Sandstone and Granite: A Geological Puzzle
Which of the following statements is correct geologically?
a. The sandstone is younger if the granite contains sandstone inclusions.
b. The granite is older if the sandstone contains pebbles of the granite.
c. The granite is older if it contains inclusions of sandstone.
d. The sandstone is younger if it shows evidence of contact metamorphism.
Answer:
The statement that is correct geologically is b) The granite is older if the sandstone contains pebbles of the granite.
Explanation: Sandstone is a sedimentary rock that contains sand-size grains which are bonded either by a cohesive rock or chemical cement. Granite, on the other hand, is an igneous rock with a larger grain size that is visible to the naked eye, containing a significant amount of feldspar and quartz.
When sandstone strata and a mass of granite are observed to be in contact, and pebbles of granite are found within the sandstone, it can be concluded geologically that the granite is older than the sandstone strata. This is based on the grain size and composition of the rocks.
Option B and D are correct:
1. The granite is older if the sandstone contains pebbles of the granite.
2. The sandstone is younger if it shows evidence of contact metamorphism.
Rock dating: The presence of rock inclusions indicates that the included rock is of older origin. In the case of volcanic activity, if rocks are carried by lava flows and become part of the newly formed rocks, the older rocks will form inclusions within the new rock as it solidifies.
Contact metamorphism: This refers to metamorphism that occurs due to contact with or proximity to an igneous intrusion. If the sandstone shows evidence of contact metamorphism, it suggests that the sandstone is younger than the surrounding granite rocks.