GEOL 101 1st Edition Outline of Last Lecture Structural Variations Physical Properties Mineral Classes Common Silicate Minerals Important Nonsilicate Minerals Magma Parent Material Of Igneous Rocks Igneous Processes Igneous Composition Outline of Current Lecture Igneous Textures Common Igneous Rocks Generating Magma from Rocks Evolution of Magma Intrusive Igneous Activity Current Lecture Igneous Textures o Texture is overall appearance of a rock based on the size shape and arrangement of interlocking minerals o Factors affecting textures Rate of cooling Slower rate fewer but larger crystals Fast rate many small crystals Very fast glass Percentage of silica present Dissolved gases o Types of textures Aphanitic fine grained Rapid rate of cooling Microscopic crystals May contain vesicles holes form gas bubbles Phaneritic coarse grained Slow cooling Large visible crystals Porphyritic Minerals form at different temperatures o Different cooling rates slow fast Larger crystal are embedded in a matrix of smaller crystals GEOL 101 1st Edition Vesicular Voids left by gas bubbles vesicles Rapid cooling pumice Glassy Very rapid cooling of lava Rock is called obsidian Pyroclastic Fragmental appearance produced by volcanic eruptions Often appear more similar to sedimentary rocks Pegmatitic Exceptionally coarse grained From magmas with lots of volatiles fluid rich enhances crystallization Common Igneous Rocks o Granite Phaneritic vs Ryholite Aphanitic felsic coarse vs fine o Diorite Phaneritic vs Andesite Aphanitic intermediate coarse vs fine o Gabbro Phaneritic vs Basalt Aphanitic mafic coarse vs fine o Peridotite vs Komatite rare ultramafic coarse vs fine o Obsidian compact and Pumice frothy glassy coarse vs fine o Tuff less than 2 mm and Volcanic breccias greater than 2 mm Pyroclastic coarse vs fine Generating Magma from Solid Rock o Increases in temperature Geothermal gradient temperature increases in upper crust Temperature increases with depth Partial melting Peridotite plus melt 1000 degrees Celsius Complete melting around 1500 degrees Celsius Rocking in lower crust and upper mantles are near their melting points o Decrease in pressure decompression melting When confining pressures drop decompression melting occurs Divergent plate boundaries o Addition of volatiles Melting at lower temperatures Water Subduction of oceanic lithosphere Evolution of Magma o A single volcano may extrude lavas exhibiting very different compositions o Bowens reaction series Minerals crystallization the composition of the liquid portion of magma continually changes Melting temperature changes composition crystallize base on melting point o Higher melting point crystallize first o Lower melting point crystallize later o Melt at same temp are found in same igneous rocks GEOL 101 1st Edition o Processes responsible for changing a magma s composition Magmatic differentiation Separation of a melt from earlier formed crystals o Earliest formed mettle settles changing the composition of the remaining melt Assimilation Incorporating surround rock bodies into a magma o Host rock is dislodged and is incorporated into a magma body Magma mixing Two chemically distinct magmas produce composition different from either o Younger magma body intrudes and mixes with older one Intrusive Igneous Activity o Most magma is emplaced at depth in the Earth As underground igneous body once cooled and solidified is a called a pluton o Classification of plutons Tabular sheet like Massive o Types of intrusive igneous features Dike discordant pluton cut others Sill a concordant pluton parallel to Laccolith Similar to sill Lens to mushroom shaped mass Arches overlying strata upward Batholith Largest intrusive body 100 square km Frequently forms the cores of mountains