01 CVE2000 4 - Unit 1 - Lab 3b - Geology - Identification of Metamorphic Rocks PDF

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CVE20004 – GeomechanicsLaboratory 3b: Identification ofMetamorphic RocksUnit 1:GeologyGeology: The Geological Rock CycleCVE20004 GeomechanicsMagma or LavaIgneous RocksSedimentary Sediments RocksMetamorphic RocksMeltingMetamorphismCompaction, Cementation or CrystalisationWeathering and Transportation...

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CVE20004 – Geomechanics

Unit 1:Geology Laboratory 3b: Identification of Metamorphic Rocks

Geology: The Geological Rock Cycle Compaction, Cementation or Crystalisation Sedimentary Rocks

Sediments

Metamorphism Weathering and Transportation

Igneous Rocks

Metamorphic Rocks Magma or Lava Melting

CVE20004 Geomechanics

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Metamorphic Rocks – Foliated Geological metamorphism is the process of a rock changing composition and texture by heat and/or pressure (without melting). This can be achieved through increases in temperature from the underlying magma rising and/or extreme overburden pressure (possibly through tectonic plate movement). If pressure is the reason for the metamorphism, new minerals are formed as old mineral grains are sheared. This creates a foliated texture. Foliation is where platy minerals develop a parallel orientation.

CVE20004 Geomechanics

General classification of groups of metamorphic rocks

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Laboratory Exercise No. 3b – Identification of Metamorphic Rocks

CVE20004 Geomechanics

Metamorphism of Igneous (Plutonic) Rocks Granite

Acidic igneous plutonic rocks (such as Granite) are largely unaffected by increases in temperature alone, but when affected by moderate temperatures and pressures together (i.e. regional metamorphism), these igneous rocks generally turn into Gneiss. CVE20004 Geomechanics

Gneiss

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Metamorphism of Igneous (Plutonic) Rocks

Gneiss A unique feature is the light and dark banding in the foliated texture.

Gneiss (Biotite) CVE20004 Geomechanics

Metamorphism of Igneous (Plutonic) Rocks Gabbro

Basic igneous rocks (such as Gabbro) are also affected by regional metamorphism but become a type of Schist, Amphibolite or Ecolgite. As these igneous rocks are mostly a single dark colour already, no banding develops during the metamorphism.

However, Schist can also develop from sedimentary type rocks.

Schist (Hornblende) CVE20004 Geomechanics

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Metamorphism of Sedimentary Rocks

Mica Schist

CVE20004 Geomechanics

This sample of Schist was likely metamorphosed from either a sandstone, siltstone or shale (under medium to high grade regional metamorphism).

Metamorphism of Sedimentary Rocks Clastic sedimentary rocks such as siltstone, shale or mudstone may metamorphose as slate: One unique feature of slate is the near perfect horizontal banding present. This is why slate has been used as a roofing tile material and the base for billiard tables.

Slate will form under low grade regional metamorphism CVE20004 Geomechanics

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Metamorphism of Sedimentary Rocks Clastic sedimentary rocks such as siltstone, shale or mudstone may metamorphose as Phyllite:

Phyllite and Slate are very closely related. Often, Phyllite is seen as a low quality slate (due to the irregular foliation present and slightly larger grain/crystal size).

Phyllite will form under low to medium grade regional metamorphism CVE20004 Geomechanics

Metamorphic Rocks – Non Foliated Clastic sedimentary rocks such as sandstone and siltstone may metamorphose into Quartzite: Here, the majority of the metamorphism has been due to an increase in temperature as no banding or foliated texture is present. Also, the grains should not be individual but well and truly interlocked together. Quartzite will form under Thermal and Regional metamorphism CVE20004 Geomechanics

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Metamorphic Rocks – Non Foliated Clastic sedimentary rocks such as limestone and dolomite may metamorphose into Marble:

Similar to the Quartzite sample, the grains should not be individual but well and truly be interlocked together.

CVE20004 Geomechanics

Metamorphic Rocks – Non Foliated (from Basic Extrusive Igneous Rocks) A basic extrusive rock such as basalt may metamorphose into Hornfels: It can be difficult to visually detect the difference between Basalt and Hornfels, but Hornfels is denser. One way is to listen to the tone or pitch of the sound when you tap it on a hard surface. They will sound different. Hornfels Basalt Basic igneous rocks (such as basalt) affected by thermal metamorphism usually become dark, compact, relatively fine grained, structureless, fairly heavy rocks that are generally called Hornfels CVE20004 Geomechanics

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Simplified Identification of Metamorphic Rocks First, how can you tell if the rock has undergone metamorphism ? Does the crystalline structure interlock or do the grains / particles interlock ? Is their evidence of layering or banding within the rock structure ? If yes, then it is probably a Metamorphic Rock If no, and the crystals are random and show little orientation, probably an igneous rock Once you have established that it has undergone metamorphism Does the rock sample possess foliation ? (i.e. are layers evident that show alternating bands of different texture) If no, Fine to very fine grains – probably Hornfels Medium grains – could be Marble – if pale in colour and reacts to HCl – could be Quartzite – if pale in colour and no reaction to HCl – could be Ecolgite – if dark in colour and no reaction to HCl If yes, Are the crystals visible ? If no – could be Slate If yes, – could be Phyllite – if Fine to Medium Grains – could be Schist – if Medium Grains – could be Gneiss – if Medium to Coarse Grains CVE20004 Geomechanics

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