Writing Earth s History

Transcription

1 Earths History

2 Writing Earth s History How is Earths History like writing in your Journal? Everyday, something happens and, with a pen, it is written down in the pages of your journal. As you continue, you add paper. Other days, you might use a large eraser to erase certain sections. Or perhaps, you or someone else might rip out some of the pages of your journal and pages are missing. Or perhaps, you might spill something on a page, or your kid brother might slobber all over it, or he might make a a rip, a wrinkle, a tear, or leave a finger print. All are a part of that journals history.

3 Question How do geologists interpret earth s complicated history? How do geologists know how old the world is? How do geologists know how life was like millions of years ago? How do geologists know that Utah s geological history has had a Shallow inland sea, glaciers, dinosaurs, a big lake, sand dunes, mountains, volcanoes & earthquakes?

4 Answer: They Interpret Geologic EVIDENCE But what is the evidence, what does it look like, and what does that evidence mean?

5 Earths Processes Leave a Mark (aka) EVIDENCE

6 How does Earth keep its History? Erosion = Eraser Deposition = Deposits = Writing History Igneous Intrusion = Writing Beadle, 2009

7 How rocks tell us the story of that region s environment, and the life it held.

8 Glaciers leave Major marks, traces or evidence. The Glaciers and later rivers in these mountains have Removed (eroded) whatever strata Was there to see the history is gone!

9 Igneous Fire Rock

10 Igneous Intrusion

11 Igneous Extrusion

12 Basalt: Basaltic Lava Flow

13 Sedimentary Formed from Sediment, or Broken down rock

14 Breccia: Flash Flood Deposits

15 Conglomerate: River Bed Deposit

16 Cross Bed Sandstone: Ancient Sand-dune

17 Sandstone: Ancient Beach Or Flood Plain Deposit

18 Sandstone/Shale

19 Mud Stone: Lake Bed

20 Alluvial Fan

21 Delta

22 Limestone: Shallow Marine Sea

23 Metamorphic Changed Rock Meta: Change Morph: Structure Osis: Action/Condition/States

24 Gneiss: Ancient Mountain Belt Gneiss Chert Don t take Gneiss for Granite

25 Quartzite: Metamorphosed Sandstone Quartzite: medium to coarse-grained, with interlocking grains of quartz; sugary appearance. Dr. Richard Busch

26 Slate

27 Marble Marble: medium to coarse-grained, with interlocking grains of calcite or dolomite; sugary appearance. Dr. Richard Busch Stop Here

28 Q: How do we see Earth s history? A: Strata (and other marks) Strata are stacked layers of rock and soil. Each layer is made of slightly different stuff: Normally, the layers are found flat And horizontal Like these There are at least three different layers in this picture: But sometimes they can be tilted like these or even have their order totally mixed up as we will see!

29 Relative Dating: We compare fossils or rock layers to determine which is older/younger, but do NOT determine the actual age.

30 Relative Dating Used to find the age of a fossil or rock compared to others! Things to help in relative dating: A) Index Fossils- If a particular species lived only in a certain time period then you know that whatever layer you find its fossil, that layer is the same age as the fossil B) Strata Comparison- We figure out which layers came first, which came last, and everything inbetween by comparing layers and where they are in relation to each other. This is not an exact method. It does not give an exact year!

31 How fossils and types of rock give us a clue into Earth s History Because the earth is constantly moving and reshaping its self, it will leave traces of that environments history there. Parts of that existing ecology are left behind Includes plant material, animal remains, rocks and minerals. (Finger prints of living things) Animal Fossils as well as the rocks tell what sort of environment was there in a specific time.

32 I. Laws of Relative Dating Principle of Original Horizontality Principle of Super Position Principle of Unconformity (gaps of time) Principle of Original Continuity Principle of Cross Cutting Relations

33 1. Principle of Original Horizontality Sediments form horizontal layers in fluids. Basically, in a lake or floodplain, sediments form in a flat layer not up and down. Even though this is just a drawing, it shows correctly how layers of strata are normally Deposited: In flat Horizontal layers You can see that different types of rock and soil are represented by different symbols

34 Horizontal Layers in a Depositional Environment Body of Water Beadle, 2009 Sedimentary Rock Layers

35 Faulting and Folding Earthquakes and tectonic events (like mountain building) can mix layers up and put them totally out of order. Top Bottom The layers are getting separated The fold has caused what should be the bottom To become the top

36 Horizontal Layers can get squished by tectonic forces and they won t look horizontal anymore: Mountain / Erosion Body of Water squeezing squeezing Beadle, 2009

37 You can see the folds caused by tectonic forces squeezing the layers of strata Strata is often messed up some and can be a bit hard to read

38 2. Principle of original continuity If you find two similar layers on both sides of a gap it s probably the same layer that has been eroded in the middle. The layers were deposited In a shallow sea, but the sea Dried up and a river cut part Of the layers away

39 Can you match up the layers? The soil and rock between these 3 locations has been eroded away. Fill in the empty spaces: Notice that layer D is totally missing in location 2 Something has caused this layer to disappear between location 2 and 4 What is the eraser process called? There are two index fossils indicated here as well. They help Us match the layers together!

40 Notice that what you see on one side of the stream, you also see on the other side of the stream. We figure that even though they are not connected anymore, the layers on either side of the river are the same! The layers used to be connected, but the river has cut them in half! Have a student try to pick out Similar layers.

41 3. Law of Superposition If one thing is placed upon another The Youngest will be on the top The Oldest will be on the bottom You cannot form a layer above if there isn t anything below it.

42 Fig c W. W. Norton

43 - The Relative Age of Rocks The Position of Rock Layers According to the law of superposition, in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it. Also note how they are in horizontal layers.

44 Super Position Youngest When sedimentary rock layers are stacked, younger layers are on top and older layers are on the bottom. Next oldest 3 rd oldest 2 nd oldest Oldest If there were no geologic forces the top layer would always be the youngest!

45 4. Unconformity Gaps in geological time caused by uplift, erosion, and weathering. That part of the rock history was erased via erosion and weathering. Erosion = History Eraser Deposition = History Maker

46 - The Relative Age of Rocks Determining Relative Age An unconformity occurs where erosion wears away layers of sedimentary rock. Other rock layers then form on top.

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48 Can you see the Unconformity? Have a student try to pick out the unconformity.

49 5. Principal of Crosscutting Relations If something goes through a layer, the layer had to be there first! If there s a fault from an earthquake, folding or an igneous intrusion that cut through the layers it s younger than the layers that were already there since you had to have something cut through it in the first place.

50 Volcanic Activity This can add an extra layer of rock in some places. Magma can flow into underground cracks Older and Layers leave a young layer between two old layers. Here is a real life example: Younger Layer You can see that magma has flowed into some cracks and became solid Younger layers are supposed to be on top!!! But here it is Between two older layers!!! ***The dark layers of volcanic rock are called intrusions!

51 Volcanic Intrusion Example: Intrusion means that magma has cut it s way through other layers that were already there! Now it has become igneous rock: You can see the sandstone layers That were already present (old) This diagonal layer of volcanic rock cut it s way through As magma after the sandstone was already there (young).

52 Principle of Crosscutting Relations Cont. - The Relative Age of Rocks Which is younger?

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56 Interpreting Cross Sections

57 15 don t forget the tree! Imperfections in the Strata Q: Can you follow the order? There Sometimes are 15 events/features the strata is difficult to read! Tilting= ? Hard to be sure