howdy everyone and thank you for continuing on with module three when we explore earth materials and in this video we're going to talk about sedimentary rocks now sedimentary rocks are kind of our classic rock they're formed by the accumulation and cementation of sediments smaller rocks and possibly biological material as well or they could also be formed by the chemical precipitation from a fluid now we can categorize sedimentary rocks as well based on their source and composition of the sediments and when we classify sedimentary rocks we can put them in these three categories we can call them salicastic or plastic made up of silica minerals another category is biological or organic made up of organic material and then a third category called chemical sedimentary rocks these are formed by chemical processes and I'll talk about each of these categories of sedimentary rocks here in a moment but before we dive into sedimentary rocks there's a lot of definitions that we're going to have to go through pretty quickly because these terms are going to be thrown out at you and I want you to make sure that you understand what we're talking about so when we talk about a sediment we're talking about the loose particles of minerals or rocks that come from the weathering and erosion or precipitation from a solution by evaporation or some organisms secrete sediments as well but sediments are the loose particles they're not cemented together yet they're not um solidified weathering when we talk about weathering this might confuse you we're not talking about sunny rain cloudy tornadoes we're talking about the process that changes rocks at the Earth's surface this is called weathering it's changing rocks at the Earth's surfaces and we have lots of different types of weathering we have chemical physical or biological weather so this is just the process that's changing rocks at the Earth's surface that's breaking them down into smaller and smaller bits then we have erosion which is an important process in this part of the rock cycle erosion is the actual physical removal or the transport of all that sediment on earth's surface and we can transport or remove the sediment by water wind ice or gravity and then we have deposition this is when that transported sediment settles or comes to rest it stops moving then we have lithification which is the consolidation of those sediments and that turns them into sedimentary rock and the lithification process includes compaction cementation and sometimes recristallization now we talked a little bit about crystallization with ignous rocks when the melted material cools solidifies and crystallizes but when we talk about crystallization in sedimentary rocks we're talking about the growth and development of mineral crystals and and chemical processes like salt crystals and evaporates in some limestones and then cement cement is the solid material that precipitates in the pore spaces the little gaps in between the sediment grains that cements them together and forms the rock now here is just a quick little diagram showing you how most of our sedimentary rocks are formed we have our sediments that are being deposited and our sediments can include lots of different minerals here we have some felt spars we have some quartz they're being settled at the surface here of course we have some gaps in between the sediment grains we call these pore spaces and over time you have more and more being deposited on top it's going to start to compact that sediment decreasing the volume decreasing the amount of pore space and over time that pore space is going to get to be filled with solid material and it's going to form our sedimentary rock we call that cementation now many sedimentary rocks get deposited like that in horizontal layers or beds and we call those bedding or stratification where the top and bottom surfaces are called bedding planes and these beds of sedimentary rocks can also contain sedimentary structures like ripple marks and mud cracks which tell us a whole lot about the environment in which they formed in another most unique characteristic of sedimentary rocks is that they often contain fossils and in fact they're pretty much the only type of rock that will contain fossils because ignous rocks too hot it's going to destroy anything and then metamorphic rocks under a lot of temperature and pressure usually is going to destroy any fossils that were once existed so really sedimentary rocks are the rock types that we're going to be looking for when we want to look for fossils and fossils are any evidence of pre-existing life forms and that can be a whole or partial organism like an entire skeleton but it can also be footprints tracks trails fecal material really anything any kind of evidence that shows that there was once pre-existing life there and fossils provide information about ancient ecosystems they tell us about the history of life and the relative ages of deposits which is so important when we're trying to study the history of Earth so sedimentary rocks are a crucial piece in understanding how earth has evolved from its creation to today now this is what we mean by bedding bedding is those horizontal layers of sedimentary rocks those sediments get deposited in these layers they get compacted lithified cemented together but they kind of continue that process as long as nothing is going to interrupt that and we get these beautiful bedding planes that you can see okay so starting with our different types of sedimentary rocks our solicastic rocks or our classic rocks these are the classic form of sedimentary rocks where we have the weathering or the breakdown of rocks on the earth's surfaces turning them into sediments those