Teacher Notes in Yellow

 

    
Castle Rock, Marblehead - teacher guide

This "Field Trip" and the others in Marblehead highlights the fact that, while the Boston area is very geologically inactive at present, this was not always the case. For much of its history, the Boston area was next to tremendous and violent geologic activity. These rocks formed in the late Precambrian - probably between 550 and 570 million years ago. This was a period of time when many volcanoes were found in our area.

This site and the others in Marblehead are good places to discuss the different types of volcanic rocks. The rocks below are extrusive rocks. These rocks, and the ones at Marblehead Light, formed on the surface of the earth. The rocks here formed from volcanic ash. (Marblehead Light also has rocks that formed from lava flows - you won't find those here.) By contrast, the rocks at Devereaux Beach formed deep below the surface of the earth - probably thousands of feet down. Yet, how is it possible that all of these rocks appear, currently, exposed at the same altitude? Discuss how uplift, erosion, faulting can alter landscapes to make this possible.

We suggest looking at this "field trip" in conjuction with other volcanic sites. You may want to have your students look at Nantasket Beach and Dane Park at the same time.

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?

 
The two types of tuff found here are both formed from volcanic ash. The dark welded tuff forms as extremely hot ash piles up - its own heat causes it to weld together into rock. The lighter ashy tuff (also called lithic tuff) forms as cooler ash settles; eventually it is lithified into rock. In both cases is common to see bits of other rock (volcanic cinders and "bombs") in the tuff.

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?