GEO1115_2021_Activity8.4S.docx

THEME 8 – IGNEOUS ROCKS [ 70/73 : A+ ] A CTIVITY 8.4S – V ISUAL IDENTIFICATION AND ASSOCIATED QUESTIONS ( /73) Reference material: - Lecture of associated theme (Brightspace) - Glossary ( https://mysite.science.uottawa.ca/dumas/ ) - Reference document “Section of the lab manual” in Brightspace Activity: Answer the questions associated to each of the following pictures. N.B. The rocks shown in the pictures below can be any of the rocks identified with a red “ X ” in Table 1 of the “Section of the lab manual” (reference document on igneous rocks). Also, the same rock (rock name, texture) can be tested more than once with different pictures. Igneous rock 1 ( /4) a) Name this rock. Obsidian b) Give the word that describes its texture. Vitreous c) What was the rate of cooling of the magma that generated this rock (slow or rapid)? Slow d) What is the name given to the type of fracture shown? Conchoidal fracture

Igneous rock 2 ( /7) a) Give the word that describes the texture of this rock. Phaneritic b) Name the dominant mineral in that rock ( hint: green colour ), and give its chemical formula. Explain how the chemical formula of this mineral relate to the composition of this rock (i.e. felsic, mafic, or ultramafic). Olivine - (Mg,Fe) 2 SiO 4 It is ultramafic due to the very high Mg and Fe content in the chemical composition in the rock

c) Qualify the viscosity (low or high) of the magma that generated this rock, and justify your answer. Low as it is mainly composed of olivine which is one of the first minerals to crystallize, then the magma is still extremely hot d) Name this rock. Peridotite

Igneous rock 3 ( /5) a) Give the word that describes the texture of this rock, and explain why you say so. Porphyritic because there is multiple distinct sizes of crystals, larger ones interspersed within ones which are smaller and invisible to the naked eye. b) What is the specific textural name given to the large crystals in that rock? Phenocrysts c) Briefly explain what this texture implies in terms of rates and depths of cooling. This occurs as rising magma cools in two stages, first lower within the Earth’s crust it cools slowly and forms the larger crystal pieces. Then as it rises and reaches a much shallower depth it cools much more quickly and crystallizes into extremely fine crystals that are invisible or nearly invisible to the naked eye.

Igneous rock 4 ( /8) a) Give two different words that describe the textures present in this rock. Aphanitic and Vesicular b) Give the word that describes its composition, and briefly comment on what this implies regarding the relative abundance of Si, Fe, and Mg in the rock (low or high). It is mafic, this means high in Fe and Mg and low in Si c) Name this rock. Vesicular Basalt d) Name two minerals that form this rock and give one diagnostic physical property for each of these two minerals. - Olivine o Very dark, generally dark green, mineral - Amphibole o A more intermediate shade e) What crystalline rock would be of equivalent composition but of different texture? Gabbro

Igneous rock 5 ( /7) a) Give the word that describes the dominant texture of this rock. Aphanitic b) What was the rate of cooling of the magma that generated this rock (slow or rapid)? Rapid c) Give the word that describes its composition, and briefly comment on what this implies regarding the relative abundance of Si, Fe, and Mg in the rock (low or high). Felsic, therefore high in Si and low in Fe and Mg

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