 Hello this is a basic video on sedimentary rocks. Today we're going to discuss how sedimentary rocks form. This is the process of weathering erosion, then sedimentation, deposition, burial, compaction, cementation. Why are sedimentary rocks matter? They cover a good chunk of the planet's surface and about 5% of our upper crust by volume is sedimentary rocks. My favorite part about sedimentary rocks though is that they tell the story of the past environments where they were deposited, also known as the environment of deposition, abbreviated by EOD. They are also an important economic resource. So how do we make sedimentary rock? Well we start by weathering an erosion. There's actually a lecture on this series devoted to weathering erosion if you want to learn more about it. Essentially it's breaking down and dissolving existing rock to make sediment pieces of existing rock. And then that sediment is carried or moved sometimes by wind, water, ice, or gravity. Those eroding agents deposit at sediment via the process of deposition and it comes to rest in its new home. It's environment of deposition, the environment where it was deposited. Deposited means laid down, set down, right? Next, the sediment is buried over time. You could have a basin that is slowly filling the sediment. You have sediment covered by other sediment and you have this process of burial. Then because of all of the sediment overlying the other sediment, it starts to squeeze it, compact sediment. It undergoes some basic chemical changes from low level amounts of heat and pressure, not enough to metamorphose that sediment, but definitely enough to change it chemically. And finally it can be glued together through this process of cementation. And all of these steps encompass what's known as lithification or turning to stone. So to be more specific, diagenesis includes increased temperatures and pressure. And it can include the deposition, the precipitation of new minerals and the dissolution of existing minerals. And this usually happens in the upper crust. Lithification, meaning turning to stone, induced here as an example, is taking unconsolidated sediments and creating sedimentary rocks from them. And this involves compacting them, squeezing them, the weight of overlying deposit sediments, helping in that process. And water, groundwater, moving through that material, crystallizing new minerals, precipitating new minerals between those grains to hold them together. Really common cementing agents are quartz and calcite. Geologists like to classify rocks based on the story that the rocks have to tell. The whole reason we're looking at these rocks is to try to decipher some useful information. A lot of times that is what happened to create those rocks. So we break sedimentary rocks into two major groups. Detrital, also known as plastic and chemical slash organic. Sometimes people break these out into a third group known as biochemical or organic. But for the sake of this lecture, I'm grouping them together. Detrital rocks contain a plastic texture, plastic meaning pieces. Detrital sedimentary rock contains pre-existing pieces of other rocks that have been re-glued together. Chemical sedimentary rocks are different because the chemicals were dissolved completely by water or some other solvent, usually always water, and then re-deposited. They re-crystallize out of solution. So you have a lot of chemical slash organic rocks that form that way. We organize plastic, aka detrital rocks by size, from the biggest to smallest, the size of the class, the size of the pre-existing pieces that they're made of. The biggest size class is your gravel size larger than two millimeters. We call these conglomerate if the class are rounded and breccia if the class are angular. We'll talk about those in more detail, but it all has to do with the environment deposition. If the size is sand size, we creatively name that sandstone. It's smaller than you can see with your eye, but still gritty between the teeth. That's silt size. We creatively name that siltstone. Finally, it has a smooth texture when gritted. We call that mudstone, and if it has fine, thin layers known as lamini, we'll call it a shale instead of a mudstone. It's a little bit more specific. All of the size is differentiated by the amount of energy in that environment where the rock was deposited, the environment deposition. Let's look at the largest size and work our way down. Conglomerates and breches. Here's a picture of a beautiful conglomerate, and it has rounded grains that are glued together with some cement, the matrix. Some possible environments of deposition is a river with high energy, and this is a misprint. Large amounts of travel. Breches are angular with gravel size sediments, and those can be an example of an environment deposition for that as a landslide. There's a lot of energy, but not very much travel. Think of a rock tumbler. The more the rock gets tumbled around, the smoother it becomes, so that's all about the amount of travel. Both types of rocks are generally poorly sorted, but there are exceptions. Next size down, sandstone with sand-sized particles. Possible environments of deposition include beaches, rivers, dunes, anywhere that you see sand in the modern world, accumulating naturally. That is a likely environment of deposition of sandstone. Quartz is the most abundant mineral within sandstone. Here's a close-up microscopic picture of a quartz aeronite sandstone with these beautiful polished quartz grains glued together. If it contains arcoes, or if it contains feldspar, you have an arcoes, and a gray wacky contains rock fragments. Next size down is shale, mudstone, siltstone. All of these are your smaller particles. Low energy environments of deposition like lakes and semi-deep ocean. Gradual settling, not a lot of energy to move things around. It allows those really fine particles to fall out of the water and accumulate into these rocks. When these things weather, they generally form gentle slopes, and it looks like a huge pile of material like this. If you have shale, you get these nice fissile layers where it breaks apart into these nice layers. That is it for the tridal sedimentary rocks, and we'll get more into rocks, chemical sedimentary rocks in the next lecture.