein was published anonymously as well. Many readers assumed Percy Shelley had written it.

The Brontë sisters realized that if they were going to try to get their work published, they would have to think of different names. They wanted to keep their initials, so they ended up with Currer for Charlotte, Ellis for Emily, and Acton for Anne. Nobody knows for certain where Bell came from , but their brother’s name was Branwell, so it could have been the first and last three letters of his name. They took these names and set out to publish their work, first in a joint collection of poems and then in novels. The names sounded masculine enough to protect them, but they were still close to their real identities.

Their first book, a collection of poems, was not successful at all. In fact, it sold only a couple of copies. The novels did much better, especially Jane Eyre, which became a major success very quickly, although Wuthering Heights and Agnes Grey had a more mixed early reception. At one point, there was confusion over whether Currer, Ellis, and Acton Bell were really different people. To prove that they were, Anne and Charlotte traveled to London and appeared in person before the publisher. This meant their father and their publisher were the only people who knew their true identities.

Their works were not widely published under their own names until 1850, after Anne and Emily had both died. Charlotte wrote a preface for a new edition of Wuthering Heights and Agnes Grey in which she revealed who the three sisters were. At first, many readers were surprised that such raw and passionate novels had been written by women. After a while, though, people got used to it and Charlotte’s fame grew. Unfortunately, she died in 1855. Their father, Patrick, outlived his wife and all six of his children by six years. Rather sad. And this is what I learned today.

Sources

https://www.andrews.edu/life/student-movement/issues/2024-03-08/a-e__the-bronteu-sisters.html

https://en.wikipedia.org/wiki/Bront%C3%AB_family

https://en.wikipedia.org/wiki/Charlotte_Bront%C3%AB

https://en.wikipedia.org/wiki/Emily_Bront%C3%AB

https://en.wikipedia.org/wiki/Anne_Bront%C3%AB

Image By Branwell Brontë – one or more third parties have made copyright claims against Wikimedia Commons in relation to the work from which this is sourced or a purely mechanical reproduction thereof. This may be due to recognition of the “sweat of the brow” doctrine, allowing works to be eligible for protection through skill and labour, and not purely by originality as is the case in the United States (where this website is hosted). These claims may or may not be valid in all jurisdictions.As such, use of this image in the jurisdiction of the claimant or other countries may be regarded as copyright infringement. Please see Commons:When to use the PD-Art tag for more information., Public Domain, https://commons.wikimedia.org/w/index.php?curid=6553830

The post #1618 Who were Currer, Ellis, and Acton Bell? appeared first on I Learned This Today.

How do salt flats form? Salt flats form when water with a high mineral content gets trapped and evaporates more quickly than it can be replenished. Salt flats are sometimes called salt pans. They are a large expanse of land that used to be a lakebed but is now covered in a hard crust of salt and other minerals mixed with fine sediment.

There are many salt flats around the world, and they range from tiny to enormous. The largest is the Salar de Uyuni in Bolivia. It is about 10,852 km², which makes it larger than a lot of cities, and it sits at an elevation of around 3,656 meters. There is also the Bonneville Salt Flats in Utah, USA, which are famous for the many land speed records that have been attempted there. They are extremely flat, which makes them perfect for going fast.

Salt flats are usually the bottoms of dried-up lakes, but they will not form in just any dried-up lake. Salt flats form when the water is salty, the drainage basin is completely enclosed, and the climate is dry enough that the water evaporates more quickly than it can be replaced. It also helps if this wet-and-dry process repeats, because each cycle leaves a little more mineral behind.

All water has dissolved minerals in it, but some water carries far more than other water. Rivers and freshwater lakes contain small amounts of salt, but it is washed downstream by the current. They are also replenished by rainfall, which dilutes the mineral content. The oceans have more salt because they lose water through evaporation and, when water evaporates, it leaves its dissolved salts behind.

