This photo of Montana’s Glacier National Park shows layers of Precambrian sedime

Question

This photo of Montana’s Glacier National Park shows layers of Precambrian sedimentary rock. The darker layer contained between the lighter sedimentary layers is of igneous origin. The lighter layers were created when the molten material that formed the igneous rock heated the adjacent rock.

a) Is the igneous layer more likely a lava flow that was laid down at the surface prior to the deposition of the layers above it, or a sill that intruded after all sedimentary layers were deposited? Explain your reasoning.

Explanation / Answer

a) Igneous rocks are formed by cooling and solidification of lava. Igneous rocks are of two types; intusive and extrusive depending upon their location either above or below the surface respectively. So, the igneous layer given in this photo of Montana’s Glacier National Park is more likely to be a lava flow that was laid down at the surface prior to the deposition of the layers above it.

b) Sedimentary rocks are formed by sedimentation and subsequent solidification of the material at earth's surface. Metamorphic rocks are formed as a result of transformation of pre-existing rocks. Therefore, the light-colored rock given in the picture belongs to sedimentary group of rocks.

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