hi I wanted to talk to you about how old a rock is or how do we figure out the age of a rock um when we're looking at sedimentary rocks and figuring out the age of a rock it really depends on what type it is so here we've got a whole bunch of sedimentary rock layers and sedimentary rock is created by sediment laying down on the uh Earth's surface it gets pressed and hardens into rock and then maybe another layer goes on top of it another layer another layer and another layer and we know that if these are older if if these are horizontal that this one is probably the oldest and this one is probably the newest if they're sedimentary and that is called the law of superposition um that works for horizontal layers of sedimentary rock now this gives us the relative age so what we're doing is we're relating one layer to another we know that this one is oldest this one is newest we don't know the exact age or absolute but we know um in general which one was here first also on this drawing there's a fault and if you remember from sixth grade earth science a fault is a crack in the Earth's surface so in this case uh we can also look at the relative age of this fault and in this case if you take a look at it the layers were clearly here before the fault shifted them so the fault hoen happened last which means it's newer and that's the how we figure out the relative age of sedimentary rocks and when you have horizontal layers the one that was closest to the bottom is the oldest the one at the top is the newest that's called the law of superposition here what we have is an ous intrusion into the same set of rocks or the same set of sedimentary layers and remember that down in the earth you have a whole bunch of um molten material and if it's down there it's called magma so this ous intrusion it uh intruded so basically it moved up this way it intruded into the existing Rock so because it basically formed this space in this rock here it has to be relatively younger than the other Rock so even though it's you know down here by this old layer remember the law of superposition says that in horizontal layers of sedimentary rock the further down it is in the layers the older it is up here it's newer even though this Ignus is down here that doesn't apply because remember the law of superposition is just for sedimentary layers this is ous now if the ous rock kept moving or that magma kept moving and it basically Spilled Out onto the surface once it breaks through it's no longer called magma it's called lava and that's not called an intrusion it's not in the earth it's an extrusion that forms a different kind of ous rock the uh other thing we should probably discuss here is this fault now if the ous intrusion or Extrusion for that matter is unaffected by the fault then it's a little trick to find out which one is older or newer relatively um if that intrusion came over this way and all of a sudden it was shifted down if the fault moved it then we know hey the fault happened after this was here so the fault's newer this is older but if the intrusion just keeps going as normal and we can see that oh that ous rock just flowed straight into this and the fault didn't move it then we know the fault happened first it's older the ous intrusion is newer