Igneous Rocks and Intrusive Igneous Activity The 3 types of rocks:! Sedimentary! Igneous! Metamorphic Marble 1
10/7/15 SEDIMENTARY ROCKS Come from rocks sediments (rock fragments, sand, silt, etc.) Fossils Siltstone IGENEOUS ROCKS Are made of volcanic material (lava or magma) Extrusive Igneous (or volcanic) rock formation Intrusive IntrusiveIgneous Igneous (or ( plutonic) plutonic) rock rockformation formation Basalt Granite 2
METAMORPHIC ROCKS Rocks have been changed from a pre-existing rock, by high-pressure/high-temperature Sedimentary Marble Sedimentary Metamorphic Igneous The Rock Cycle Igneous Rock High Temp. Sediments Pressure and Metamorphic Rock Sedimentary Rock 3
Igneous rocks " Igneous rocks make up large parts of the continents and all the oceanic crust - indicate plate boundaries 4
Composition of Magma " Silica is the primary constituent of magmas " Magmas are differentiated based on the relative proportions of silica, iron, and magnesium. Felsic silica rich, iron and magnesium poor Intermediate intermediate between felsic and mafic Mafic silica poor, iron and magnesium rich Bowen s reaction series " Bowen s reaction series describes the sequence of mineral crystallization in a cooling magma. " There are two branches in the reaction series: Discontinuous series. - produces ferromagnesian minerals Continuous series - produces a variety of plagioclase feldspars 5
Igneous Rock Textures " Minerals begin to crystallize from magma and lava after small crystal nuclei form and grow. Intrusive (plutonic) Extrusive (or volcanic) Igneous Rock Textures " Minerals begin to crystallize from magma and lava after small crystal nuclei form and grow. " Two broad groups of igneous rocks are: Extrusive (or volcanic) Form in the exterior of the earth. Rapid cooling = Aphanitic texture =mineral grains are not easily visible Intrusive (plutonic) Form in underground. Slow cooling = Phaneritic texture: mineral grains that are easily visible Also we have: Rocks with more complex cooling histories are characterized by porphyritic textures. Mineral grains with different sizes. 6
Examples Igneous Extrusive Rocks Obsidian is usually black in color though it can also be red or have a greenish tint. It is a dense volcanic glass, usually composed of rhyolite, rich in iron and magnesium. Obsidian is formed when the lava cools so quickly that crystals do not have time to grow. Obsidian fractures with very sharp edges. It was used by Stone Age cultures for making knives, arrowheads, and other tools where sharp edges are important. Basalt is a dark-colored, fine-grained rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. Pumice is light and porous. It forms during explosive eruptions. Pumice is full of holes caused by expanding volcanic gases. It is composed of volcanic glass and minerals, and can form in all types of magma: basalt, andesite, dacite, and rhyolite. Examples Igneous Intrusive Rocks Granit is a light-colored rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below Earth s surface. Granite is composed mainly of quartz and feldspar with minor amounts of mica, amphiboles and other minerals. This mineral composition usually gives granite a red, pink, gray or white color with dark mineral grains visible throughout the rock. Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende and sometimes 7
Crystals vs. Sediments (Granit = Igneous intrusive) (Conglomerate = sedimentary) Intrusive Igneous Bodies - Plutons " Plutons are bodies of igneous rock which have been intruded in country rock or have formed in place far beneath the surface. 8
Volcanic pipes and necks " Volcanic pipes are magma- filled, cylindrical feeder channels beneath volcanoes. " Pipes can become volcanic necks with deep erosion. Geo-inSight 3., p. 100 Laccoliths Geo-inSight 6. and 7., p. 101 9
10/7/15 Dikes and sills " Dikes are discordant features (meaning they cut across layering in the country rock) " Sills are concordant (parallel to the rock layers). Batholiths and Stocks " Batholiths are plutons that have more than 100 km² in area of exposure. " Stocks are somewhat smaller plutonic bodies. 10