Volcanics in Dane Park, Brookline

When you think of the Boston area, you don't usually think of volcanoes. But, indeed, about 600 million years ago there were many volcanoes in the Boston area! Because of millions of years of geological activity, much of the evidence for these volcanoes has been eroded away, or remains hidden.

One place where evidence of these volcanoes remains is in Dane Park in Brookline. Here you can see rocks formed from ancient ash deposits (tuff) and other rocks formed from cooling lava flows (basalt). Because these rocks are so old, much of this rock has been metamorphosed.

Observe the pictures below - try to imagine how each rock formed. Can you figure out which are basalt and which are tuff?
<pictures of basalt> <pictures of tuff>
Often when a volcano erupts, blobs of lava get ejected - as they fly through the air, they cool and harden. They get deposited with the hot ash, which eventually all gets cemented together to form a solid rock. These bits of rock found in the ash tuff are known as volcanic "bombs".
<bomb pictures>  
The picture below shows an outcropping of the volcanic tuff (rock formed from ash). Notice the ridges running across it. What ideas do you have about what might cause those ridges to form?  
<another picture showing the whole "mound">