Please answer these questions thank you Question 5 (13 marks more Felsic a High
ID: 117773 • Letter: P
Question
Please answer these questions thank you
Question 5 (13 marks more Felsic a High eer mic felsic Ghedm)-> Figure 3 See photograph of the hand specimen in Figure 3 hand specimen shows relatively large crystals of K-feldspar and smaller unidentifiable dark minerals. Both the k feldspar and the dark mineral occur in a very fine grained matrix. Quartz and olivine are notably absent This ..fal_what is the appropriate name for the texture of this rock? [2 marks). Describe and explain the cooling history of this rock (4 marks]. (c) B this rock intrusive or extrusive? Explain your answer, 12 marks] Basalt8ltk ndesite Latite Trachyte Dacite Rhyoihe ndestendrute agjocass Not present Pyroxene . Amphibole s Biotite Not present Not present K-feldspar Not present Quartz Not present .often present,.frequently present, rarely present what could the unidentifiable black mineral possibly be? Explain your answer 12 marks ) Using the rock description and the table above, identifly this rock. (1 marks) (d) Question 5 (6 marks]Explanation / Answer
a) The texture of this rock rock is porphyritic texture.
The porphyritic texture forms when magma sits and is cooled a bit below the Earth's surface, thus giving time for the large crystals to grow, before erupting onto the surface and cooling very quickly. The large crystals are termed phenocrysts while the aphanitic rest of rock is called the groundmass. Here, the k-feldspar form the phenocrysts whereas the unidentifiable black minerals are groundmass.
b) The cooling history of this rock-
Porphyritic rocks are formed when a column of rising magma is cooled in two stages. Phenocrysts or coarser grains are embedded in a fine-grained matrix in a porphyritic igneous rock. Porphyritic texture in a rock indicates that a rock has undergone two stage cooling. Slow cooling initially and rapid cooling in later stages perhaps.
c) This rock is an extrusive rock.
Extrusive igneous rocks cool from lava rapidly because they form at the surface, so they have small crystals.As the groundmass of this rock is fine-grained which is a result of rapid cooling on the surface, it is an extrusive rock.
e) The rock description given in the table shows that the rock is Trachyte as it has k-feldspar and absence of Quartz and Olivine.