Chapter 4 Answers

Review Question Answers:

1. As the temperature decreases minerals that formed early (e.g., olivine) may react with the remaining magma to form new minerals (e.g., pyroxene).

2. Calcium-rich plagioclase forms early on in the cooling process of a magma, but as the temperature drops, a more sodium-rich variety forms around the existing crystals.

3. Some minerals must begin to form while melt is still present. Early-forming minerals, which are typically quite dense (e.g., olivine), will sink to the bottom of the magma chamber if the magma is not too viscous, thus becoming separated from the rest of the magma. The composition of the remaining magma will be more felsic than before.

4. Phaneritic texture means that individual crystals can be distinguished by the naked eye. Aphanitic texture means that individual crystals cannot be distinguished without a microscope. The dividing line is somewhere between 0.1 and 1 mm, depending on the minerals.

5. a) basalt; b) diorite; c) granite

6. A concordant body (a sill) is parallel to any pre-existing layering (e.g., bedding or foliation) in the country rock is. A discordant body (a dike) cuts across any pre-existing layering, or has intruded in country rock without layering (e.g., in granite).

7. When the hot magma intrudes into cold country rock, the margins cool quickly and small crystals form, whereas magma that is not in contact with the cool country rock will cool more slowly, and larger crystals will form. The chilled margin is the band of small crystals along the edge of the dike.

8. A batholith has an area of 100 km2 or greater, whereas a stock is smaller.

9. Batholiths (or stocks) intrude into existing rock by (a) melting through the country rock, or (b) causing the country rock to break and fall into the magma (stoping), or (c) pushing the country rock aside.

10. Compositional layering forms when early-crystallizing minerals sink toward the bottom of a magma chamber. This can only happen in non-viscous magma. Mafic magma is typically much less viscous than felsic magma.

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