Castle Rock, Marblehead

The rocks here are from hardened volcanic ash. There are two distinct types of rock here - dark-colored welded tuff and light-colored ashy tuff. Can you tell, from these pictures, which type of rock formed first? What evidence can you see, or would you look for, to answer this question?


Here you can see both the boundary between the two tuffs. You can see that the rocks actually consist of volcanic ash mixed with other particles in it. Why is this type of rock so varied in composition? Why do you see small rocks of other types mixed in?


Close up of the welded tuff showing other particles mixed in. Close up of the ashy tuff showing other particles mixed in.

A granite xenolith inside the welded tuff.

How do you think this piece of granite got here?