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1 Igneous Rock Questions Read “Exercises in Physical Geology” pages 24-37 and answer the following: [1] Define igneous rock. ________________________________________________________________________ ________________________________________________________________________ [2] How does lava differ from magma? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ [3] Igneous rocks are usually classified according to their _________________________ and _________________________. [4] What is meant by the texture of igneous rock? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ [5] How does the rate of cooling influence the crystallization process? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ [6] The statements that follow relate to terms describing igneous rock textures. For each statement, identify the appropriate term. [7] (a) Openings produced by escaping gases. (b) Obsidian exhibits this texture. (c) A matrix of fine crystals surrounding phenocrysts. (d) Crystals are too small to be seen with the unaided eye. (e) A texture characterized by two distinctly different crystal sizes (f) Coarse grained, with crystals of roughly equal size. What does a porphyritic texture indicate about igneous rock? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 2 [8] How are granite and rhyolite different? In what way are they similar? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ [9] A simple field method of identifying igneous rocks is to classify them according to whether they are ____________________ or _____________________ coloured. If the rock is light-coloured (white, gray, pink, red) it is composed largely of the silicaaluminum (silicic) group of minerals. The four most common minerals in this group are _____________________, _____________________, _____________________ and _____________________. The rock also will be relatively _____________________ in weight. If the rock is dark-coloured (black, dark gray, dark green) it is composed largely of the iron-magnesium-calcium (mafic) group of minerals. The four most common minerals in this group are _____________________, _____________________, _____________________ and _____________________. The rock also will be relatively _____________________ in weight. [10] The above eight minerals are the main constituents of igneous rocks. The size of the mineral grains (crystals) in the rock is known as _____________________. The _____________________ of cooling of the magma determines the size of the mineral grains. If the cooling of magma is slow, the chemical compounds within the magma had sufficient time to develop into _____________________ and form a rock with a _____________________ texture. Rocks with easily visible minerals were formed _____________________ the earth’s surface. Rocks which cooled on or near the surface cooled more rapidly, so their mineral grains are too small to be seen. Such rocks appear very dense and have an_____________________ texture. If the magma cooled so rapidly that no crystals developed at all, the rocks have a _____________________ texture. Igneous rocks which formed by the cooling and hardening of magma below the surface are called _____________________ while those that formed on the surface are called _____________________.