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Weathering breaks down rocks and creates sediments which become the raw materials for other rocks and the formation of our soils. And we call the process of moving that sediment erosion. In today's episode, we're just going to focus on the weathering part. We'll discuss mechanical, chemical, and biological weathering and take you on a tour of some of the landscapes they help shape from landslides and sinkholes to caves!

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#CrashCourse #Geography #Weathering
As Earth’s plates diverge, collide, subduct, and  slide past each other, oceans open and close,   continents reassemble, and earthquakes,  volcanoes, and mountains heave the crust.

All of which bring together two  key physical geography players:   global tectonic systems and climate  systems. Especially through weathering,   which is when rocks break down by physically  disintegrating or chemically decomposing.

The weathering process actually  removes carbon dioxide from the   atmosphere by converting it into  chemical compounds called carbonates   that can be dissolved in water  and eventually turned into rock. And young mountains like the Himalayas which   formed just 40-ish million  years ago seem timeless,   but they actually weather and erode quickly. Which  might affect the balance of the Earth systems.

In fact, some geomorphologists, who study  landforms and surface processes that shape   the Earth, think that weathering of the  Himalayas when they first rose -- along   with the Rockies and Alps -- affected the  atmosphere enough to change global climates. Of course, flattening entire mountain ranges  happens over millions or tens of millions   of years. But we can still see weathering in  action.

At time scales you and I can comprehend,   weathering lends that reddish  brown tinge to my bike left outside   and eats away at the facades of old buildings. But no matter where we look, weathering  is the first essential step in wearing   down the landscape, shaping  the Earth slowly but surely. I’m Alizé Carrère, and this  is Crash Course Geography.

INTRO. Like we learned a few episodes ago, the Earth’s  topography or the shape of the land comes from   a whole bunch of different processes --  some that originate inside the Earth and   some on it. And the rock cycle -- which is the  continuous creation and destruction of rocks   and minerals -- is driven by both those internal,  or endogenic, and external, or exogenic, forces.

Many rocks form under high temperatures  and pressures deep in the Earth’s crust.   And when they’re exposed to the lower temperatures  and pressures at the Earth’s surface and all the   air, water, and organisms, they start to decay. Which is just part of the rock life cycle. Though   each of the three rock types have distinctive  characteristics based on their formation,   all rocks eventually break down into finer  material.

Which is where weathering comes in. Weathering breaks down the rocks  and creates fragmented materials or   sediments which become the raw material for  other rocks and for the formation of soils.   So one reason why weathering is so important  is because it’s the principal source of   inorganic material in the soil, which  vegetation needs to grow. If it weren’t   for weathering the Earth’s continental  surface would just be solid bedrock.

Now once those sediments are created, they’re  often picked up and moved somewhere else by wind,   running water, ice, and waves -- and we have  a different word for that process: erosion. So, we can think of weathering as prepping  the rock materials to be moved and erosion   as when the rock actually gets moved. Both  make up the first stage of denudation,   which is the general word we use to talk about  all the processes that wear away the landscape.

Though sometimes the weathered rock can move  without the help of an erosional agent like water.   Like we call the pieces on top of the  solid bedrock that haven’t moved yet   the weathered mantle, and it  might just stay where it is. Or gravity could make it creep or slide or slump  or flow downhill, which we call a mass movement.   These are the mudslides, debris flows,  and landslides we hear about that can be   really dangerous and often become headline news.  But before any transport can happen  -- from gravity or erosion -- the land   has to actually be broken down through  one of three main types of weathering. Basically, we have to think about how rocks can  break.

So let’s start with using just water. When water turns into ice it actually increases  in volume by 9% -- which is how your water   pipes can burst when it gets really cold. The  pressure builds up and [*pipe bursting noise*].

The same thing happens in a rock. Water  that’s been absorbed into the pores within   the rock expands when it freezes, building  up stress and causing the rock to break. This is frost action, which can break off  anything from small grains to large boulders   and is an important process in cold environments  where freeze-thaw cycles are common.   The debris that comes from frost action  has a special name too.

We call it scree,   and we can find it at the base of cliffs in  mountain ranges like the Alps and Rockies. Rocks can also get roughed up in  coastal and arid environments and   during dry weather periods when there’s a lot  of evaporation through salt crystal growth. As water evaporates, crystals can grow from the  minerals that were dissolved in the water.

The   crystals fill up the pore spaces and other gaps  in the rock and exert pressure, which forces apart   mineral grains. When they fall out of rocks like  sandstone, we call this granular disintegration. Like in Arizona’s Canyon de Chelly we find  villages built into cliff sides used by the  .

Ancestral Pueblo culture about 1000 years  ago. These cliff dwellings protected them   from the elements and from enemy attack  and were a whole web of interconnected   residences and ceremonial centers located in  deep indentations formed in the sandstone cliffs.   The weathered niches were partially  formed by salt crystal growth. So whether rock is weathered with salt  crystal growth or frost action -- or   other processes we won’t get into  like thermal weathering, unloading,   and slaking -- when rock breaks down  into progressively smaller fragments   without being altered by chemicals,  it’s called mechanical weathering.

Ultimately mechanical weathering is what  turns solid rock into regolith -- which   is the important bottom layer of soil  profiles -- and it also helps our next   type of weathering by providing more surface  area for weathering processes to operate. So, let’s talk about that bike  I left out overnight [ahem,   maybe actually for many days]. The reddish  brown tinge on it is from oxidation,   which is when certain metallic elements  combine with oxygen to form oxides.  The iron in my bike frame is being drawn out to  form iron oxide or rust, weakening it.

