The photomicrograph to the right is of part (approx. 4 mm times 3 mm) of a thin
ID: 299061 • Letter: T
Question
The photomicrograph to the right is of part (approx. 4 mm times 3 mm) of a thin section of an igneous rock seen in plane-polarised light. The rock is composed of crystals of minerals A and B in a fine-grained matrix C. Which term best describes the texture of the rock? (Tick one of the one boxes below): What term best describes components A and B? (Tick one of the nine boxes below): Which of the following best describes the cooling history of the magma represented by the rock specimen? (Tick two of the nine boxes below): A. B and C formed by very slow cooling (1000s of years) A, B and C formed by moderately slow cooling (weeks to months) A. B and C formed by fast cooling (hours) A. B and C formed by rapid quenching (seconds) A and B formed by slow cooling (months). C by rapid cooling (hours) C formed by slow cooling (months). A and B by rapid cooling (hours) A and B crystallised first, followed by C C crystallised first followed by A and B A. B and C all crystallised at the same time From which of the following was the sample most likely collected? (Tick one box only): Plutonic igneous body Lava flow Pyroclastic flow Laccolith Pegmatite body Hydrothermal vein Layered intrusion The Earth's mantleExplanation / Answer
a. Porphyritic texture- As the Crystals A and B are well developed where as C is formed in the ground mass
b. Phenocryst – These are well developed crystals with well defined boundaries as it’s clear for the both the crystals.
c. A and B crystallized first and followed by C since the A and B have developed well and there might have been any sudden cooling for the C as don’t have time to develop it’s crystal boundaries.
d. As looking from this section it’ll be difficult to say which one would be the correct sample place as the matrix part is more but the crystals with A and B are well defined border line so it suggest it can be taken from any Laccolith type of the location.
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