Teacher Notes in Yellow

This "Field Trip" 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.

On the South side of Nantasket Beach is Atlantic Hill. The exposed rock along the shore shows some really interesting volcanic rocks.

Wait! Volcanoes??? Here in the Boston area? Well, these rocks probably were formed between 550 and 570 million years ago. Look at this map and find New England. Aside from the fact that New England is near the South Pole(!!), you may notice that it's right on a subduction zone. Why is a subduction zone associated with the presence of volcanic rocks?

What you can see in this site are several layers of different volcanic activity. On the bottom you see andesite - quickly cooling lava - that probably formed under water. You can see evidence of this in the "pillow" lava formations that occur in very rapidly cooling lava (such as lava cooling under water). On top of that you see layers of tuff - rock formed from volcanic ash. Here's an interesting question - do you think tuff should be considered an igneous rock or a sedimentary rock? While it formed from volcanic ash, it is composed of piles of ash that compressed and hardened together into rock.

View looking South towards Atlantic Hill

View of the top of Atlantic Hill, showing layered tuff in tilted bands,
overlaying andesite lava (where "pillow" lava can be found!)

The pictures above show tuff, which is formed from volcanic ash. When tuff forms from ash, sometimes bits of other rock get embedded. Sometimes you can also see clear layering of the ash.

The pictures below show quickly cooling andesite. The features highlighted in this picture are known as pillow lavas, which form as lava cools very rapidly underwater.

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