Topic Assignment: Magmas and Igneous Rocks
Objective: To learn basic principles of magmas and their formation of igneous rocks.
Materials: hand samples of igneous rocks, textbook
Instructions: Answer the questions below and submit via Blackboard. See Calendar for recommended due date.
- What is magma/lava? Is it accurate to say that all the material in a lava flowing out of a volcano is melted?
- The three basic causes of melting of hot solid rock deep in Earth’s interior are (1) decompression melting, (2) flux melting (by addition of volatiles such as water), and (3) heat transfer. See pg. 142-143 in EPP (pg. 100-101 in EG). Give a brief explanation about each one.
- Heat transfer
- Four types of magma exist in general based on their composition. See table at right. What do mafic magmas have in greater abundance than felsic magmas?
- A wide range of magmas exist. Several factors explain this: (1) source rock composition, (2) partial melting, (3) assimilation, and (4) magma mixing. Choose one of these and describe how it contributes to differences in magma composition.
- Three factors affect the cooling time of a magma before it becomes completely solid: (1) depth, (2) shape and size of the magma body, and (3) presence of groundwater.
- How does depth affect cooling time?
- Take two magmas of equal size and but differing shapes (spherical vs. disc-like). Which one will cool more quickly and why?
- Why is groundwater a factor in cooling?
- What are intrusive as opposed to extrusive igneous rocks?
- Draw a simple sketch for each igneous rock body listed below:
- Texture and composition are used to classify igneous rocks. Texture is the size and relationship of minerals and composition is shown by the presence of indicator minerals or the percentage of silica or other compounds in the rock. Rocks that have visible or microscopic minerals are classified according to the figure below. Rocks with few to no minerals are classified differently (see textbook discussion on fragmental/pyroclastic rocks and glassy rocks).
- What percentage of plagioclase is in diorite? Give a range.
- Which rock has the most olivine?
- If minerals such as pyroxene, amphibole and Ca-plagioclase are darker in color, which rocks on the chart below are usually dark in color.
- For each square below, first guess the percentage of space that is occupied by dark squares, which will represent the percentage of mafic minerals present in an igneous rock, then count the squares to get the real percentage. Each block contains 100 smaller squares.
1 2 3 4 5
- Which square in question 3 above best represents diorite (an intermediate rock)?
- Which square in question 3 above best represents granite (a felsic rock)?
- Which square in question 3 above best represents basalt (a mafic rock)?
- Texture is affected by cooling rate, which is a function of depth. Fill in the blank: Magma that cools slowly at depth tends to produce rocks with __________ minerals.
- The classification chart above also shows the classification by texture. Provide the chemically equivalent but different textured rock name for these three rocks here: rhyolite, andesite and basalt.
- Go to Google Earth. Pan to Hawaii Volcanoes National Park. Based on the color of these volcanic mountains, which type of lava is present here?
- Now pan to Mt. Pinatubo in the Philippines. Based on the color of these volcanic mountains, which type of lava is present here?
- Now go to Yellowstone National Park. Explore the photos here or see the grand canyon of the yellowstone river. What kind of lava created these yellowish rocks?
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