Chapter 5 Igneous Environments Texture of Igneous Rocks • Igneous ...

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    

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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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Stress

Movement of Magma       

Viscous magma does not flow easily while a fluid magma flows more easily Viscous magma strongly resists flowing and when erupts on the surface, it does not spread out but piles up, forming mounds of lava Fluid magma flows more easily and may spread out in thin layers on the surface. Can travel longer distances form its source and cover large areas with lava Low temperature: magma at low temperatures flows only with great difficulty- very viscous Abundant silicate chains make the magmas very viscous Volatiles: water and other volatiles decrease viscosity Abundant crystals: magma with abundant crystals is more viscous than a magma with fewer

Magmatic Activity at Hot Spots  

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Hot spot: a site of intense magmatic activity that has certain types of igneous activity that cannot be explained easily by its plate-tectonic setting Hot Spots and Mantle Plumes: o plumes may begin at the core-mantle boundary and ascend all the way through the lower mantle and into the asthenosphere o The plume and surrounding mantle are both solid o A mantle plume rises because it is hotter and less dense than material around it Hot spots in Oceans o When magma generated by a mantle plume encounters the lithosphere, it spreads out along the boundary o There, it causes melting of the overlying lithosphere Hot spots in Continents o The mantle plume’s high temperatures cause melting