Parts of a rock body judged to be a lava flow display a crystalline texture with needles of amphibole all lined up parallel to one another. Is such an oriented crystalline texture compatible with igneous origin? HINT: Some lava flows are described as 'streams of lava'.
A lava flow is made up of glass in which are embedded half-inch long, well-formed crystals of feldspar. How might such a texture originate? HINT: Grain size in an igneous rock is greatly affected by the rate at which the melt from which it formed cooled. Does the rate at which lava cools change from when the lava is still underground to when it spreads out over the surface of the earth?
On the face of a vertical cliff a series of off-white, horizontal sedimentary rock layers are exposed. Near the top of the series is a black layer sandwiched in between the off-white layers. Along the bottom contact with the black layer, the top part of the off-white layer is dark grey. Near the middle of the series is another black layer sandwiched in between the off-white layers. Along both its contacts with the off-white layers, the parts of the off-white layers adjacent to the black layer are dark grey. Neither the black layers nor the dark-grey zones can be examined directly, as they are too high above the ground. A geologist decides the upper black layer is a lava flow and the lower black layer is a sill. What evidence can the geologist supply to support those claims? HINT: First, draw and label a sketch to represent the arrangement of the rocks as seen on the cliff face.
HINT: What is above and beneath a sill when it cools? What is above and beneath a lava flow when it cools.

© 2001, David J. Leveson