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3. ROCKS

3. ROCKS. Rocks are compact, semi-hard to hard mass composed of one or several minerals and/or mineraloids. On the basis of their mode of origin , rocks can be divided into three basic types: Igneous Sedimentary Metamorphic. ROCK CYCLE.

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3. ROCKS

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  1. 3. ROCKS

  2. Rocks are compact, semi-hard to hard mass composed of one or several minerals and/or mineraloids. • On the basis of their mode of origin, rocks can be divided into three basic types: • Igneous • Sedimentary • Metamorphic

  3. ROCK CYCLE The formation cycle of different types of rock and the processes associated with them. This is called therock cycle. Sedimentary Rock The rock cycle is important in any discussions of soil formation, especially if we know that soils are nothing but disintegrated rocks. Metamorphic Rock Igneous Rock

  4. A. Igneous Rocks • They are those that have solidified from a molten or partially molten material called Magma or Melt. • Magma has originated well bellow the surface and ascended towards the surface, and has crystallized as solid rock either on the surface or deep within the earth's crust as temperature fell. • The types of igneous rock formed by the cooling of magma depends on the i) composition of the magma; ii) the rate of cooling associated with it.

  5. Bowen’s Reaction Series Bowen’s reaction principle—describes the sequence by which new minerals are formed as magma cools. different chemical compositions with similar crystalline structures different in their chemical composition and crystalline structure Depending on the proportions of minerals available, different types of igneous rock are formed.

  6. Bowen’s Reaction Series Temperature of Crystallization Two series of minerals formed during crystallization of magma Low Silica Magma Intrus. Extrus. Gabbro Basalt Diorite Andesite Granite Rhyolite 1200oC 1000o Frame- Double Single Isolated work Sheet Chain Chain 750o High Silica Magma

  7. Mineralogy of Igneous Rocks Many different types of minerals occur in igneous rocks, but only about eightare normally present as essential constituents of a rock. They are: • Quartz • Orthoclase • Plagioclase • Muscovite • Biotite • Hornblende • Augite • Olivine Which of the eight are present is controlled primarily by the composition of themagma.

  8. Classification of Igneous Rocks I. Classification based on content of silica The content of silica (Sio2) in igneous rocks varies from over 80% to about 40%. Rocks containing much silica were originally called ACID, and those with less silica were called BASIC.

  9. Igneous Rocks and Silicates Frame- Double Single Iso. work Sheet Chain Chain The Igneous Rock Classification parallels the systematic Silicate Mineral Classification Mineral Percentage

  10. II. Classification Based on Mode of Occurrence A. Volcanic or Extrusive Rocks They are formed when magma reaches the surface, along either wide vertical fissures or pipe-like openings in the earth crust. B. Plutonic or Intrusive Rocks Those rocks are formed when the magma rise through the earth’s crust but does not reach the surface. Intrusions can be divided into two types: 1. Major Intrusions 2. Minor Intrusions • Stocks • Batholiths • Sheets • Dykes • Sills • Laccolith • phacolith

  11. Extrusive/Volcanic: Igneous rocks that form due to the freezing of melts above the surface of the Earth. Remark: In general Acidic lavas do not flow from their place of eruption in contrast to Basic lavas.

  12. Intrusive/Plutonic: Form by freezing of melts belowthe surface of the Earth.

  13. Igneous Intrusions

  14. Types of Igneous Intrusions Discordant:cut across pre-existing fabric of rock layers Dikes: are small igneous intrusions that cut across rocks into which the magma intrudes. They are commonly sheet-like, only a few meters wide, but possibly laterally extensive. Stocks: are fairly large (10’s of miles) igneous intrusions that cut across pre-existing rock layers. In size, they are on the order of an individual mountain peak. Batholiths: are huge igneous intrusions made of many stocks. Their size is on the scale of an entire mountain range (100’s of miles).

