Aim: How do Scientists Read Rocks? Do Now: In your notes, answer the following question. Of the three types of rocks we have discussed (Igneous, Sedimentary and Metamorphic), which one do you think would be able to hold clues to Earth’s past? Why?

Outcrop – section of rock exposed at the surface that shows a geologic sequence (layers).

I. Reading Earth’s History Uniformitarianism - processes we observe today are most likely the same processes that occurred millions of years ago. When in Earth’s history does this painting depict? Triassic ~200 mya Today (2015) Devonian ~360 mya

*Does not tell us the actual age or use numbers to express time. Relative dating - tells us the sequence in which events occurred. *Does not tell us the actual age or use numbers to express time.

II. Rules for Relative Dating: Law of Superposition - sedimentary layers are created flat and even. the oldest layers are at the bottom since they were deposited first. How many layers of rock are here? Which is the oldest?

B. Law of Original Horizontality - folds/faults disturb layers after they are Lithified. (caused by plate tectonics). How did the outcrop change in pictures below? What caused this change? When must have the change occurred? Original Outcrop Changed Outcrop 5 - folding

C. Law of Cross-Cutting Relationships - layers can be cut after they form by intrusions and faults. - Intrusions are layers of magma that melt through pre-existing rock and then solidify. What happened in the changed outcrop? When must this change have occurred? Original Outcrop Changed Outcrop 4 * 3.5 - Intrusion *

Igneous Intrusion cuts through the pre-existing rock. - Which came first, the intrusion or the rock it is in? - Which principle lets us know this?

- Contact Metamorphism – a symbol geologist use to help show which rocks have been affected by the intruding magma (intrusions). - indicates the rocks have been affected by the intrusion when it was magma (hot!). Analyze these two geologic cross sections, describe how they are different?

III. Important Features Ash Layers – Created by an erupting volcano, occurs at one specific time in Earth’s history. Do not copy This layer is 65 million years old and is called the KT ash boundary. Below this layer, scientists find fossils of dinosaurs. Above this layer, there are no dinosaur fossils. What does this layer tell us? This layer marks the extinction of the Dinosaurs

Dragonfly, North America Fossils Remains of ancient life preserved in sedimentary rock. Knowing when the organisms lived, helps geologist put the rocks containing them in order. Why are fossils found only in Sedimentary rock and not Igneous or Metamorphic? Turtle fossil, China Dragonfly, North America

- a gap in the sedimentary rock record. Unconformity (UNC)- - a gap in the sedimentary rock record. - caused by erosion VIDEO Unconformity (UNC)

Closure 1: Place the letters in relative order from oldest to youngest.

Closure 2: Place the letters in relative order from oldest to youngest.

Closure 3: Place the letters in relative order from oldest to youngest.

Closure 4: Get the back worksheet, top bin