Igneous Rocks

Question 1

Compare and contrast granite and rhyolite.

Revolt · Accepted Answer

Granite and rhyolite are both felsic igneous rocks with similar mineral compositions, including abundant quartz and feldspar. Granite forms underground from slowly cooling magma, giving it a coarse-grained texture with large visible crystals. Rhyolite forms at or near the surface from rapidly cooling lava, giving it a fine-grained texture, although it can sometimes be glassy or porphyritic.

Question 2

The texture of obsidian, shown below, is best described as glassy.Why does obsidian contain few or no mineral grains?

Revolt · Accepted Answer

Obsidian contains few or no mineral grains because its lava cooled so rapidly that there was not enough time for mineral crystals to form. Instead, the lava solidified as volcanic glass.

Question 3

Identify the property of magma that causes it to be forced upward towards Earth’s surface.

Revolt · Accepted Answer

Magma is less dense than the surrounding solid rock, so buoyancy causes it to rise toward Earth’s surface.

Question 4

Use the information about igneous rocks A–D to classify each one as intrusive or extrusive and basaltic or granitic. Fill in the chart with A, B, C, or D.Rock A—dark-colored large grainsRock B—large crystals, high percentage of silicaRock C—fine-grained texture, light-coloredRock D—from Hawaiian volcano area, no visible crystals ExtrusiveIntrusiveBasaltic1.2.Granitic3.4.

Revolt · Accepted Answer

Extrusive Basaltic: D; Intrusive Basaltic: A; Extrusive Granitic: C; Intrusive Granitic: B

Question 5

Identify each statement as true or false. Rewrite false statements to make them correct.

Revolt · Accepted Answer

The statements to evaluate were not provided, so I cannot identify them as true or false or rewrite the false statements.

Question 6

Directions: For each item, tell which event would occur first.

Revolt · Accepted Answer

The events to compare were not provided, so I cannot determine which occurs first.

Question 7

Directions: For each item, tell which event would occur first.

Revolt · Accepted Answer

The events to compare were not provided, so I cannot determine which occurs first.

Question 8

Explain why some types of magma form igneous rocks that are dark colored and dense.

Revolt · Accepted Answer

Some magma is low in silica and rich in iron and magnesium. These elements form dark-colored, dense minerals, so the igneous rocks produced are dark and dense.

Question 9

If you were shown one photograph of pumice and one of granite, how could you distinguish between the two rocks?

Revolt · Accepted Answer

Pumice is lightweight and full of holes or bubbles, while granite is dense and has large, visible crystals. Pumice usually has a frothy texture, whereas granite has a coarse, grainy texture.

Question 10

Suppose you found an igneous rock that had almost even amounts of silica, iron, and magnesium. How would you classify this rock? Why?

Revolt · Accepted Answer

I would classify it as an intermediate igneous rock because it has nearly equal amounts of silica, iron, and magnesium. If it were fine-grained, it would be andesite; if it had large crystals, it would be diorite.

Igneous Rocks Answers

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