  15. Concordant:follow pre-existing fabric of rock layers without interruption of layering. Sills: are also small igneous intrusions. They are sheets of rock that, unlike dikes, are parallel to pre-existing rocks. Think of magma invading sedimentary rocks by spreading out between rock layers. That magma would cool to form a sill. Laccoliths: are small intrusions having a flat floor and doomed roof. Phacoliths: are small intrusions having both a curved roof and floor. Lopoliths: are also large, inverted mushroom-shaped (spoon-shaped) intrusions that “sagdown” in the middle because of dense rocks. Xenoliths are blocks of countryrocks contained or enclosed into batholith rock.

  16. Igneous Intrusions Remarks: • Dyke Swarm: is a group of parallel or radiating dykes. • One has to be careful when making site investigation not to confuse dykes or sills with major intrusions.

  17. Tabular Intrusions: Plutons

  18. Non-Tabular Intrusions: Plutons • Laccolith: a dome-like sill that bends the layers above it into a dome shape

  19. Non-Tabular Intrusions: Plutons • Pluton: Irregular blob-shaped discordant intrusions that range in size from 10’s of m, to 100’s of km • Laccolith:a dome-like sill that bends the layers above it into a dome shape • Batholith: A pluton that is > 100 km2 in surface exposure • Stock: A pluton that is <100 km2 in surface exposure

  20. III. Classification of igneous rocks based on composition and mode of occurrence

  21. Igneous Rock Classification (Summary) Intrusive (Plutonic) Extrusive (Volcanic) Mineral Percentage

  22. Classification of igneous rocks (Summary)

  23. Igneous Rock Classification Silicic IntermediateMafic Granite Diorite Gabbro Rhyolite Andesite Basalt (Porphyritic) Intrusive Extrusive

  24. B. Sedimentary Rocks Sedimentary rocks are widely spread over the surface of earth. They constitute about 75% of the area of the continents. Development • Weathering reduces the exposed rock mass to fragmented particles which can be more easily transported more easily by wind, water, and ice. • When dropped by the agents of transportation, they are called SEDEMENTS. • Sediments are typically deposited in layers or beds termed STRATA. • When compacted and cemented together (a process called LITHIFICATION), they formed sedimentary rocks.

  25. The process through which sediments are converted into sedimentary rocks is called DIAGENSIS. It includes the following phases: 1. CEMENTATION Water percolating through the voids (or pores) between the particles of sediment carries mineral matter which coats the grain and acts as cement that bind them together. 2. COMPACTION The weight of top layers compacts sediments and expels water out. 3. CRYSTALLIZATION Sometimes grains of sediments are joined together due to crystallization of some of their constituents due to pressure.

  26. Mineralogy of Sedimentary Rocks The composition of a given sedimentary rock will depend on: • The source of the waste material • The weathering process (Mechanical vs. Chemical) • The distance traveled Minerals from pre-existing rock • Quartz • Feldspar Mineral form in the area of deposition • Carbonates • Sulphates • Iron oxides • Silica

  27. Classification of Sedimentary Rocks Sedimentary rocks are classified into two major categories: • Detrital Sedimentary Rocks They formed from minerals or rock fragments derived from the breakdown of pre-existing rock

  28. Conglomerate This is a rock consists of cemented boulder, cobbles, pebbles and granules. Sandstone This rock is composed essentially of pressure- cemented grains of sand. The cement agent determines the degree of indurations, or hardness, of sandstone. Shale It is formed from silts and clays which have hardened into rock, with the principal indurations agent being pressure.

  29. REMARKS 1. Breccia is a variety of conglomerate, which has angular rather than rounded fragments. 2. When the grains in sandstone are practically all quartz, the rock is referred to as orthoquartzite. 3. For Sandstone, the three chief kinds of cement, in the order of their importance are: • Silica • Iron oxides • Carbonates 4. Mudstone has a blocky aspect; whereas, in the case of shale, the rock is split into platy slabs.

  30. 5. In mudstone and shale, size of the particles are generally less than 1/16 mm. 6. Shale is the most abundant of the sedimentary rocks. Nearly half of all the sedimentary rocks are shale. 7. Marl is a calcareous mudstone (expansive). 8. From engineering point of view, sedimentary rocks are the most important and relevant because: • They are the most abundant, and hence encountered. • They are the most problematic.

