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Stratigraphy Notes: Terms, Laws, and Practice Name: ___________________________________KEY – Return to Ms. Renée Date: ___________ Hour: ________ Stratigraphy Notes: Terms, Laws, and Practice Good video links: https://www.youtube.com/watch?v=M2Ex5DIjtfU (4.10 minutes) https://www.youtube.com/watch?v=CiL8WCbhqmg (4.39 minutes) https://www.youtube.com/watch?v=RZcGtrP29pA (11.14 minutes) https://www.youtube.com/watch?v=fYSeM63Fv0s (26.27 minutes – after notes) Stratigraphy is a branch of geology which studies rock layering in an effort to understand Earth’s history. Graphs or maps are made and analyzed to determine the relative age of rock layers and geological events, like faults. Relative Age: How old one thing is compared to another thing. Absolute Age: Using a number to describe how old something is. Practice relative vs. absolute age Use the pictures below to describe the people’s relative and absolute age. To determine their ages, assume the year is 2000 – makes the math easier! PETER ALAN ROY (born in 1975) (born in 1999) (born in 1920) Relative age: _______________Roy is the oldest and _______________Alan is the youngest. Peter is older than _______________Alan but younger than _______________.Roy Absolute age: Peter is _______25 years old, Alan is _______1 year old, and Roy is _______80 1 2 rock salt years old. 3 shale or siltstone 4 limestone Stratigraphy uses relative age, so we will be determining which rock layers are older than other rock layers and whether features like faults and intrusions are older or younger than the rock layers in the picture. Symbols are often used to identify types of rocks and physical features. You will need to use your Stratigraphy Symbols sheet while working through this unit. The pictures or graphs used in Stratigraphy are often cross sections or outcrops. A cross section shows what is inside the Earth – it is like slicing an orange in half and looking inside. An outcrop shows exposed layers of the Earth that are visible at the surface. These layers have been exposed through uplift and/or weathering. There are several Laws and terms involved in Stratigraphy that will help you correctly analyze these maps/graphs. We will look at their definitions and analyze examples. 1. Law of Horizontality: All sedimentary layers were originally deposited in horizontal layers. If the layers do not appear horizontal, they were later deforms by uplift, tilting, folding, or other geological processes. Figure 1: Original layers – deposited Figure 2: Layers after uplift and tilting – horizontally the original horizontal layers were tilted 2. Law of Superposition: In any horizontal, undisturbed layers of sedimentary rock, the oldest layer is at the bottom and the youngest layer is at the top. Figure 3: Law of Superposition Example: Figure 3 Fill in the blanks to correctly analyze the picture in Figure 3. Always start with the oldest layer and work your way up to the youngest. Use the YOUNGEST symbols and the Law of Superposition to help you. Youngest: # ______1 made of ___________________gray shale # ______2 made of ___________________rock salt # ______3 made of ___________________shale or siltstone OLDEST Oldest: # ______4 made of ___________________limestone 3. Extrusion: When molten rock reaches the surface of the Earth it is called lava. An extrusion shows where lava has cooled and solidified on the surface. (igneous rock) Figure 4: Extrusion – a layer of Figure 5: Extrusions can get lava on the surface of the Earth covered by additional layers and no longer be at the surface. Extrusion – layer of lava What type of rock is the extrusion? _____________basalt 4. Intrusion: When magma cools and solidifies Figure 6: Intrusion – magma that before it reaches the surface. (igneous rock) never makes it to the surface Intrusions will cut through other layers of rock. The intrusion will be younger than the rocks it cuts through, because the original rock layers had to be there first for the intrusion to cut through them. What type of rock is the Intrusion – cuts intrusion? through layers of other rock _____________basalt There are three major types of intrusions: Figure 7: Label the three types of intrusions 1) Sill: A sill forms when magma injects itself between the rock layers, sill forming a horizontal ________: horizontal sheet of igneous rock. sheet of igneous 2) Dyke: A dyke forms when rock magma pushes up towards the surface through cracks in the rock, forming a vertical sheet of igneous rock. 3) Batholith: A batholith is a magma chamber or large pool ________:dyke ______________:batholith of magma that cools vertical sheet chamber of below the surface. of igneous rock igneous rock Look at Figure 5 on the previous page. It could be really hard to tell whether the igneous rock is an intrusion or extrusion if we were not told what it was. But, when symbols for contact metamorphism are added, it makes it much easier. Contact metamorphism provides evidence of whether a feature is an intrusion or extrusion. 