And the   same thing happens in rocks. When oxygen dissolved  in water comes into contact with iron in a rock,   the iron is removed, weakening the  structural integrity of the rock. This is a form of chemical weathering which  occurs when some or all the minerals in the   rock are altered or decayed by agents like  water, oxygen, carbon, and other organic acids.

We humans have actually contributed  to a lot of chemical weathering.   Like on the columns of the Parthenon  which is a temple dedicated to the   goddess Athena that’s stood for over 2,500  years on the Acropolis of Athens, Greece. Now there’s a black crust on the columns from   sulphur dioxide getting released in the  air from burning coal in power plants,   smoke emissions, burning oil, and other  industrial processes. Add a little moisture,   and that sulphur dioxide forms sulphuric  acid that corrodes certain stone surfaces.

But there’s plenty of chemical  weathering that happens without us too.   Water is the universal solvent --  meaning it can dissolve almost anything,   which can then get absorbed into a rock -- so  it’s very efficient at chemical weathering.  Especially combined with carbon dioxide  to form the chemical compound carbonic   acid that dissolves many minerals and forms  carbonates in a process called carbonation.   This is the same process that gives the zing and  sparkle to fizzy beverages...and even forms caves. Carbon dioxide dissolves in rainwater,  and especially in humid climates   there’s a lot of carbonation happening  in rocks like limestone where the main   mineral making up the rock is  calcite or calcium carbonate. Calcium carbonate reacts with carbonic acid  to form calcium bicarbonate which can be   dissolved away in water.

So just like oxidation  weakened my bike frame, carbonation weakens the   limestone. And when entire regions are composed of  limestone -- and there are a lot because there’s a   lot of limestone on Earth -- it creates karst  topography, which is where the caves come in. The word karst comes from the Karst Plateau in  Slovenia where this particular landscape was first   described.

Though you can also find karst scenery  in southern China, Japan, Puerto Rico, Cuba,   and the Yucatan Peninsula, as well as in US states  like Kentucky, Indiana, New Mexico, and Florida. In a karst area, streams on the surface get  diverted underground through openings in the rocks   created by carbonation, which creates disappearing  streams and dry valleys. There can be steep sided   gorges pitted with cavities called sinkholes  which can often collapse catastrophically.

When cavities in the rock get large  enough for humans to enter and explore,   they officially become caves. And if  they’re formed by limestone rock, we call   them solution caves, which usually form between  two different rocks, like limestone and shale. As we enter a solution cave to go spelunking,  or cave exploring, we’re kind of entering a   subterranean world that only exists because of  chemical weathering.

We might see sediment from   glaciers mixed with bones of dead animals or  even humans. And if we’re lucky, we might see   artifacts like pottery sherds and spearheads  from the Neolithic, Bronze, or Iron Ages. A little further in, we might walk  into a roughly dome shaped chamber   large enough to hold a tall building that  was probably hollowed out by meltwater   streaming through the cave at  the end of the last ice age.

Next we find ourselves in a narrower  passage and see beautiful and delicate   formations called stalactites  growing down from the ceiling.   They’re usually made of calcium carbonate  that got weathered then carried and   deposited by groundwater one drip at  a time from cave ceilings and walls. The similar looking deposits growing  from the floor are stalagmites.   Depending on how fast the water drips,  how many minerals the water contains,   and the cave atmosphere, they can  look more like columns or cones. And where a stalactite joins a stalagmite, we  get a column.

Many stalactites, stalagmites,   and columns get stained beautiful colors as iron  and manganese are oxidized through oxidation. Moving through the cave, the wild scenery isn’t  the only difference we notice from the surface.   Caves are also unique habitats with fragile,   self-contained ecosystems with simple  food chains and a distinct climate. For instance, bacteria down here can synthesize  inorganic elements in total darkness -- unlike   plants on the surface -- and produce  organic compounds that feed other cave life   like algae, small invertebrates,  slimy salamanders, fish and BATS!

Caves can be just a single passage a few feet  long or a large multi-level extensive maze.   Mammoth Cave in Kentucky in the  US is the longest in the world   with over 640 kilometers of mapped  subterranean hollows and passages.   And worldwide, it’s estimated that ninety  percent of possible caves lie undiscovered. Which might not be all bad since cave  tourism is seriously impacting these   otherworldly places that took millions  of years to weather. So when we emerge   from our cave tour, we have to take precautions.

Like being careful to not reuse our gear in other  caves without extensive decontamination or even at   all to avoid the spread of white nose syndrome,  a fungal disease killing bats in North America. Finally, the action of animals and plants  bring about biological weathering that   helps both mechanical and chemical weathering.  Like the teenage girl in Harbin from episode 3   who almost twisted her ankle on the curb  while jogging. It might have been that   the curb was uneven because tree roots  exerted pressure to heave the sidewalks.

Other organisms bore into rocks, and bacteria,  algae, fungi, and lichens can chemically alter   minerals in rocks. When dead lichens leave  dark stains on rocks these spots can absorb   more thermal radiation than the lighter  rock, which encourages weathering. And we can’t forget us humans who have  exposed bedrock by building quarries,   mines, roads and railways.

And our  agricultural practices have greatly   changed soil and weathering processes  in many regions throughout history. Weathering is a complex phenomenon and  there are no firm rules as to how different   rocks will weather in different climatic  conditions because minor differences in   their characteristics and how much moisture  is available can also control outcomes. But weathering itself doesn’t work alone and  depends on other processes to transport sediment   and expose fresh rock.

On a hillslope  the processes of weathering, transport,   and erosion are very much mutually interdependent  and form one unified system shaping the Earth. Which we’ll continue discussing next time,   when we talk about erosion and  how rivers shape our landscapes. Thanks for watching this episode of Crash Course  Geography which is filmed at the Team Sandoval  .

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