card 9 ANDESITE

Andesite forms from the eruption of intermediate-composition lava. Such eruptions often begin in an explosive manner, so deposits of layers of pyroclasts are common on and around the volcano. The explosive phase of the eruption is then followed by fluid flow of lava from the volcano which cools, hardens and protects the underlying layer of pyroclasts from erosion. The volcanos resulting from a series of such eruptions are layered (stratified), and so are referred to as strato- or composite volcanos. Picture 1
shows a stratovolcano, Mt. Jackson, whose original shape has been somewhat modified by glacial erosion since its last active phase of eruption.

Picture 2 shows a typical sample of andesite. Note the light gray color which is characteristic of this igneous extrusive rock.

Picture 3 illustrates the porphyritic texture which is common in the andesitic flows comprising stratovolcanos. The large white plagioclase feldspar crystals began forming when the molten rock was still underground. The gray matrix, the main constituent of this sample, formed as the lava flow cooled rapidly at the surface. The matrix is composed of microscopic plagioclase feldspar and amphibole crystals.

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Mt. Jackson stratovolcano	andesite	porphoritic andesite

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1. INTRODUCTION

2. INTRUSIVE IGNEOUS ROCKS

3. GRANITE

4. DIORITE

5. GABBRO

6. IGNEOUS INTRUSIONS

7. EXTRUSIVE IGNEOUS ROCKS

8. RHYOLITE

10. BASALT

11. IGNEOUS ROCKS QUIZ