IGNEOUS ROCKS by Charina Cameron Part 2: Textures

TEXTURES Aphanitic (finely crystalline) Porphyritic (phenocrysts) Phaneritic (coarse)

TEXTURES Cooling rates control crystal sizes

VOLCANIC GLASS A glassy texture results from lava cooling very fast at Earth's surface Obsidian From a felsic magma Black in thick pieces from traces of mafic minerals Conchoidal fracture like window glass

INTRUSIVES - PHANERITIC TEXTURE Felsic - granite Intermediate - diorite Mafic Phaneritic - gabbro Aphanitic - basalt

VOLCANICS - APHANITIC TEXTURE Felsic - rhyolite Intermediate - andesite Mafic - basalt

PORPHYRITIC TEXTURES Phenocrysts = isolated crystals in a finer matrix Rhyolite porphyry Andesite porphyry Basalt porphyry

PYROCLASTICS Ash (very fine, like flour) Lapilli (sandy to pebbly) Bombs (cobble -like & larger) Volcanic ash Mount St. Helens (1980)

PYROCLASTICS Volcanic ash Lahars very fine-grained can cover everything Lahars rain soaks ash mudflows

CONSOLIDATED PYROCLASTICS Tuff Consolidated volcanic ash Welded tuff = hot ashfused Banded tuff (with glass)

EVIDENCE OF VOLCANIC GAS Pumice Vesicles in lava Felsic magma Scoria Vesicles in basalt Mafic magma Hawaii Amygdaloidal basalt Vesicles filled with zeolite minerals Larger & elongated vesicles near top of flow