Engineering Geology Igneous rocks Hussien Al - deeky 1
Engineering Geology The Geology Definition of Rocks In Geology Rock is defined as the solid material forming the outer rocky shell or crust of the earth. There are three major groups of rocks by its origin: ( 1)Igneous rocks : cooled from a molten state ; e.g., granite, basalt (2)Sedimentary rocks : deposited from fluid medium; the products of weathering of other rocks in water ; e.g., sandstone, mudstone…; (3)Metamorphic rocks : formed from pre-existing rocks by the action of heat and pressure . e.g., dolomite, marble …; 2
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Engineering Geology Rock Cycles 4
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Engineering Geology The Engineering Definition of Rocks Thus, in pure geological sense rock is defined as the essential part of the earth’s crust. Geologists concern about the origin, classification, history, and the spatial aspects of rocks. So, geologically speaking, ice, sand, marble, coal, basalt, can be simply regarded as rocks . However, the Engineering Geologists have a different, and relatively narrower view of rocks . The Engineering Definition of Rocks Rock is the hard and durable material By an excavation point of view, Rocks are the earth materials that cannot be excavated without blasting . This definition clearly excludes other kinds of earth materials such as soils, and glacial till s, etc. Here is another engineering definition of rocks : The earth materials that do not slake when soaked into water . For example, a thick loess deposit is regarded as rock geologically and regarded as soil in engineering . 6
Engineering Geology Igneous rocks • Igneous rocks form by cooling and crystallization of molten rock • Molten material residing below Earth’s surface is known as magma , whereas the same material at the surface is called lava. • Igneous rocks formed of cooled lava or volcanic ejected are common, but most molten material cools below Earth’s surface, producing bodies of igneous rock known as plutons . • Intrusive and Extrusive 7
Engineering Geology Properties of Magma and Lava • All igneous rocks derive either directly or indirectly from magma. Lava is magma that has reached Earth’s surface. • Plutonic (intrusive) igneous rocks form as magma cools and crystallizes within Earth. • Volcanic (extrusive) igneous rocks form by cooling and crystallization of lava or by consolidation of pyroclastic material, such as volcanic ash, ejected from volcanoes It is not possible to study magma directly • However, studying lavas can tell us a lot • Magmas have a range of compositions • Characterized by high temperatures • Have the ability to flow 8
Engineering Geology Magma Composition • Silicate minerals are by far the most abundant minerals in the crust and silica is the most abundant constituent of magma. • The bulk chemical composition of magma is dominated by the most abundant minerals Si - Al - Fe - Ca - Mg - Na - K - H - O • These major elements occur as oxides (SiO 2 ) • SiO 2 = ~45 to 75 % of rocks • Water and CO2 make up 0.2 - 3 % • Minor and trace elements make up the remainder How hot are magma and lava? • Erupting lavas range in temperature from 1000 ° to 1200 ° C . Magma must be even hotter, but direct measurements are not possible. • Rock is a poor heat conductor. 9
Engineering Geology Cooling rates • Intrusive (plutonic) rocks cool slowly while extrusive (volcanic) rocks cool quickly • The cooling rate determines whether or not crystals form • So cooling and crystallization determine the texture of the rock 10
Engineering Geology Texture • Texture refers to the size, shape and arrangement of minerals’ grains and is an important characteristic of igneous rocks. Grain size records cooling history. • 1- An aphanitic texture consists of an aggregate of very small mineral grains , too small to be seen clearly with the naked eye. Aphanitic textures record rapid cooling at or very near Earth’s surface and are characteristic of extrusive (volcanic) igneous rocks . 11
Engineering Geology 2-Plutonic textures: Phaneritic • A phaneritic texture consists of an aggregate of large mineral grains , easily visible without magnification . Phaneritic textures record slow cooling within Earth and are characteristic of intrusive ( plutonic) igneous rocks
Engineering Geology 3-Glassy texture • Very rapid cooling of lava produces a “glassy texture”. The lava cools so quickly that atoms do not have time to arrange in an ordered three dimensional network typical of minerals. The result is natural glass, or obsidian
Engineering Geology 4-Vesicular textures • Gases trapped in cooling lava can result in numerous small cavities, vesicles, in the solidified rock . 14
Engineering Geology 5-Pyroclastic texture ( fragmental textures) • Igneous rocks formed of mineral and rock fragments ejected from volcanoes by explosive eruptions have Pyroclastic ( fragmental) textures . The ejected ash and other debris eventually settles to the surface where it is consolidated to form a Pyroclastic igneous rock. • Much of this material consists of angular pieces of volcanic glass measuring up to 2mm 15
Engineering Geology 6-Porphyritic texture • Igneous rocks comprised of minerals of two or more markedly different grain sizes have a porphyritic texture. The coarser grains are called phenocrysts and the smaller grains groundmass. Porphyritic textures result from changes in cooling rate and include both aphanitic porphyrys and phaneritic porphyrys. 16
Engineering Geology Classifying Igneous Rocks • Most igneous rocks can be classified on the basis of Texture ,mineral composition and color. • Compositional equivalents
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Engineering Geology Engineering Considerations of Igneous Rocks (1)Fine-grained igneous rocks cannot be used as aggregates in Portland cement due to volume expansion caused by the Alkali-silica reaction. Solutions include:(a)Can be used in low alkali cement;(b)Non-reactive aggregates go with the high alkali cement;(c)Add pozzolans, coal-ashes, etc. in the aggregate-cement mixture to minimize the reaction. (2)Coarse-grained igneous rocks (e.g., granite, syenite, etc.) are not for aggregates for constructions because its low abrasion resistance; but fine-grained igneous rocks (e.g., basalt) are good for aggregates (e.g., basalt as paving aggregates goes with asphalt. (3)Sitting of foundations needs to avoid weathered rocks (e.g., dams, bridge piers, etc.); (4)Igneous rocks are good for dimension stone (tombstone etc.) because their resistance to weathering but need avoid fractures 19
Engineering Geology Gabbro and basalt 20
Engineering Geology Diorite and Andesite 21
Engineering Geology Granite and Rhyolite 22
Engineering Geology Pumice Red Obsidian Tuff Scoria 23
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