Some rivers pick up more dissolved minerals than others depending on the rocks they run through. If the surrounding rock is highly weathered, it breaks down more easily and minerals leach into the water. Those rivers can carry salts and other dissolved ions into a lake, slowly making it more and more salty. This is the first step in the formation of a salt flat.

The second step is to have a lake that does not drain anywhere. Most lakes are fed by rivers at one end and drain into other rivers at the other end. For a salt flat to form, the lake cannot have an outlet. The only way water can leave is by soaking into the ground or evaporating into the air.

The third step is to cut off, or at least reduce, the rivers that feed the lake. In very arid regions this can happen naturally because the climate shifts, the sources of water dry up, and the rivers weaken or disappear. At that point, little new water is flowing into the lake and none of the water is flowing out of it. The remaining water becomes a concentrated brine.

If the area is hot and dry enough, the water in the trapped lake starts to evaporate. The main idea is that water evaporates easily but dissolved minerals do not. Table salt (sodium chloride) has a boiling point well above 1,000°C (around 1,400°C), so it is not going anywhere at normal temperatures. As the water evaporates, the brine becomes more concentrated, and eventually minerals begin to crystallize out and settle. Over time, this builds up an evaporite layer: mostly salt in many places, but often with other minerals as well.

Many salt flats do not form in a single, dramatic drying event. They grow through cycles. During wetter periods, a shallow lake can return and dissolve some of the surface crust. Then, when the water evaporates again, a fresh layer of crystals forms. Brine can also rise slowly through the sediment, evaporate at the surface, and leave another thin deposit. Repetition smooths the surface, fills small dips, and produces the wide, level plains that make salt flats so striking. The polygon patterns that appear on some flats are thought to form as the crust shrinks, cracks, and is re-cemented over time.

Salt flats are often popular tourist destinations. The Salar de Uyuni in Bolivia can turn into a mirror when it rains. A thin layer of rainwater covers the flat salt and creates near-perfect reflections, especially when the air is calm.

However, salt flats are also used as a resource. People have used surface salt for food in some places, but much of the salt from large natural flats is used for industrial purposes because it can be contaminated with sand, dirt, and other minerals. In some regions there can also be trace elements such as arsenic, depending on the local geology. If salt is intended for food, it usually needs careful processing, which can be expensive. Salt for consumption can be produced in controlled areas that are kept very clean, where salty water is spread out in shallow ponds and left to evaporate. And this is what I learned today?

Sources

https://en.wikipedia.org/wiki/Salt_flat

https://en.wikipedia.org/wiki/Salar_de_Uyuni

https://www.thoughtco.com/salt-flats-geography-1435836

https://www.echemi.com/community/can-you-eat-salt-from-the-great-salt-flats-in-utah_mjart2204152579_201.html

https://my.nsta.org/resource/6277/science-101-why-are-oceans-salty-and-lakes-and-rivers-not

Photo by Leonardo Rossatti: https://www.pexels.com/photo/uyuni-salt-flat-2613110/

The post #1607 How do salt flats form? appeared first on I Learned This Today.

What is a lava tube? A lava tube is a tunnel made by flowing lava that sets on the outside but continues to flow on the inside. They can range from very small to enormous.

For lava tubes to form, you obviously need lava. Not all volcanic eruptions produce lava. Some of the bigger, more explosive volcanic eruptions don’t produce as much lava. The pressure builds up in them until they explode, and then they blast rock, ash, and debris into the air. They can also create pyroclastic flows, which are when superheated gas and ash pour down the side of the volcano, almost like a river. It can travel at over 150 km per hour and can burn or destroy everything in its path. Pompeii was buried by a pyroclastic flow. This kind of volcanic eruption doesn’t produce lava or lava tunnels.

Lava is magma that has reached the surface. It is a very hot stream of almost liquid, molten rock. It is made up of minerals and gases. These minerals have become molten because of the immense pressure in the Earth. When something is pressurized, its melting point is raised, which means it can get very, very hot without melting. This is the rock deep in the Earth. As the rock rises, the pressure drops, and the melting temperature of the rock drops to a point lower than the temperature it is at, so it melts. The minerals are not completely melted, and the magma is a viscous fluid. Sometimes the magma comes to the surface and flows out of the volcano. At this point, it becomes lava.

