110 Lab - Igneous Rock ID F23

GENERAL GEOLOGY Igneous composition and textures Chapter 4 1 All igneous rocks are formed as magma or lava cools. Minerals crystalize as this molten material “freezes”. When examining igneous rocks, the variety of mineral sizes tell a story of the conditions the rock was exposed to while it formed. This assortment of mineral sizes – whether they are visible, microscopic or absent, and if rocks are composed of ash or if it has bubbles within it is referred to as texture . The variety of minerals present in the rock is controlled by the elemental constituents available as is solidifies. The availability of elements is referred to as composition . Composition: Igneous rock composition in the simplest terms can be divided into categories dependent on the relative abundances of mafic and sialic (felsic) minerals. Sialic rocks are rich in silica, potassium and sodium; these minerals tend to be light- colored. Sialic minerals have a lower melting point which means they will crystallize at lower temperatures. Mafic minerals are rich in magnesium, iron, and calcium; these tend to be dark-colored minerals. Mafic minerals have a higher melting point and will crystallize at high temperatures. A quick note on mineral colors: in igneous rocks greens are nearly always a dark color and pinks are nearly always light colors. Sialic rocks consist mainly of light-colored minerals with few dark-colored minerals which have formed from lower temperature magmas or lavas. Mafic rocks consist of mainly dark- colored minerals (black, dark-brown, or green) which have formed from high temperature magmas or lavas. If the rock is composed of a near equal mix of light and dark minerals the rock is intermediate in composition. If the rock is completely composed of green minerals (or green with a few black minerals) the composition is ultramafic . Early in 1900’s Norman Levi Bowen ran experiments to verify observations that rocks with differing compositions seemed to solidify from the same magma. To do this Bowen placed samples of peridotite into pressurized ovens where the pressure and temperature could be controlled. The samples would be heated until the samples melted then subsequently cooled incrementally and then quenched. Doing this Bowen recognized two distinct series of minerals formed in a predictable and repeatable order. Bowen named these series the continuous series and discontinuous series. The continuous series consists of plagioclase feldspars whose composition changes as a function of temperature of formation. The plagioclase changes from calcium-rich at high temperatures to more sodium-rich at low temperatures as the mineral grows during cooling. This continuous change in composition can result in a mineral with a calcium-rich core becoming progressively more sodium-rich at the rim. The discontinuous series consists of the minerals olivine, augite (pyroxene group), hornblende (amphibole group) and biotite mica. Initially as magma cools from high temperatures olivine will crystallize; as the magma cools the olivine will react with the melt and augite will form replacing the olivine. If the magma continues to cool the augite will react with the melt forming amphibole, finally the amphibole will react with the melt forming biotite mica.

GENERAL GEOLOGY Igneous composition and textures 2 Only at low temperatures will the minerals orthoclase feldspars, muscovite mica and quartz form. Viscosity is the resistance to flow. A lava or magma with a low viscosity can flow long distances while high viscosity magmas may only flow for a short distance if they flow at all. Mafic and ultramafic are low viscosity (like water or ketchup). Intermediate and sialic are high viscosity (like peanut butter or molasses). Composition is the main factor that controls the viscosity. Compositions with high weight percent (wt.%) silica will have high viscosity; compositions with lower wt.% silica will have lower viscosity. This is commonly confused with the temperature. While low viscosity magmas are typically higher temperature, if a sialic magma is raised to the same temperature as a mafic magma the sialic will still be more viscous. Variations in viscosity are evident in volcanism, both in the shape of the volcano and the type of volcanism. Volcanoes with high viscosity lavas tend to have steep upper portions and gentler slopes at the base; they will also be more explosive and violent when they erupt. Volcanoes with low viscosity lavas will be very broad and gently sloping with effusive (non- explosive) eruptions. Composition Silica wt.% Viscosity Ultramafic < 45% Low High Mafic 45% - 53% Intermediate 53% - 69% Sialic > 69% Sialic (Felsic) 700°C Muscovite Quartz Ultramafic Mafic Orthoclase Intermediate 1400°C Bowen’s Reaction Series 1200°C 1000°C 800°C High melting point (first to crystallize) Low melting point (last to crystallize)

GENERAL GEOLOGY Igneous composition and textures 4 Porphyro-phaneritic: Rock composed of visible phenocrysts and a phaneritic (visible) matrix Formed deep within the Earth Porphyro-aphanitic: Rock composed of visible phenocrysts and a aphanitic (microscopic) matrix Started to form deep within the Earth but subsequently erupted to the surface Aphanitic: Rocks with mineral which are too small to see with the unaided eye (microscopic) Formed at or near the Earth’s surface Glassy: Rocks which has cooled very rapidly therefore no minerals have formed Formed at the Earth’s surface Vesicular: rock composed of glass with an abundance of bubbles Formed at the Earth’s surface or Vesicular: rock composed of microscopic minerals with an abundance of bubbles Formed at the Earth’s surface Pyroclastic: Rock composed of ash and rock fragments smaller than 2mm Formed at the Earth’s surface or

GENERAL GEOLOGY Igneous composition and textures 5 Pyroclastic: Rock composed of ash and rock fragments greater than 2mm Formed at the Earth’s surface or Pyroclastic: Rock composed of compressed ash and fiamme (black glass) Formed at the Earth’s surface Go to the bins that contain the class Mineral Collection. Pick one sample of each of the minerals listed in the table above, but two of the plagioclase feldspars (one light and one dark) and lay them out on your tabletop. Arrange the minerals according to their composition (Ultramafic, Mafic, Intermediate, and Sialic). 7.0 Conchoidal fracture Green (forsterite), Orange (fayalite) Luster vitreous; granular massive; streak white to pale green Olivine 7.0 Conchoidal fracture Gray, brown, purple (amethyst) Luster vitreous; greasy; prismatic and pyramidal, amorphous; streak white Quartz 6.0 Excellent, two directions (prismatic) nearly 90° Dark gray, blue, black, white Luster vitreous; habit prismatic; streak white; striations on cleavage plains Plagioclase feldspar 6.0 Excellent, two directions (prismatic) nearly 90° Translucent, brown, gray, green, red Luster vitreous; habit prismatic; streak white; exsolution lamellae Orthoclase feldspar 5.5-6.0 Excellent, two directions (prismatic) ~56° and ~124° Black, dark green Luster vitreous, pearly, dull; habit prismatic; streak white Hornblende (amphibole) 5.5-6.0 Excellent, two directions (prismatic) nearly 90° Black, dark green Luster vitreous, resinous, dull; habit columnar; streak greenish gray Augite (pyroxene) 2.5-3.0 Perfect, one direction (basal) Brown, green brown, black Luster vitreous, resinous, pearly; habit micaceous; streak gray; transparent brown as single sheets Biotite 2.0-2.5 Excellent, one direction (basal) Colorless to silvery white Luster vitreous to pearly; habit micaceous; streaks white; occurs in books of many sheets; books look more metallic while single sheets are colorless Muscovite Mica

GENERAL GEOLOGY Igneous composition and textures 7 Use your observations from the previous page and the table above to determine the appropriate igneous rock name for the following samples: 1: Gabbro 2: Granite 3: rhyolite 4:scoria 5: Andesite 6: Granite 7: Olivine 8: Rhyolite 12: volcanic Tuff 13: Gabbro 16: Diorite 18: Pumice 19: Diorite Pegamite