Rocks formed from . Igneous rocks that are characterized by porphyritic texture are known as porphyry. 5.4.1 Magmatic Viscosity Magmatic viscosity defines the degree of fluidity of the magma, e.g. Igneous rocks with a fine-grained texture (grains generally tiny or too small to see) are volcanic in origin. Geology Laboratory: Igneous Rocks and Processes Revised on 10/13/2011 Page 5 of 10 Fine-grained (or Aphanitic): An igneous rock (or a part of an igneous rock) with a fine-grained or aphanitic texture has crystals, but they are too small to recognized with the unaided eye. Atlas of igneous rocks and their textures A companion volume to the Atlas of Rock-forming Minerals in Thin Section, this full-colour handbook is designed to be used as a laboratory manual both by elementary students of earth sciences undertaking a study of igneous rocks in thin section under the microscope, and by more advanced students and teachers as a reference work. This pile of igneous rocks contains a wide range of igneous rock compositions and textures and represents a dozen different rock names and histories. The five columns represent the five basic textures. Igneous rocks are classified based on texture and composition. Textures of Igneous Rocks. Igneous rock texture Texture is anything that you can see in a rock that can give you clues as to how that rock formed. that crystallizes at the surface. In igneous rocks, these textures are shown by granites and felsites and hence are also often named as granitic and felsitic textures. The textures of volcanic rocks will be discussed in Chapter 4, so here we’ll only look at the different textures of intrusive igneous rocks. Porphyritic texture tells you that the igneous rock underwent two stages of cooling, first slow, then fast. Igneous textures are used by geologists in determining the mode of origin igneous rocks and are used in rock classification. Textures and structures. The texture of a rock provides a clue whether the magma cooled fast or slowly and where the rock was formed. Igneous rocks are formed through the crystallisation of magma as it cools either in the mantle or on the surface after a volcanic eruption. 3.4 IGNEOUS ROCK TEXTURE The classification of igneous rocks is based not just on composition, but also on texture. In practice, this means that the average dimension of the crystals is less than 1 millimeter. As mentioned earlier, texture refers to the features that we see in the rock such as the mineral sizes or the presence of glass, fragmented material, or vesicles (holes) in the igneous rock. lava. The four rows, the four basic compositions. Following is a brief account of these textures of igneous rocks: I. Equigranular Textures: All those textures in which majority of constituent crystals of a rock are broadly equal in size are described as equigranular textures. Igneous rocks are classified by texture and chemical composition Texture of Igneous Rocks Depends on size of mineral grains: Coarse-grained (larger grains) • Large grains easily visible to naked eye • Cooled slowly, below earth’s surface — time for minerals to grow large • Called “intrusive” or “plutonic” rocks. Textures of plutonic rocks: Refer to the lab handout on volcanic rocks because many of the same terms apply to plutonic rocks, e.g., crystallinity, grain size, crystal form, shapes and habits. Vulcan – god of fire Intrusive, or Plutonic rocks. Textures in Igneous Rocks: The term ‘texture’ is defined as the mutual relationship among the minerals and the glassy materials in a rock. of igneous rocks Crystal development euhedral: grains bounded by its own perfect to near-perfect crystal growth faces subhedral: partly bound by its own growth faces, or growth faces only moderately well developed anhedral: irregular; little or no evidence for its own growth faces. 3. Sieve texture, a term sometimes reserved for metamorphic rock textures, refers in an igneous context to numerous small inclusions of one component within a crystal, a common example being the numerous inclusions of glassy groundmass material within "corroded" plagioclase phenocrysts of some andésites and related rock types. Rocks formed from . 13 Ahmed Essam Ayad 14. The texture in igneous rocks depends on the following four factors: i) Viscosity of magma ii) Rate of cooling iii) The order of crystallization of the constituent minerals. The key to identifying rocks is careful examination. UNESCO – EOLSS SAMPLE CHAPTERS GEOLOGY – Vol. Textures of Igneous Rocks: Texture of a rock is the appearance of the rock and how one feels touching it. Building Stones 2 – Igneous rocks What are the differences between igneous rocks commonly used as building stones? that crystallizes at depth. Igneous Rocks: Texture and Composition. This relates to the cooling history of the molten magma from which it came. The following textural terms are commonly used for plutonic rocks. Texture describes the physical characteristics of the minerals, such as grain size. Give each small group of pupils a set of the photographs of igneous rocks, printed onto card and trimmed to remove the names. Rhyolite Felsic lava 800°-1000°C Lower temp and higher silica = more viscous. All igneous rock began as molten rock formed deep within the earth called magma. In