Lava and ash erupted from volcanoes can also accumulate as layers. These materials are considered 'igneous'.

Here, a geologist climbs a loose, rubbly slope. The yellow arrow points to where layering can be seen clearly. In the background looms Mount Hood, one of the Cascade Range volcanoes located in Oregon. Is the layering related to distant Mount Hood?

The Deccan Plateau, in India, is an enormous area underlain by dark layered rocks. No volcanic mountain is visible. What is the origin of these layered rocks?
Consider the sequence of events in the volcanic region shown in the diagram below:


From the discussion above, it seems that layering may develop in both sedimentary and igneous (volcanic) environments. If layers may be traced to a volcanic cone, then we know they are igneous. But if erosion has removed the cone or separated the layers from the cone, how can it be known if they are volcanic or sedimentary? Is there something that is part of the layers themselves that will serve to distinguish sedimentary from volcanic layers?

© 2001, David J. Leveson