Question
I recently went hiking in the Aravaipa area and saw several reddish-brown rocks with white "scratch" marks on them. The rocks were of various sizes ranging from palm-sized to the size of a loaf of bread. The entire surface of the rock was covered in small holes similar to pumice but with only a fraction of the holes. The "scratches" are in random directions and cover the entire rock. They are about 1/4 to 1/2 inch long. I am curious as to what would cause the white marks--they do not seem to be a different mineral within the rock. I have a picture I can send in if you would like it. Ric
Answer
Hello Nic,
This is a volcanic rock, probably an andesite or dacite. The white linear features are mineral crystals of the feldspar mineral family – the latter is a guess but I’m pretty confident. The small half-sphere indentations appear to be vesicles. They occur when gas – H20 mostly – escapes from the cooling lava.
This rock has porphyritic texture – larger mineral grains in a groundmass of finer grained minerals.
Thanks for sharing the image.
Mike