There are three types of lava, and they contain different minerals, which affect their flow in different ways. There is basaltic lava, which has lots of iron, magnesium, and calcium. It is the hottest type of lava, generally between 1000 and 1200℃, which means it can generally travel the furthest from its source. Then there is andesitic lava, which has a moderate amount of all these minerals and is cooler than basaltic lava by about 200℃. Last, there is rhyolitic lava, which has a lot of potassium and sodium, but not much iron, magnesium, and calcium. It is about 650 to 800℃, which makes it the coolest of all the types of lava. This is the most viscous of all the types of lava.

Lava tubes are made by basaltic lava because it has the lowest viscosity, which means it can travel the furthest. It also has a surface called pahoehoe, which sets very easily. When the lava comes out of the ground, it contains a lot of gas, which slowly rises to the top. This makes the lava more like foam, and it allows heat to be carried away from the outer layer of the lava more quickly, cooling it. As the lava flows away from the volcano, the outer surface continues to cool, forming a fairly hard roof. Underneath this, the rest of the liquid lava continues to flow. If the slope the lava is flowing down is not very steep, then the lava will slowly cool down and solidify until the whole tube is full of solid rock. However, if the slope is steeper, the lava will continue to flow, ending up in the sea, or going into a hole in the ground. That will leave the lava tube completely empty, giving us the tubes that we have today. Lava in lava tubes actually flows further than lava exposed to the air because the roof of the tube provides insulation, which stops the lava from cooling and keeps it flowing. Observations in Hawaii have shown that the temperature of lava in a lava tube only drops by a few degrees over the entire length of the tube.  

Some lava tubes can be over 20 meters wide, and they can stretch for tens of kilometers. They also give a good sign of what volcanic activity has occurred because subsequent lava flows will follow the same path. Volcanologists can see the height that the lava flow reached marked on the wall of the tube. There are a lot of lava tubes on other planets that we can see as well. There are some on the moon and many on Mars. A lot of planets, such as Mars, don’t have tectonic plates, so volcanoes there can grow to enormous sizes. Olympus Mons, on Mars, is 2.5 times taller than Mount Everest. It also puts out a lot of lava, which can be seen from the lava tubes. They travel a long way out from the volcano. The lower gravity also means they can be bigger, and some of these tubes are hundreds of meters wide. And this is what I learned today.

Sources

https://en.wikipedia.org/wiki/Lava_tube

https://volcano.oregonstate.edu/faq/what-lava-made

https://education.nationalgeographic.org/resource/magma

https://perlan.is/articles/what-is-lava

https://www.usgs.gov/news/volcano-watch-how-does-pahoehoe-flow

The post #1514 What is a lava tube? appeared first on I Learned This Today.

What is a sinkhole? A sinkhole is a place where the ground has collapsed to leave a hole, caused by material below the ground being removed. Sinkholes can range from just a few meters deep to enormously deep, and they can be caused naturally or by human activity. The largest sinkhole in the world is in China. It is called Xiaoxhai Tiankeng, which means “heavenly pit”. It is 660 m deep, and it has a volume of 130 million cubic meters. Both natural and human caused sinkholes happen because material underneath the ground has been removed. It is usually removed by water, but it can also be removed by mining or pumping.

A lot of sinkholes form in areas where the rock is limestone. If there is a lot of rainfall or an underground river flowing into the limestone, it can start to dissolve. Water dissolves limestone very easily because, as it flows through the soil, the water absorbs carbon dioxide, which makes it slightly acidic. The carbon dioxide comes from all of the microbes and bacteria that are breaking down the dead matter in the soil. When carbon dioxide dissolves in water, it is called carbonic acid and, interestingly, is the reason why carbonated water tastes slightly sour. Carbonic acid is not strong enough to dissolve most types of rock, but it can dissolve limestone. The carbonated water flows into cracks in the limestone. It takes a very long time, but it slowly dissolves the limestone and makes the cracks bigger. Over thousands, sometimes tens of thousands of years, the carbonated water can dissolve enough of the limestone that a huge hole develops. This will be a sinkhole.

