Answer to Brain Drain

Click on images to enlarge

Question:
These rock structures from the southwest look similar at first glance. What major rock type makes up each structure - igneous, sedimentary, or metamorphic?

Answer:
Image on left:
Sedimentary - If you look closely at the enlarged image, you will find horizontal layering - a strong indicator of sedimentary origin. Look near the bases of the largest structures in the foreground. This is Monument Valley near Mexican Hat, Utah.

Image on right:
Igneous - This structure, called Shiprock, is located in New Mexico. If you look closely at the larger image, you won't find clear horizontal layering. Neither the tallest structure in the upper righthand side of the photo, nor the long ridge show layering.

The taller vertical structures are the remains of volcanic necks. Volcanic necks are the remains of magma that solidified in the main vertical shaft underneath the central crater.

The long low-lying ridges are the remains of magma that solidified in cracks or dikes radiating around the volcano.

The volcanic necks and dikes are now exposed because less resistant material has been eroded away. We are looking at the inside of an ancient volcano.

Photographs by Louis Maher, images used with permission.

Related links:
Monument Valley Tribal Park

The Shiprock Landform
New Mexico Bureau of Mines and Mineral Resources