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 years old. 2 rock salt 3 shale or siltstone
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.
Example: Figure 3 Figure 3: Law of Superposition 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 of ______siltstone and is ______sedimentary rock.
2. Letter D is made of ______sandstone and is ______sedimentary rock.
3. Letter A is made of ______igneous rock.
4. Which layers (letters) would be affected (burned) by contact metamorphism?
______C, E, D, and B
5. We can conclude that Letter A is an ______.intrusion
Figure 14:
Youngest: ______A ______B ______D ______E Oldest: ______C
Look at Figure 15 below to answer these questions:
6. Letter C is made of ______metamorphic______rock.
7. Letter M is a ______fault ______line .
8. Which letters were affected by M? ______B, C, and D
9. Letter A is made of ______igneous rock.
10. Letter A is an ______.intrusion
11. Which layers did A cut through? ______B, C, and D
12. J (the curvy line) shows an ______.unconformity
Figure 15:
Youngest: ______H ______J ______G J ______F ______E ______A ______M ______D ______C Hint: Write in letter A at the first Oldest: ______B time it could have happened; make it as old as it could possibly be.