geology help?

[ohsnapsitzjustin]

what is the evidence for distinguishing a boundary at the base of the crust? what is that boundary called?

[godheadslament]

The boundary is the Mohorovicic Discontinuity; the Moho for short. It marks the boundary between the crust and the mantle. There are infact 2 mohos, the geophysical moho and the petrological moho, which are slightly apart. For the sake of simplicity, you can assume they are in the same place (there's not a lot of distance in it, relatively speaking).

Geophysical methods are mostly used for direct study of the Moho. There are 2 kinds of seismic waves relevant in this study, P & S - P waves can travel though liquids, whereas S waves cannot. As such, the transition to 0 velocity in S waves marks the Moho.

The petrological evidence of the Moho is mostly indirect. Sometimes, ocean crust and some mantle is obducted onto the continents, called ophiolites (notable examples are the Troodos Ophiolite of Cyprus, and the Oman Ophiolite). The Moho is often visible in these crustal / mantle remnants. The sequence is (basically) as follows -

Hemipelagic sediment
Pillow lavas
Sheeted dyke complex
Gabbro & Plagiogranite
Mantle peridotite.

The Moho is the boundary between the gabbro and peridotite.

The main reason you would distinguish a boundary here is that it marks the transition from mostly brittle rocks to those that flow slowly, due to heat and pressure gradients.