describing texture we are generally referring to the average size of the mineral grains present, but other important characteristics include the relative sizes (i.e., whether a mineral is present in large grains relative to other minerals) and the presence or absence of cavities. Texture describes the physical characteristics of the minerals, such as grain size. The size and shape of the mineral grains or crystals and the pattern of their arrangement give a texture to the rock. Principal minerals are Potassium Feldspar, Biotite Mica, and Quartz. Types of Lava. Classification based on rock texture Extrusive igneous rocks This texture is commonly found in igneous rocks in which phenocrysts are embedded in a fine-grained groundmass. The main factor that determines the texture of an igneous rock is the cooling rate (dT/dt) Other factors involved are: The diffusion rate - the rate at which atoms or molecules can move (diffuse) through the liquid. Textures are best observed under a microscope, because many textures are microscopic. COARSE GRAINED TEXTURE , mineral grains easily visible (grains several mm in size or larger) A hand specimen of granite with phaneritic (coarse grained) texture. Magma is a form of molten rock that exists below the Earth’s surface. There are two major classifications of igneous rocks: Intrusive and To identify rocks, you must not be shy about taking a hand lens to carefully examine a rock before announcing its identity. The ejected ash and other debris eventually settles to the surface where it is consolidated to form a pyroclastic igneous rock. Igneous Rock Textures. iv) The relative rates of growth of the constituent minerals. Igneous rocks with a porphyritic texture (large grains - phenocrysts - surrounded by a finer-grained groundmass) cooled in a two-stage process, first slowly and then more rapidly; this occurs in both plutonic and volcanic rocks. magma. The larger grains formed and grew during the early, slow cooling, for example while magma sits in a volcanic magma chamber. There are two major states of molten rock: Magma and Lava. Textures of Igneous Rocks The main factor that determines the texture of an igneous rock is the cooling rate (dT/dt) Other factors involved are: The diffusion rate - the rate at which atoms or molecules can move (diffuse) through the liquid. Pluto – god of the underworld. I. Igneous Textures A. If the magma starts to raise toward the surface it will start to cool. Then, use your table to answer the questions at the end of the lab. TEXTURES OF THE IGNEOUS ROCKS* Structure - large-scale features recognizable in the field, such as banding, lineation, jointing, and vesicularity. Two Types of Igneous Rocks Extrusive, or Volcanic rocks . Symplectic intergrowth connotes an intimate fine-grained … Types Of Igneous Rock Textures B Inequigranular Texture: I. Porphyritic Porphyritic Texture: a relationship between large crystals called Phenocryst and ground mass, according to the nature of ground mass, porphyritic texture can be classified into 4 main types and 2 other main types based on Phenocryst which are usually characterize Extrusive rocks. The cooling history of these rocks is reflected by their grain size mixture (Le Maitre, 87). Igneous rocks are classified according to their texture (coarse-grained vs. fine-grained) and their chemical and mineralogical compositions. 6.1.1. Igneous Rocks For this lab, you will be making observations and interpretations about igneous rocks. Igneous rocks are classified according to their texture and their composition. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic. Types of Lava. Sherlock Holmes is famous for his amazing deductions, but his real gift was his ability to observe small but critical details. Point out that the photographs are all at natural scale (the 1p coin is 2 cm across). This table shows how we classify and distinguish among the variety of igneous rock types by texture and composition. 4.1.3: Igneous Rock Bodies; References; Igneous rocks are classified based on texture and composition. The finer-grained matrix formed during later, but more rapid cooling, for example after the magma erupted onto the surface. Also answer these questions on our online class site so that you can get credit for the laboratory exercise. II - Occurrence, Texture, and Classification of Igneous Rocks - Gezahegn Yirgu ©Encyclopedia of Life Support Systems (EOLSS) 2. Volcanic textures: Pyroclastic • Igneous rocks formed of mineral and rock fragments ejected from volcanoes by explosive eruptions have pyroclastic (fragmental) textures. Lava is the term given to magma once it reaches the Earth’s surface, usually in the form of a volcanic eruption. We will cover mineral crystal sizes and vesicles in this section. Mode of Occurrence of Igneous Rocks Magmas erupted from volcanoes are either poured out as coherent fluidal lava flows or I recommend that you print the data table provided and fill it in as you analyze each sample. Cooling history is also related to changes that can occur to the composition of igneous rocks. Igneous Rocks Igneous rocks are formed by the cooling of molten rock. This relates to the cooling history of the molten magma from which it came. 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