Historical Geology Geologic Time Igenous

so in the last let's take moment of this chapter want to talk very very briefly about which rocks we can use for our radioactive age dating know I've got the slide okay in there slide 18 to talk about which rocks but want you to to be sure why to understand why we use those rocks so obviously you already know the answer because hopefully you've looked at the lecture but what we want to talk to you about really quickly are the three different rock types and which ones we can use or which ones we we have how they form and why we can use certain ones so we have three rock types our first rock type is sedimentary and we've talked little bit about these right especially in our last chapter now sedimentary rocks form mainly from the erosion of other rocks so if you pick up handful of sand each little grain of sand Right comes from somewhere else right as it erods so what's actually pretty neat is if you pick up handful of sand you might be holding 20 different states in your hand well now here's the problem if take an age dating sample of my beach sand right if take sample in here my my sand grains each little grain is going to give me different date right cuz they're coming from different different places so cannot use sedimentary rocks right to get my age dates it's going to give me very unreliable date our next one are metamorphic now metamorphic rocks form they they start out as pre-existing rocks so they're already there and then they undergo pressure and heat so they undergo metamorphosis like caterpillar to butterfly so when they do that they turn into new rock now the problem with that this pressure and heat that pressure and heat restarts the geologic time clock so because of that we cannot use our metamorphic rocks so that leaves us with our third type and that's ous ous rocks form from melted rock all right so when that melted rock cools the second it cools that geologic clock starts right so since that geologic clock starts and it doesn't stay unaltered this is the kind of rock we like to use so here we go we have winner right this is the rock type we like to use now very specifically okay with our ous rocks we like to use rocks that come out of our volcanoes right our lava flows now when volcano erupts let's say it lasts 3 months now that might feel forever for us but in the grand scheme of things that's very quick 3 months do not take long at all so we like to use right Ash layers because they come down right the ash sits up in the sky and it eventually falls down makes layer down here that Ash layer only took couple of days to form so it's going to give us very specific age date so this is why we like to use ous rocks very specifically our Ash layers so hopefully that helped you kind of understand why we use Ignus rocks and why they are the most reliable