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Chapter 5

EESA06H3 Chapter Notes - Chapter 5: Mantle Plume, Asthenosphere, Magma Chamber

Environmental Science
Course Code
Lisa Tutty

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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 aphaniticformed 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 that is slow
enough to allow large crystals to grow
Determinants of magma rising towards Earth’s surface
Magma pressure
Density: mafic magma generated from partial melting of the mantle is less dense than the
surrounding solid rocks and so rises
Gas pressure: when gases form bubbles in the magma, the density of the e magma decreases
and the magma rises faster
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