sediments are dominantly composed of silicut minerals which is why we sometimes call it salsa um and then those sediments over time get compressed cemented together forming that sedimentary rock now sedimentary rocks so salicastic ones we we classify them based on the size of those individual grains so we could have big individual grains that are at least 2 millimeters in diameter they're easily seen with the naked eye we call these gravels and when we have gravel cemented together this forms conglomerates or breches now if those grains get smaller less than 2 millimeters but greater than a sixeenth of a millimeter these are these are our sandized grains you've probably seen sand before at the beach so you you kind of know what we're dealing with but when we have sand cemented together this forms a rock called sandstone so it's a pretty easy naming convention but when those grains get really small less than a 16th in diameter um that's when we call them mud and mud gets cemented together and creates mud rocks and we have different types of mud rocks depending on how small those little mud grains are silt and clay we could have silt stones and clay stones mud stones are also called shells shell is probably a common one you've heard of now here are some examples of the different types of solicit sedimentary rocks again our gravelized grains cemented together form conglomerates our sandized grains cemented together form our sandstones and our mudsized grains cemented together form our shells here's uh another example of conglomerates here at the earth's surface you can see those large gravelsized grains cemented together and they're also difficult to weather down they're a little bit resistant to weathering processes so usually they kind of stick out of the earth's surfaces with these outcrops same with sandstone sandston tend to be pretty resistant to weathering as well they form cliffs a lot of times these are some red sandstones here's our silt and mud stones these can be weathered down a little bit easier than our sand and conglomerate so they usually form gentler slopes they don't usually form um many cliffs here's another example of shale this shell has been deformed and bent but you can still see the layers being preserved now again our solicassic sedimentary rocks are classified based on those grain sizes again gravel size form conglomerates sand size forms sandstone mud size forms mud rock shells next we have our organic or sometimes they're just called biological sedimentary rocks and these are pretty easy because they're sediments that consist of biologic material such as shells which I've shown here with coina these are shells biological material that has been cemented together into a rock that's a biological sedimentary rock and sometimes it's not as obvious like coal coal is actually almost completely biological it is made of decomposed plant material that has been cemented and solidified together so coal is a perfect example of an organic sedimentary rock as well so here's some examples of coal um within rock so here's a coal seam sticks out very easily really dark black if you ever driven through Kentucky West Virginia Western Pennsylvania you can see these coal seams sometimes on the side of the roads now of course this one is not really economically viable we're not going to spend money to extract this coal seam but sometimes these coal seams can be hundreds of feet thick and in that case they are economically viable and we would go in and mine them this is another organic sedimentary rock this one's called chalk you might recognize this as the white cliffs of Dover chalk yes that is indeed almost 100% organic material chalk forms from a fine grain marine sediment known as ooze and this is formed when forminifra marine algae or other tiny little organisms living on the bottom or in the water actually die in the ocean and their remains sink to the bottom and accumulate as this ooze and over time you get enough of that d dead dead microscopic creatures and it forms this chalk deposit so this chalk deposit is actually millions and millions of tiny little dead creatures which is kind of creepy to think about all right and finally our last type of sedimentary rock are called chemical sedimentary rocks these are formed by inorganic chemical reactions typically these are caused when seawater evaporates seawater is very salty and the water evaporates leaving behind the salt the salt can accumulate as sediment be lithified and cemented just like our silicassics but we're forming a rock salt a chemical sedimentary rock a great example of a chemical sedimentary rock is gypsum like we see here in White Sands New Mexico this is the evaporation of an ancient lake leaving behind the sediments and then the most common type of um bioassic or chemical sedimentary rock is limestone limestone's a little tricky because we can consider it biological or chemical depending on how it forms so that's why it kind of gets bioclassic and or chemical um it gets a little bit confusing to categorize but it all depends on how it's formed limestone is is very common and it's composed mostly of the mineral calite and calite is calcium carbonate now calsite is produced by crystallization from water so it can um be formed by just minerals precipitating out of solution have you ever seen the crusty stuff on an old kitchen faucet or an old bathroom faucet that crusty stuff on the faucet's actually limestone it's calcium carbonate it's being precipitated out of the water over time and that is formed in a chemical process so that would be a chemical version of limestone but calcium carbonate is also formed by organisms so shells um creatures that have shells are