Sinkholes can emerge in three different ways. They all begin with carbonated rainwater dissolving the limestone under the surface. In the first case, solution sinkholes, the limestone starts to dissolve at the surface, and a depression is formed. Over time, more rainfall falls, and there is a gradual increase in the depression until it fills with water. At this point, the water can flow down into the limestone and continue the erosion, or it can get blocked with soil and just become a lake. The second type is called cover-subsidence sinkholes. Rainwater comes down through the soil and dissolves the limestone. Soil and sand from the surface slowly trickle down into the expanding hole, and the surface gradually drops. This is a slow process and not that damaging. It might slow down as the soil fills the gaps. The third type is called cover-collapse sinkholes, and these are the most dangerous and the most damaging type. They begin in the same way as the second type of sinkhole. Carbonated water dissolves the limestone, and the sand and soil from the surface start to trickle down. The difference here is that the top layer of the ground stays in place, forming an arch. The limestone hole gets bigger, more soil falls down, and the top of the arch gets thinner and thinner until it can’t support itself anymore. At that point, it collapses into the very deep hole that has formed underneath it. This can cause a lot of damage and even fatalities.

Sinkholes can also form in different types of ground. Salt is another common mineral that sinkholes form in because it can be dissolved very easily. There are some areas where sinkholes have formed because the salt has been mined or pumped out of the ground, and this is where human caused sinkholes form. There are many ways of making salt, but one of the methods is to mine it from under the ground. If a large area of salt is found, it can be mined by pumping in water, which dissolves the salt, and then pumping out the water. The salt can then be extracted from the water. The problem with this is that the salt was supporting the ground, and if you remove it, the ground can collapse. The land around the Dead Sea has a lot of sinkholes because it is an extremely salty area with lots of rock salt. Humans can cause sinkholes in other ways as well. They could come from underground water systems that begin to leak. Lots of water leaks into the ground, which dissolves the bedrock, and sinkholes form. Mining can also cause sinkholes because it removes large amounts of earth, which can weaken the ground. They also alter the water table sometimes. Sinkholes can appear in cities sometimes as well because buildings can funnel rainwater and concentrate it in certain places, where it starts to sink into the ground. Sinkholes in cities or built-up areas can be disastrous and result in loss of life. And this is what I learned today.

Sources

https://www.usgs.gov/faqs/what-difference-between-a-sinkhole-and-land-subsidence

https://www.usgs.gov/faqs/what-difference-between-a-sinkhole-and-a-pothole

https://en.wikipedia.org/wiki/Sinkhole

https://education.nationalgeographic.org/resource/sinkhole

https://www.bbc.com/travel/article/20221117-xiaoxhai-tiankeng-the-worlds-biggest-sinkhole

https://igws.iu.edu/outreach/lessonplans/dissolving

https://en.wikipedia.org/wiki/Carbonated_water

Photo by Robert Senz: https://www.pexels.com/photo/view-of-a-cave-15319512/

The post #1513 What is a sinkhole? appeared first on I Learned This Today.

How did they find the Chicxulub crater? They found the Chicxulub crater, the remains of the asteroid that wiped out the dinosaurs, when they were looking for oil.

The Chicxulub crater was made when an asteroid ten kilometers in diameter hit Earth. It landed just over 66 million years ago and left a crater that is 200 kilometers wide and 30 kilometers deep. The crater has an outer edge that reaches 300 kilometers.  The actual age has been calculated at 66,043,000 years, plus or minus 11,000 years. The area where the asteroid hit is currently half on land and half at sea, but when it impacted, the whole area was under the sea. The asteroid was traveling at 20 kilometers per second. The impact caused winds of 1,000 km/h and tsunamis that might have reached 1.5 km in height. It ripped up the seabed and thrust hot ash, dust, and other materials into the atmosphere. Most of North and Central America would have been decimated by waves, earthquakes, and fires. The rest of the dinosaurs and a lot of other life were wiped out by the nuclear winter that followed.

