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EESA06H3 (234)
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Chapter 5

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University of Toronto Scarborough
Environmental Science
Lisa Tutty

Chapter 5 Igneous Environments Texture of Igneous Rocks  Igneous rocks form by solidification of magma  Have millimeter-to centimeter-sized crystals or some have meter-long crystals and others are noncrystalline glass  Vary from white to black or have mixed colors  May contain holes, fragments, or compacted ash  Rocks with crystals visible to the eye are called phaneritic  Rocks with crystals not visible to the eye are called aphanitic—formed from magma that solidifies too rapidly  Very coarse igneous rocks are called pegmatite  Many volcanic rocks contain small holes called vesicles referred to as vesicular  Some volcanic rocks contain angular fragments and are called a volcanic breccia Settings where different Igneous textures form  The different textures of igneous rocks reflect the environment in which the magma solidified  Magma can solidify at depth, erupt onto the surface as molten lava, or be explosively erupted a volcanic ash  Vesicles form when gases dissolved in magma accumulate as bubbles. They can form only under low pressures on the surface or very near the surface  Volcanic breccia can form from explosive eruptions of ash and rock fragments, from a lava flow that breaks apart, or from volcano-triggered mudflows and landslides on the steep and unstable slopes of the volcano  Volcanic glass forms when magma erupts on the surface and cools so quickly that crystals do not have time to form  Porphyritic texture forms when magma needs sufficient time in a subsurface magma chamber to grow visible crystals  Pegmatite forms if magma is water rich  Volcanic ash erupts vertically in a column and settles back to Earth  Fine-grained igneous rocks form if the magma only has enough time to grow small crystals  Coarse-grained igneous rocks form at greater depths, where magma cools at a rate
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