3. Considering the relationship between cooling rate and crystal size in igneous

Question

3. Considering the relationship between cooling rate and crystal size in igneous rocks, describe in your own words what would you expect to see in igneous rock textures in the following situations. An excavation in a rock quarry permits you to see the full width of an igneous dike that has intruded into surrounding rock. Describe the possible change(s) in texture from the center of the dike to the edge of the dike (where it is in contact with the surrounding rock). In this instance, the dike is 3 meters wide a.

Explanation / Answer

3. a) As a general principle between cooling rate and crystallization rate it is well established that faster is the cooling lesser is the crystal size growth. In case of the 3m dike, the central portion will comparatively cool slower than the parts of the dike which are in direct contact with the surrounding country rock. This will create a condition in which the crystal growth in the peripheral parts of dike will be less but overall number of crystals will be large this will give rise to aphanitic igneous texture whereas in the central portion of the dike becasue of slow cooling we may have phaneritic texture where the crystal growth will be uniform but number of crystals will be lesser. In the transitional zone from center of the dike to periphery gradually on both sides of center the fine grained crystal will increase and coarse grained crystals will decrease and this will give rise to porphyritic texture.

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