The asteroid theory for the extinction of the dinosaurs is pretty widely accepted today, but it was only introduced in 1980. Two scientists, Luis Walter Alvarez and his son Walter, had discovered a layer of iridium-rich clay that appeared to circle the Earth. They found it in every place they looked and always at the same depth. Iridium is very rare on Earth, but it is very common on asteroids. The fact that there was a layer of iridium soil all over the Earth at the same depth implied that it had been put down at the same time and most likely by an asteroid impact that was large enough to spread it all over the world. The depth of the iridium corresponded to the Cretaceous-Tertiary extinction, and the Alvarez theory was born, but it was doubted by a lot of people. The only thing that could help prove it would be to find the crater, but that was unlikely. The majority of asteroids come down in the sea, which makes sense because 71% of the surface of Earth is sea.

Finding the impact site proved tricky. The Alvarez’s discovery sparked worldwide interest in the geology community, and they started hunting. An international conference, called Snowbird, was called in 1981 to coordinate all of the search efforts. Unbeknownst to all of these crater hunters, it had already been found. The Mexican state-owned oil company Petroleos Mexicanos had surveyed the area and even dug up core samples. The core samples had been dug up in the 1950s and stored. The survey had taken place in 1978, conducted by two geophysicists called Glen Penfield and Antonio Camargo. They were using magnetic data, and they found unusual readings which were consistent with an asteroid impact. Penfield took the results to his company, but they told him he couldn’t release them. Oil companies keep their geological data very secret, for fear of competition. Strangely, the oil company said he couldn’t release the data, but they did say he could present some of his results at the 1981 Society of Exploration Geophysicists conference. There were very few people at the conference to hear Penfield’s presentation about the impact crater because all of the experts were at the Snowbird conference talking about how they could find the impact crater. Penfield tried to contact Alvarez, but was unable. He had a story in a newspaper, but nobody saw it. Meanwhile, all of the geologists kept searching. They found shocked quartz, which is caused by an impact. They found weathered glass beads that were formed in the intense heat of an impact. Yet, they were looking in the wrong place. It was only in 1990, 12 years after Penfield found the crater and 10 years after Alvarez published his theory, that the people who knew where the crater was and the people who were looking for it came together. In this interconnected age of the Internet, that seems amazing. A group of geologists analyzed it, and they published their findings in 1990. They named the crater Chicxulub after a nearby town.

You might think you would be able to see the crater if you looked on Google Maps. That’s what I thought. You can’t see it, though, because millions of years of sediment and rock have completely covered it over. 66 million years is a lot of time for the land to change. The original land that was there when the asteroid landed is under at least a kilometer of new rock and soil. The land is slightly depressed, but not something you can see on Google Maps. There is also a ring of lakes called cenotes around the edge of the impact crater. The rock was fractured by the impact, and this lets water get in. The water dissolved the limestone and left caves and fissures that have been filled with water. And this is what I learned today.

Sources

https://www.nhm.ac.uk/discover/how-an-asteroid-caused-extinction-of-dinosaurs.html

https://en.wikipedia.org/wiki/Chicxulub_crater

https://www.lpi.usra.edu/science/kring/Chicxulub/discovery

https://www.ebsco.com/research-starters/geology/chicxulub-crater

https://yucatantoday.com/en/blog/the-chicxulub-crater#:~:text=The%20Chicxulub%20Crater%20itself%20isn,advance%20via%20their%20Facebook%20page)

Photo by Francesco Ungaro: https://www.pexels.com/photo/brown-mountain-crate-96457/

The post #1491 How did they find the Chicxulub crater? appeared first on I Learned This Today.