  31. II. Chemical Sedimentary rocks Sedimentary rock also can be formed by chemical processes. Rocks of this type are classified as chemical sedimentary rock. Examples of Chemical Sedimentary Rocks:

  32. Remarks • If a carbonate rock contains more than 50% of sand –sized quartz grains, the rock is termed a CALCAREOUS SANDSTONE. • If it contains more than 50% clay, the rock is termed a CALCAREOUS SHALE. • If It contains more than 50% calcite, the rock is called LIMESTONE. • If It contains more than 50% dolomite, the rock is called DOLOMITE. • The name dolomite is used to designate both a mineral and a rock. In order to avoid the confusion, some label the rock DOLOSTONE.

  33. Textures of Sedimentary Rocks There are two main types of textures of Sedimentary Rocks: 1. Clastic Texture Rocks formed by mechanical weathering have clastic texture 2. Non-Clastic Texture Most chemical sedimentary rocks have a non clastic texture. These rocks have somewhat the same appearance as igneous rocks with crystalline texture.

  34. Summary of Sedimentary Rocks

  35. Features of Sedimentary Rocks Sedimentary rocks have several key features. 1. Sorting Well-sorted versus poorly sorted. Common examples: Graded Bedding, Cross Bedding

  36. Features of Sedimentary Rocks (cont.) 2. Stratification • Most sedimentary rocks are layered and the layers range in thickness from mm to meters. • Such depositional layers are called STRATA, an individual layer is a STRATUM. • In everyday language, the layers are called BEDS, if the layer thickness is > 1 cm. if < 1cm they are called LAMINE.

  37. 3. Roundness of grains

  38. 4. Ripple Marks

  39. C. Metamorphic Rocks • Metamorphic rocks are formed if a rock is subjected to increase in temperature, pressure, or both, to such degree that a new TEXTURE or possibly a new MINERAL composition is produced. • The process of change is referred to as METAMORPHISM. It is derived from the Greek META which means change and MORPH, shape or form. • The original rock may be igneous, sedimentary or metamorphic.

  40. The origin of metamorphic rocks is the leastclear among the three classes of rocks. Because no one has ever seen a metamorphic rock been formed, and for that reason many of our ideas about them are pure conjecture. • During metamorphism the minerals do not melt, but remain largely in the solid state. • The metamorphism process is isothermal process, i.e. the overall chemical composition of the rock is nearly the same before and after recrystallization.

  41. Classes of Metamorphism There are three broad classes of metamorphism depending on the control exercised by temperature and pressure. 1. Thermal or Contact Metamorphism Increased temperature is the dominant agent producing change. (Caused by igneous activity) 2. Dynamic or Dislocation Metamorphism The dominant control is stress, extra heat is relatively unimportant. (Associated with faults & earthquake zones) 3. Regional Metamorphism Both temperature and pressure have operated over a large (regional) area. (Associated with tectonic plate activity).

  42. The degree of metamorphism is related to the conditions of temperature and pressure under which the new metamorphic rock has formed, and may be assessed by the appearance of certain minerals. • The index minerals each of which indicates a particular temperature and pressure at the time it formed are used to define the metamorphic GRADE of the rock in which they occur. • One important characteristic related to the metamorphic grade is that grain size gets coarser with higher grade, i.e. rocks formed at higher temperature have coarse grained particle like GNEISS, and vise versa.

  43. Some metamorphic rocks and their original rocks

  44. CLASSIFICATION OF METAMORPHIC ROCKS I. Classification Based on Type of Metamorphism • Contact Metamorphic Rocks • Dynamic Metamorphic Rocks • Regional Metamorphic Rocks II. Classification According to Degree of Foliation (Textures)

  45. Foliated • Rock has distinct banding or layering • Formed under direct pressure Non-Foliated • No distinct layering character • Formed under uniform pressure

  46. Foliated Metamorphic Rocks slate phyllite schist gneiss

  47. iii. Classification of metamorphic rocks according to metamorphic grade

  48. iv. Classification of Metamorphic Rocks based on their Composition 1. Monomineralic Metamorphic rock composed predominantly of single mineral For example, Marble: calcite and Quartzite: quartz 2. Multimineralic Metamorphic rock composed predominantly of two or more minerals For example, Gneisses: felsic and darker mineral layers

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