Sedimentary Rock Classification

odds are you already know thing or two about sedimentary rock classification sandstone silt stone shale limestone dolostone breccia conglomerate coal but if you spend enough time reading and talking about these various types of rocks you are bound to learn that there is no one classification scheme for them and more troublingly there are lot of different words that mean the same thing in this video i'd like to guide you through several alternative ways of classifying sedimentary rocks and help you sort through some potentially confusing terminology let's begin one of the easiest ways to classify rocks is to separate clastic rocks from non-clastic rocks clast is particle or grain of lithogenous sediment it is particle derived from weathering and erosion of pre-existing rock therefore the term clasts is sometimes used interchangeably lithic fragments detritus and detrital sediment clasts range in size from boulders which are meters across to small clay-sized particles that you can only see with the use of sophisticated electron microscopes in any case if you look closely at clastic sedimentary rock you will find evidence of grains produced from weathering and erosion not surprisingly non-clastic rocks generally don't consist of grains that said the distinction between classic and non-classic rocks is not always clear one consider the case of carbonate rocks carbonate rocks consist of minerals made of the carbonate anion co3 these minerals include calcites aragonites and dolomites the two main types of carbonate rock are limestone and dolostone some carbonate rocks are truly non-clastic for example some limestones consist entirely of micrite mikrite are particles of calcium carbonate that are less than four microns in diameter or four micrometers in diameter these particles form through chemical reactions in water in other words micrite is type of hydrogenous sediment but some carbonate rocks consist of one or more types of clasts generally speaking we can distinguish between three types of clasts lithoclasts fit the classic definition of clast they are lithogenous sediment derived from pre-existing rock in the case of limestone lithoclast is reworked carbonate fragment usually derived from pre-existing rocks with sizes greater than two millimeters bioclasts are particles grains or pieces of organisms such as fragments of shells or bone put another way bioclasts are grains of biogenous sediment seashells which are made of calcite and aragonite are common bioclasts in clastic limestone you will also sometimes see the term intraclass intraclasts are irregularly shaped grains that form from erosion of partially lithified or soft sediment intraclasts can be lithogenous sediment biogenous sediment hydrogenous sediment or some combination of all of the above specific example of intraclass are called rip-up clasts rip-up class are gravel-sized pieces of clay or mud that are created when an erosional event removes pieces of clay-rich sediment and transports them some distance because clay sticks together and is cohesive the pieces can be transported and subsequently preserved when they are finally deposited in place in any case carbonate rocks are neither clastic nor non-clastic they are somewhere in between for this reason carbonate rocks are usually treated as one of number of different categories one of the other most important categories of sedimentary rocks is sometimes called terrigenous terrigenous rocks include brechus conglomerates sandstones silt stones and mudstones and shales although they sometimes contain biogenous or hydrogenous sediment the textbook examples of these rocks are made almost entirely of lithic fragments of igneous metamorphic and other sedimentary rocks so they consist of lithogenous sediment and are definitely clastic rocks they are called pteryginus because they consist of class derived from weathering and erosion of the earth but you will often see them called something else salicylastic this is because terrigenous rocks consist almost entirely of silica in the form of silicate minerals like quartz feldspar mica and clay terrigenous and carbonate rocks are by far the most common types of sedimentary rocks overall terrigenous rock accounts for about 80 to 85 of all sedimentary rocks by volume carbonate rock represents another 10 to 15 percent of the sedimentary rock volume in the crust of course this does not mean the other types of sedimentary rocks aren't important another major group of sedimentary rocks are volcanic plastic rocks as the name suggests volcanic plastic rocks consist of clasts produced by volcanoes they are also sometimes called pyroclastic rocks however whereas pyroclastic refers only to detritus ejected forcefully by volcanic eruptions volcanic clastic represents all class produced by volcanoes so it is the more inclusive and the more preferred term it is important to recognize that volcanic eruptions produce both igneous and sedimentary rock naturally volcanoes often release magma in the form of molten lava which can cool and become extrusive igneous rock like basalt but when volcanoes erupt explosively they also eject variety of other materials which we collectively call tephra volcanic ash is just one component of tephra volcanic ash consists of class of igneous rock as well as particles and grains of volcanic glass and amorphous and non-crystalline material produced by the rapid cooling of magma tephra also includes much larger lithic fragments which were forcefully ejected from the volcano as cool blocks or partly molten chunks of magma after soft tephra becomes hard sedimentary rock through the process of lithification we call it tough tough is type of volcanoclastic rock overall volcanic plastic rocks are clastic rocks consisting of various fragments of igneous rocks and glassy minerals there are number of other important types of sedimentary rocks these include amber coal phosphorite chert evaporite and iron stone among others although all of these rocks are non-clastic they form under very different circumstances coal and amber only form in the presence of life in environments where plants provide biogenous sediment and biogenic material evaporite and contrast does not require the presence of life whatsoever it consists of hydrogenous sediment produced by natural chemical reactions others chert phosphorite and iron stone can also form from hydrogenous sediment in the absence of life that said it is known that life forms play critical roles in the formation of all of these rocks it is helpful to think about these sedimentary rocks collectively as loose category of biochemical rocks chemical reactions including reactions catalyzed by living organisms are necessary for the formation of all of these rocks in summation classifying sedimentary rocks is somewhat messy affair there is no perfect way to do it we just have to recognize that some rocks may be similar in one way and different in another as rule whenever you think about rock you need to be mindful that it may actually be peanut butter cup not entirely chocolate not entirely peanut butter something in between moving forward you should concentrate on committing these sedimentary rocks to memory learning how to identify them by sights and feel and understanding the circumstances under which they are deposited you