made up of calcium carbonate so when we get an accumulation of shells or shell fragments that get cemented together that's when we kind of call it bioplastic right um so limestone is a sedimentary rock made of mainly calite um which can be made up of skeletons of microscopic organisms as well now sedimentary rocks give us a glimpse at what the environment was hundreds or thousands or even millions of years ago from when it formed and an environment and deposition or a sedimentary environment refers to the geographic setting where the sediment accumulated and they determine the nature of the sediments like the grain size the grain shape and the composition now there are three um categories of sedimentary environments and we quite simply call them continental marine or transitional now continental environments are dominated by stream erosion and deposition because streams are the dominant agent in changing the landscape we also see glacial deposits here which are typically those very poorly sorted sediments that range from clay to boulder sized and then we have Eolian deposits and Eolian deposits are just a fancy word for wind deposits that are typically well sorted and fine grained now I really like this figure here because it nicely summarizes what we can expect to find at many continental environments for example in caves we would expect to find limestone this is that same stuff that crusty stuff in your faucets forms the same way this is where calcium carbonate precipitates from the groundwater and creates rocks like those dripstones shown and in deserts we can expect sand dunes consisted of well sorted sand grains deposited by winds and we could even find sedimentary structures like cross bedding then we have aluvial fans which can be found at the base of mountains this consists of coarser grain sediments that have been dunked down the mountains through streams and maybe even landslides that settle at the base we could see breas and maybe even conglomerates there that contain those gravelsized grains that haven't traveled relatively too far far from their source and then we have inland seas and lakes and dry environments that are places we would expect to find those evaporates like rock salt and gypson similar to what we see at White Sands National Park and again white sands used to be a large lake rich in calcium sulfate that has since been evaporated completely now landslides can produce an unsorted jumble of a mess so we could expect breches being found there swamps and bogs are great places for coal formation because of the mud and decayed plant material that accumulates so we expect to find coal stream transports a variety of sediment sizes but once they reach low-lying areas the sediments left are often mudsized so silt and clays and and sand in streams in the lowlands we'd expect to find sandstone silt stones and mudstones we might even find ripple marks that might tell us which way the stream was flowing so very important in telling us all about the history of the environment and then finally glacial deposits can be found where we have glaciers and often consists of poorly sorted mixture of many different sized particles that have been picked up by the moving ice and dumped somewhere else so as you can see when we find sedimentary rocks we have a pretty good story to tell because depending on the grain size and the shape of those grains we can get a good idea on what kind of environment they must have formed in even if the environment where they are today looks completely different thanks to plate tectonics now marine depositional environments are divided according to the water depth and these environments border the world's continents and receive huge amounts of terrestrial sediments meaning sediments that have eroded from the continents so usually these sediments are rich in silica so here we tend to find plastic rocks like sandstones and silt stones now warm tropical seas that don't receive very much sediment from the continents often produce carbonate rich muds and debris from coral reefs and form limestones deep marine environments that lie seawward of the continental shelf and beyond so like the abyssal plane usually in very deep water deeper than 700 ft um the farther out we go from the coast the less and less sediment gets accumulated and the smaller and smaller the grains actually become this is why we primarily find fine grain sediments like clay on the ocean floor that produce shells now sometimes there's exceptions like when we have tubertity currents or submarine landslides those processes can carry coarser grain sediment deeper out into the ocean so when we find coarse grain sediment and deep marine environments then we know something chaotic has happened transitional environments consist of the shoreline where it is both continental and marine like a beach or a tidal flat or barrier islands lagoons and deltas there we can find deposits of sand or gravel on beaches mud on tidal flats and even reefs that create lagoons now this figure very nicely summarizes what we can expect to find in some marine and transitional environments on beaches that have strong wave activity like the northwest United States the beaches there consist mainly of pebbles and cobbles and can form conglomerates but beaches and bars along lowlying coasts like the east coast and the Gulf Coast are typically composed of well sorted sand and shell fragments creating sandstones and maybe even limestones like coina now tidal flats and lagoons are areas where we expect to find fine grain clay or carbonate rich muds forming shells and limestones way out in deep marine environments we mainly expect to find shells and limestones but sometimes can find sand