There are a number of other primary structures that may be useful in distinguishing between layered sedimentary and layered volcanic rocks.

The back-and-forth motion of waves in shallow water throws the bottom sediment into a series of ripples.

Similar looking ripples appear on the surface of solid rock. How did they form?

Drying mud forms a network of cracks.

Similar looking cracks appear on the surface of solid rock. How did they form?

A sand dune has been excavated for commercial purposes. Sets of inclined, parallel layers intersect each other at distinct angles. (Geologists call this 'cross bedding'.)

Similar looking structures appear in solid rock. How did they form?

Shells lie scattered along a beach.

Imprints of shells appear in solid rock. How did they form?
Here's an explanation of how ripples might be preserved in rock:

Question 1: How might mudcracks be preserved in rock? Crossbedding? Shell impressions?
Question 2: Would the presence of such features serve to distinguish between sedimentary and volcanic layers?


In volcanic areas, layers that are clearly lava flows commonly display an orderly network of planar fractures oriented perpendicular to the tops and bottoms of the layers. Geologists call organized planar fractures "joints". The joints divide the rock up into a series of parallel, elongate, vertical columns. Seen in side view on the face of a cliff, the columns give the rock face the appearance of a fence with vertical slats.

Here, the columns are seen closer up. Detached columns accumulate at the base of the cliff. Their three-dimensional, columnar character can be seen clearly.

The top of a lava flow shows the typical polgonal (commonly hexagonal) cross sections of the columns. (Photo: U.S. Fish and Wildlife Service.)


Here's an explanation for columnar jointing:

Question 1: Look at the two pictures under 'Useful Structures' that show mudcracks. Is the origin of mudcracks similar to the origin of columnar jointing? Explain. (Hint: Mudcracks form due to shrinkage, but the shrinkage in NOT due to cooling. What might the shrinkage be due to?)
Question 2: Can columnar jointing be used to identify volcanic layers in the absence of volcanic cones?
Question 3: Both mudcracks and lava that has undergone columnar jointing both have networks of fractures. How might mudcracks and lava flows be distinguished?

To check your answers and get additional hints and information, click on the link.

© 2001, David J. Leveson