Why am I doing this? A primer.

Magmas interacting- see the sharp contact between the grey
stuff and the whitish stuff?

Each summer I get a little better explaining this, so I’ll try again.  My PhD focuses on the deep plumbing of volcanoes.  You and volcanoes both exist up on the Earth’s surface.  My research studies what is going on about 5-30 km below where you are now standing.  Unless you are in Tibet, where the Earth’s crust is up to 70 km thick (!!!).  Then we’re playing a new ballgame. However, I digress.  Back to 5-30 km beneath the surface. Here, magma doesn’t become basaltic lava (imagine Hawaii) it becomes granite (like countertops!) because it cools so slowly- the crystals have lots of time to grow nice and big before they get cold.  There are entire mountains made of basalt (again, Hawaii) and entire mountains made of granite (Sierra Nevada is my favorite example).  What I am trying to understand is how all this granite gets put in the Earth’s crust- as a big blob? As little pieces? Something in between?  What is the link between the deep granitic stuff and the magma that gets erupted (if any)?   

 The North Cascades are a good place to study this because they have enjoyed a long history of tectonic activity- all this enjoyable activity has brought some rather deep rocks to the surface for us to study.  In particular, I’m looking at 3 different granitic-type bodies from 3 different crustal levels, let’s say deep, middle, and shallow. I’m using geochronology (exactly what it sounds like, dating rocks) and field work to understand how these 90 million year old bodies were assembled.