5. Contact Metamorphism: When rock is changed by coming in contact with magma. The hot magma will burn the original rock, changing its texture and minerals, creating a new, different type of rock. The symbol for contact metamorphism looks like this: Contrast Figure 8 and 9 below. Describe how they are different: ________________ ______________________________________________________________________Figure 9 shows contact metamorphisis above the basalt, Figure 8 does not. ______________________________________________________________________ Figure 8: Extrusion Figure 9: Intrusion We know because the top We know because the top layer has not been burned – it layer has been burned – it was wasn’t there to be burned there to be burned Look at figure 8. Note there was no contact metamorphism above the basalt (igneous rock) – only below it; the top layer of sandstone was NOT burned. This tells us the top layer was NOT there when the molten rock came in; therefore it was lava – an extrusion. Look at figure 9. Note there WAS contact metamorphism above the basalt (igneous rock) as well as below it; the top layer of sandstone WAS burned. This tells us the top layer WAS there when the molten rock came in; therefore it was magma – an intrusion. 6. Cross Cutting Law: Any feature like an intrusion or fault that cuts across one or more rock layers is younger than the layers it cuts across. This is logical - the rock layers must have already been there for the igneous intrusion or fault to cut across them. Youngest: ________I Figure 8: ________G I ________H H G ________F F ________E E D ________D C ________C B ________B A Oldest: ________A Youngest: ________G Figure 9: ________H I ________I H ________F G F ________E E ________D D ________C C B ________B A Oldest: ________A 7. Fault Line: A fault is a fracture, or crack, in the rock along which movement happens. Rock can only move along a fault if the rock was there when the fault happened, therefore a fault line will be younger than the rock layers it affected. Youngest: ________G Figure 10: Fault Line ________F ________E ________D ________C G ________B Oldest: ________A Figure 11: Fault Line Plus…. G Youngest: ________H which is an ________________intrusion made of _______________granite ________R which is made of __________________________shale ________M which is made of __________________________sandstone ________G which is a ________________________________Fault line ________F which is made of ___________________________limestone ________B which is made of _________________shale __________ Oldest: ________I which is made of siltstone 8. Unconformity: Where rock was uplifted, parts were weathered and eroded away then the rock probably sunk back down into water and was covered by more layers of sediments. This causes missing information in the rock record. This is symbolized by an uneven line BELOW the surface. *** Note this is different than uneven line at the surface. This type of line at the surface just shows weathering and erosion. Figure 12: Unconformity F G In Figure 12 above, letter F shows an unconformity. We cannot see the top of layer B – it was weathered away, so we cannot tell if it was an extrusion or an intrusion. We do not know what happened to the top of layers E and B before layer C formed on top of them. Letter G’s symbol is a little different than the symbol I gave you, but you can easily infer that it shows _______________________contact _________________________________.metamorphism Therefore we know letter B was either an __________________________extrusion or an __________intrusion______________, and is therefore younger than layer ___________E which it cut through. Youngest: ________A which is ___________shale , a type of _______sedimentary_______________ rock. ________D which is ___________,sandstone a type of ______________________sedimentary rock. ________C which is ___________,limestone a type of ______________________sedimentary rock. ________F which is a(n) ______________________unconformity_____ ___________ line. ________G which is a(n) ___________________________contact metamorphism_____ ______ line. ________B which is ___________,granite a type of ______________________igneous rock. Oldest: ________E which is ___________,schist a type of ______________________metamorphic rock. Figure 13: Unconformity, Fault Line, In/Extrusion Youngest: ________S S ________R R ________O Q ________N ________Q P ________P M Oldest: ________M M N O ********** ON YOUR OWN ********** ON YOUR OWN ********** ON YOUR OWN ********** Look at Figure 14 below to answer these questions: 1. Letter B is made
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