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Home » People » Gary Jacobson » Answers to Study Questions » Geology 110 » GeoAns 4 Igneous Rocks
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1. Molten rock below ground is magma, above ground it is lava.

2. The principle volatile components in magma are water and carbon dioxide.

3. The slower a magma or lava cools, the larger the crystals in the resulting rock.

4. Other factors that influence crystallization include the silica and water content of the magma. If there is a lot of silica then the magma becomes very viscous, ion mobility is reduced, and crystal growth is hindered. If the magma contains much water then it’s fluidity improves, ions become more mobile, and crystal growth accelerates.

5. The classification of igneous rocks is based upon the texture and composition of the rock.

6. a. Openings produced by escaping gases. VESICLES

    b. Obsidian exhibits this texture. GLASSY

    c. A matrix of fine crystals surrounding phenocrysts. GROUNDMASS

    d. Crystals are to small to be seen with the unaided eye. APHANITIC

    e. A texture characterized by two distinctively different crystal sizes. PORPHYRITIC

     f. Crystals of roughly equal size, all large enough to be seen with the unaided eye.               ........PHANERITIC

7. A porphyritic texture indicates that the magma from which the rock formed cooled at two different rates. This usually implies that the magma started to cool deep underground where cooling rates are slow, but before the magma was completely crystallized, it moved towards the surface where the remaining magma crystallized quickly.

8. Magmatic differentiation is the separation of a "parent" magma into magmas of different compositions.  In order for magmatic differention to occur, early formed crystals must be separated from the melt from which they formed so that they can't react with the melt.

Magmatic differentiation can occur in nature via crystal settling and "squeezing".

Magmatic differentiation might lead to the formation of several different igneous rocks from a single parent magma as follows: A magma cools. The first minerals to crystallize are low in silica but rich in metals, so the residual melt will become enriched in silica and depleted in metals.. If the early formed minerals are separated from the residual melt by crystal settling or "squeezing", then they will not be able to react with the melt and they will remain low silica/ high metal minerals. These early formed minerals will form a rock with a lower concentration of silica than that of the parent magma, but the residual melt will eventually crystallize to form a rock with a higher concentration of silica than that of the parent magma. Thus, rocks of different compositions can be formed from the same parent magma.

If the early-formed crystals are not separated from the melt during the crystallization of a magma, they will remain in contact with the melt. They will therefore react with the melt to form new minerals until all of the melt is used-up and the rock thus formed will have a composition identical to that of the parent magma.

9. Igneous rocks are classified in part by the minerals they contain. Bowen's reaction series tells us which minerals are compatible and thus are found together in igneous rocks.

10. Granite is phaneritic but rhyolite is aphanitic.

       Both contain a relatively high concentration of silica.

11. The crystals in pegmatites are very large because they formed from water-rich magmas which cooled slowly. Water greatly enhances ion mobility in a magma and therefore crystals grow faster.

12. a. Granite and diorite are both phaneritic, but granite has more silica.

       b. Basalt and gabbro are both relatively low in silica, but basalt is aphanitic whereas gabbro               is phaneritic.

       c. Andesite and rhyolite are both aphanitic, but rhyolite has more silica.

13. Tuff and volcanic breccia are both pyroclastic rocks.

14. Both laccoliths and sills form when magma is injected concordantly between rock layers. Because this can only happen fairly close the surface and because laccoliths and sills are both relatively small plutons,  cooling rates tend to be fairly rapid, so both pluton types tend to be made of aphanitic rocks. The two plutons have different compositions, however,because the dome-like shape of the laccolith implies a viscous, silica-rich magma, while the sheet like structure of sills implies a more fluid, low silica magma. Thus laccoliths are aphanitic with high silica and thus are usually rhyolite (sometimes andesite). Sills are aphanitic with low silica and are therefore usually made of basalt.

15. What is the largest of all intrusive igneous bodies? batholith Is it tabular or massive? massive Concordant or discordant?_discordant


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Last Updated: 01/13/2015
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