Lab 2- Igneous rocks

LAB 2: IGNEOUS ROCK IDENTIFICATION INTRODUCTION Our focus in this lab is to study the characteristics of igneous rocks so that we can identify them in hand specimen. We will also work to relate these characteristics to the formation of these rocks so that we can understand the geological processes involved in their formation. Lastly, we’ll try to keep in mind what types of mineral resources are obtained from igneous rocks and minerals. Objectives Identify the common members of igneous rocks Igneous Rocks and Texture Igneous rocks are one of the three types of rock. They are the first or primary rock in the rock cycle and form from the direct crystallization of magma. Igneous rocks that crystalize above ground are known as volcanic or extrusive rocks whereas rocks that crystalize below ground are known as intrusive or plutonic rocks. Extrusive and intrusive igneous rocks can be distinguished from each other by their texture. Textures Texturally the two main groups of igneous rocks are very different with intrusive being course grained and extrusive being fine grained. Intrusive igneous rocks cool very slowly due to the insulating effects of being underground, allowing for medium to large visible crystals to form and grow throughout the entire rock. This texture is known as phaneritic . If the crystals are particularly large, the intrusive igneous rock can be said to have a pegmatitic texture. Extrusive rocks crystalize relatively quickly at the surface of the Earth as lava flows and consequently are mostly made of crystals too small to see with the unaided eye. If the entire rock is composed of crystals that are too small to see without a microscope than it is considered to have an aphanitic or microcrystalline texture. Sometimes however a few large crystals may begin to form in the magma underground before an eruption. When erupted, the rest of this magma cools rapidly encasing the few already formed large crystals ( phenocrysts ) in a microcrystalline ground mass producing a rock with a porphyritic texture. In other words a porphyritic texture is a mixture of very fined grained crystals surrounding a few larger phenocryst crystals. When igneous lava cools so quickly that there is no time for any crystals to form a glassy texture results. Obsidian is a common glassy volcanic rock and can be distinguished by its conchoidal fracture. Extrusive igneous rocks also can have a unique series of textures that result from the volcanic eruption process. As a magma erupts the pressure on a magma is released allowing for the dissolved gasses to come out of solution forming gas bubbles in the magma or lava. Please note that it is this expansion of gas bubbles that helps power a 1

volcanic eruption. As the lava cools the gas bubbles are encased in the crystalized lava giving the rock a vesicular texture where large gas bubbles vesicles are intermittently scattered throughout the rock. Magmas with high gas contents can produce moderately explosive eruptions where lava flies through air. The high gas content in the magma will produce frothy textures which are characterized by a high density of wall to wall small bubbles (like a sponge). If the rock is frothy, has a high silica content and is glassy than it has a pumaceous texture is known as pumice . If the rock is frothy but mafic in mineral content than it has a scoriaceous texture and is known a scoria or cinder . Both scoria and pumice rocks are so frothy that they will often float on water. Moderately explosive eruptions can also produce lava bombs which are aerodynamically textured and shaped blobs of a lava. They form when the outside of a lave bomb crystalized in flight creating a distinctly textured crust that provided evidence of the flight direction. Finally extremely explosive eruptions can shred pumice in to volcanic ash and through this atmosphere very high into the atmosphere. When the ash falls down to earth it can form a deposit known as tuff , an ashy volcanic rick. If the ash deposit is particularly thick and very hot when deposited it can fuse together to form welded tuff (ignimbrite) . A key sign of welded tuff is squished pumice that have been welded in tear drop shaped obsidian lobes.  Texture Terminology Summary  Intrusive -slow cooling  Consists of medium to large coarse sized crystals  Medium-to-course grained = Phaneritic texture  Extremely course grained (crystals larger than 2.5 cm) = Pegmatitic texture  Extrusive -fast cooling  Consists of small to invisible crystals or no crystals at all  Fine grained, crystals are too small to see with unaided eye = Aphanitic texture  A few medium or small visible crystals embedded in an aphanitic groundmass = Porphyritic texture  Accumulation of pyroclastic material, ashy = Pyroclastic texture Pyroclastic Often looks like sedimentary rock, it is formed when fragments erupt from a volcano and gets welded together.  Scattered large bubbles (swiss chesse) = Vesicular texture  Wall to wall bubbles (spongey) = Pumaceous or Scoriaceous texture based on silica content  No crystals at all = Glassy texture  Mineral Content Summary:  Silica rich Silicate Minerals  Quartz, Plagioclase Feldspar (both Albite and Anorthite), Orthoclase Feldspar, Muscovite 2

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Sample # Chemical Composition ( Felsic, Intermediate, Mafic, or Ultramafic ) Visible Minerals (if any) Texture Description Rock Type (Extrusive/ intrusive) Rock Name i1 Felsic N/A Phaneritic Intrusive Granite (comes in various colors) i 2 Intermediate Quartz, Biotite, Sodium Phaneritic Intrusive Diorite i 3 Felsic Potassium Feldspar, Quartz, Sodium, Biotite, Hornblende. Aphanitic Extrusive Rhyolite i 4 Felsic Glass Glassy Extrusive Obsidian i 5 Felsic Glass Pyroclastic Extrusive Tuff i 6 Mafic Quartz, feldspar Phaneritic Intrusive Pegmatite 7

i 7 Mafic Pyroxene, Calcium Porphyritic Extrusive Vesicular basalt i 8 Felsic Glass Vesicular Extrusive Pumice i 9 Mafic Glass Vesicular Extrusive Scoria i 10 Felsic Glass Pyroclastic Extrusive Welded tuff i 11 Intermediate Sodium, Biotite, Hornblende Aphanitic Extrusive Andesite i 12 Mafic Olivine, Pyroxene, Calcium Aphanitic Extrusive Basalt i 13 Ultramafic Olivine Phaneritic Intrusive Dunnite i 14 Ultramafic Pyroxene, Olivine Phaneritic Intrusive Peridotite I 15 8