and gravels where sediment has been carried out by those submarine landslides which I know a lot about shallow marine environments is where we expect to find sand clay and carbonate rich muds forming sandstone shells and limestones and sometimes they have ripple marks that preserve the wave activity and then our coral reefs are massive limestone structures that form in shallow clear tropical seas and consists of material secreted by the corals themselves and other material life so again what kind of sedimentary rock we find tells us a whole lot about what the environment was like now not all sediments have made it to the lithification stage and are still just as important for us to study sediment that is still loose and not yet rock is called unlithified or unconsolidated sediment and we have aluvium which are stream deposits that are unlithified sediment and we also have talis which are sediment deposited by the gravity at the base of a mountain slope the weathering erosion and sediment transport and deposition are dynamic processes that occur on the Earth's surface every single day therefore we have lots of unlified or unconsolidated sediment that haven't reached the rock cycle yet so why study sediments and sedimentary rocks if you aren't convinced at how useful they are at telling us how the environment has changed well fossil fuels occur in sediments and sedimentary rocks so hydrocarbons coal and pete are all found in sediments and sedimentary rocks and so are a whole bunch of other economic minerals and materials such as uranium iron ore sulfur we use sand the silicon sand for glass and as an aggregate we use gravel and construction limestone and sandstones as building stone gypsums we use for salts and fertilizers phosphates and nitrates are important those are found in sediments and sedimentary rocks and it's super important for agriculture and just our everyday life and again they provide information on the geological history of a region the fossils tell us about the ancient ecosystems the history of life they even help us age date some things so we can figure out how old these deposits are and they also tell us a lot about the environment in which they formed was it a stream was it an ocean was it a desert and of course our sediments and sedimentary rocks are important groundwater resources groundwater exists in the pore spaces between the sediments and sedimentary rock which is crucial for human life in a lot of areas even in our own country today all right so to end sedimentary rocks let's practice sedimentary rocks that was a lot of information we went over so digest it take your time slow your thinking let's take a look at this rock now remember we have three different types of sedimentary rocks we have plastic we have biological and we have chemical so just your first instinct your first observation of looking at this rock what category would you probably put this in what does it look like well it looks like little bits of rock cemented together right what kind of rock would that make it plastic right clastic is sediments or other rock fragments cemented together so this is a plastic or salastic rock all right so remember our plastic rocks are classified based on the size of their grains what size would you think this is these are pretty large grains right consider them gravel size gravelized rock cemented together okay that means it's going to be a conglomerate or a brechup now we didn't really talk about this but the difference between conglomerate and brea are the grain shape conglomerates have round gravel sized pieces bretches have angular gravelized pieces what do you think this looks like this has angular sized pieces this makes it a brecha now if you remember from the depositional environments the grain size tells us a lot about how far it's traveled so the larger the grain the less distance it has traveled so this probably hasn't traveled very far from its source but the grain shape tells us a lot too as the grains travel they get smooth and rounded out but this is still angular so it really probably hasn't traveled very far so this is a brecha gravel size solicastic rock didn't travel very far from its source was probably formed by a chaotic hazardous process such as a landslide landslide broke up all this rock brought it down slope eventually over time relithified and solidified back into a rock so this could be a landslide deposit see you told me a story about a rock just by looking at it again it's mindblowing all right so let's practice with this rock over here on the right remember our three categories of sedimentary rocks: plastic biological chemical what's the first thing that sticks out to you about this rock probably the shell imprint all these little pieces of shell fragments so you're probably thinking biological right yes this is a biological rock and this is a bioplastic limestone so this is made up of calcium carbonate and you can tell that because it is shell fragments and shell pieces so if you saw this rock on a hike let's say you're hiking in the Guadalupe Mountains West Texas and you came across this rock it's so dry hot desert what What would this tell you are there shells in the desert not today right well if you came across this rock in the desert in the Guadalupe Mountains you can infer an entire story that there was once a tropical sea that existed in West Texas that formed this deposit doesn't exist anymore but that just tells you how much the region has changed throughout Earth's history thanks to plate tectonics so another really cool story all right hopefully I've convinced you enough why rocks are super cold i will meet you in our final video of the module to talk about metamorphic rocks