The texture and chemical or mineralogical compositions of an igneous rock can give us information on where and how the rock formed. Rocks formed from . 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. The components of rock are minerals and each mineral forms, or crystallises, under a certain temperature and pressure. that crystallizes at the surface. Give each small group of pupils a set of the photographs of igneous rocks, printed onto card and trimmed to remove the names. To identify rocks, you must not be shy about taking a hand lens to carefully examine a rock before announcing its identity. The four rows, the four basic compositions. 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. Point out that the photographs are all at natural scale (the 1p coin is 2 cm across). Rocks formed from . There are two major classifications of igneous rocks: Intrusive and Classification based on rock texture Extrusive igneous rocks Igneous Rocks: Texture and Composition. 4.1.3: Igneous Rock Bodies; References; Igneous rocks are classified based on texture and composition. Texture describes the physical characteristics of the minerals, such as grain size. Sherlock Holmes is famous for his amazing deductions, but his real gift was his ability to observe small but critical details. Magma is a form of molten rock that exists below the Earth’s surface. lava. There are two major states of molten rock: Magma and Lava. Vulcan – god of fire Intrusive, or Plutonic rocks. 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. Textures of Igneous Rocks: Texture of a rock is the appearance of the rock and how one feels touching it. Principal minerals are Potassium Feldspar, Biotite Mica, and Quartz. Lava is the term given to magma once it reaches the Earth’s surface, usually in the form of a volcanic eruption. If the magma starts to raise toward the surface it will start to cool. We will cover mineral crystal sizes and vesicles in this section. The ejected ash and other debris eventually settles to the surface where it is consolidated to form a pyroclastic igneous rock. In practice, this means that the average dimension of the crystals is less than 1 millimeter. 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. The finer-grained matrix formed during later, but more rapid cooling, for example after the magma erupted onto the surface. Symplectic intergrowth connotes an intimate fine-grained … I. Igneous Textures A. Types of Lava. 3.4 IGNEOUS ROCK TEXTURE The classification of igneous rocks is based not just on composition, but also on texture. Textures in Igneous Rocks: The term ‘texture’ is defined as the mutual relationship among the minerals and the glassy materials in a rock. Texture describes the physical characteristics of the minerals, such as grain size. Pluto – god of the underworld. Igneous Rocks For this lab, you will be making observations and interpretations about igneous rocks. 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. 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. magma. I recommend that you print the data table provided and fill it in as you analyze each sample. Porphyritic texture tells you that the igneous rock underwent two stages of cooling, first slow, then fast. Rhyolite Felsic lava 800°-1000°C Lower temp and higher silica = more viscous. TEXTURES OF THE IGNEOUS ROCKS* Structure - large-scale features recognizable in the field, such as banding, lineation, jointing, and vesicularity. Igneous rocks are formed through the crystallisation of magma as it cools either in the mantle or on the surface after a volcanic eruption. II - Occurrence, Texture, and Classification of Igneous Rocks - Gezahegn Yirgu ©Encyclopedia of Life Support Systems (EOLSS) 2. 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. 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. Igneous Rocks Igneous rocks are formed by the cooling of molten rock. This table shows how we classify and distinguish among the variety of igneous rock types by texture and composition. Igneous rocks are classified based on texture and composition. 5.4.1 Magmatic Viscosity Magmatic viscosity defines the degree of fluidity of the magma, e.g. This texture is commonly found in igneous rocks in which phenocrysts are embedded in a fine-grained groundmass. 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. Igneous rocks that are characterized by porphyritic texture are known as porphyry. This relates to the cooling history of the molten magma from which it came. This pile of igneous rocks contains a wide range of igneous rock compositions and textures and represents a dozen different rock names and histories. UNESCO – EOLSS SAMPLE CHAPTERS GEOLOGY – Vol. 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. Also answer these questions on our online class site so that you can get credit for the laboratory exercise. The cooling history of these rocks is reflected by their grain size mixture (Le Maitre, 87). The following textures of igneous rocks are important from subject point of view: 1. Igneous rocks are also classified according to their textures. 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. Cooling history is also related to changes that can occur to the composition of igneous rocks. Then, use your table to answer the questions at the end of the lab. Igneous rocks with a fine-grained texture (grains generally tiny or too small to see) are volcanic in origin. 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. Igneous Rock Textures. Igneous rocks are classified according to their texture (coarse-grained vs. fine-grained) and their chemical and mineralogical compositions. Types of Lava. Texture - refers to degree of crystallinity, grain size, and geometrical relationships between the cons tituents of a rock (fabric). The following textural terms are commonly used for plutonic rocks. The five columns represent the five basic textures. The size and shape of the mineral grains or crystals and the pattern of their arrangement give a texture to the rock. iv) The relative rates of growth of the constituent minerals. The texture of a rock provides a clue whether the magma cooled fast or slowly and where the rock was formed. The key to identifying rocks is careful examination. All igneous rock began as molten rock formed deep within the earth called magma. 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. Volcanic textures: Pyroclastic • Igneous rocks formed of mineral and rock fragments ejected from volcanoes by explosive eruptions have pyroclastic (fragmental) textures. Textures and structures. 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. Textures of Igneous Rocks. Igneous textures include the rock textures occurring in igneous rocks. 3. 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. that crystallizes at depth. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic. Composition refers to the rock’s specific mineralogy and chemical composition. Igneous Rocks. Igneous textures are used by geologists in determining the mode of origin igneous rocks and are used in rock classification. This relates to the cooling history of the molten magma from which it came. 6.1.1. The larger grains formed and grew during the early, slow cooling, for example while magma sits in a volcanic magma chamber. 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. In igneous rocks, these textures are shown by granites and felsites and hence are also often named as granitic and felsitic textures. Two Types of Igneous Rocks Extrusive, or Volcanic rocks . 13 Ahmed Essam Ayad 14. Mode of Occurrence of Igneous Rocks Magmas erupted from volcanoes are either poured out as coherent fluidal lava flows or Building Stones 2 – Igneous rocks What are the differences between igneous rocks commonly used as building stones? Igneous rocks are classified according to their texture and their composition. Igneous rock texture Texture is anything that you can see in a rock that can give you clues as to how that rock formed.
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