Some magmas cool so fast or are so viscous that crystal seeds can\'t form. Inste

Question

Some magmas cool so fast or are so viscous that crystal seeds can't form. Instead, atoms in the melt are frozen in place, arranged hap hazardly rather than in the orderly arrangement required for min- eral grains. Because haphazard atomic arrangement is also found in window glass, we call a rock that has cooled this quickly igneous glass, and it is said to have a glassy texture (FIG. 5.4). Like thick glass bottles, volcanic glass fractures conchoidally. Vokcanic glass commonly looks black, beut impurities may cause it to be red-brown or streaked. Most igneous glass forms at the Earth's surface when lava exposed to air or water cools very quickly, but some may form just below the surface in the throat of the volcano. In addition, much of the ash blasted into the air during explosive eruptions is made of volcanic glass, formed when tiny particles of magma freeze instantly in the air. EXERCISE 5.4 Interpreting Porphyritic Textures Name: Course: Section: Dater (a) Based on what you have deduced about magma cooling, how does a porphyritic texture form? (b) Describe the cooling histories of the rocks shown in Figure 5.3 in as much detail as you can 5.3a 5.3b 5.3c (continued) CHAPTER S USING IGNEOUS ROCKS TO INTERPRET EARTH HISTORY

Explanation / Answer

a) When magma crystallizes below the earth surface there is enough time to form crystals within magma. But when it erupts to the surface incompletely crystalzed magma may be appeared on the surface. But we may fine some crystal called phenocrysts. These phenocrysta are embeded in the fine grained particles called porphiritic texture.

b) 5.3a) Here pyroxene crystals are denoted as phenocrysts. The pyroxenes are formed after olivine minerals formed in crystalization of mafic magma. These may be at very high temparatures but when erupts to the surface the temparature of magma gradually decreased and finally porphiritic texture formed.

5.3b) Like the above cooling history, By Bowen's discontinuous reaction series Hornblende is crystallized after pyroxene crystals. So this magma may has some time to crystallize than the above rock.

5.3c) The dark coloured minerals are generally formed at early stage of magma crystallization. After the formation of the dark coloured minerals the magma is erupted to the surface. Then the dark mineral phenocrysts are appeared in fine grained ground mass.

Related Questions

Navigate

• Browse (All)
• Browse S
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.