4. Three major rock types are exposed in the air photo shown: (i) Metamorphosed

Question

4. Three major rock types are exposed in the air photo shown: (i) Metamorphosed sediments (dark grey); (i) granite (light tones); and (ii) dikes (long, narrow bands of dark-grey tones) The very dark tones are water bodies) (a) Study the contacts between the granite and the dikes, and state which of these features is the youngest. (b) Think about the processes that most likely lead to the formation of the granite bodies and the metamorphosed sediments, then comment on the relative age of these rock bodies. (c) Which is most resistant to weathering: the metamorphic rocks or the granite? Briefly explain your answer. (d) Think about the properties of these types of rocks. How can you explain the presence of so much surface water in this area? (And you're not allowed to explain this based on heavy rainfalls; annual precipitation in this area is not particularly high.) ada) 0 1 mile

Explanation / Answer

a) The dykes are younger than the granites as is evident from the contact relationships between the granites and dykes. The dykes have intruded into the granite.

b) Granites are the primary rocks as they have formed from the process of intrusion of magma into the country rocks at depths from the surface. The metamorphosed rocks are formed by the mineralogical and chemical changes of the primary igneous rocks (i.e. granite in this context). Thus, the metamorphosed rocks are ounger than the granites.

c) Granite is more resistant to weathering than the metamophosed rocks. This is beacuse granite is abundant in the mineral quartz which is highly resistant to weathering. Moreover, granite is impermeable and does not allow water to pass through it as pores are not present. Thus, water as the agent of weathering is not effective.

d) The presence of surface water is due to the reaction procedures during metamorphism. Metamorphism involves chemical and mineralogical changes which may involve dehydration i.e. exudation of water. Moreover, it may be due to the magmatic water (involved in the granite formation) or circulating groundwater.

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