1. Tectonic plate - tectonic plates are segments of the crust that move around on a semi-molten layer within the Earth called the mantle. You can dress the part by using common paper plates to construct a garment or simply make a t-shirt with the name of your favorite plate (mine is Juan-de-fuca). The key to this costume is that once you are at the party you need to move really slowly around the room. BONUS - if you need to get away from a conversation you can always re-enact "continental drift" by slowly wandering in a different direction.
2. Overturned bed - an overturned bed occurs when a layer of rock has been folded in such a way that it is upside down. You can create this by re-constituting an old baby mattress or other box shaped object and attaching a small pillow/blanket upside down. You could even paint the mattress to look like a particular rock type.
3. Rock bugs - take a t-shirt and attach small photos of rocks with Velcro. This would work best if the rocks had pipe-cleaner legs attached to them so they look like spiders (or any kind of bug). When you see someone you want to talk to (or think is attractive) you can pretend there are bugs crawling all over you (the better the freak out the more believable) and have that person help you get the rock-bugs off of you. After that is done you can graciously thank them for helping you get your rocks off. Ha!
4. Piece of schist - schist is a metamorphic rock that forms at relatively high degrees of pressure and temperature and contains abundant mica minerals that can resemble pieces of sequence. Glue pieces of sequence to a t-shirt using alternating bands of sequence in shades of brown and silver/green. For added effect you can shape outline of bands like a shape of poo (maybe even use a character like Mr. Hanky from South Park). When anyone asks, you are a real piece of schist.
For our physics friends........
5. Light wave - so many ways to do this one. One is to wear an article of clothing from a show like "Biggest Loser" and then proceed to wave at people all evening long. Another idea is to dress as a lightbulb and carry a surfboard. A "sciencey" way to go would be to dress as a photon (let's say that looks like an electron or similar particle) and carry the surfboard. You could ask people where you can catch a "light wave".
6. Black hole - dress all in black and consume all the food you are able to. At random times through the party call a group of people to you and temporarily wrap a large hola hoop (or similar) around everyone as your event horizon. Refer to anyone to gets away from the hula hoop as Hawking Radiation
7. Pumpkin pie - this is a great one for people who want a simple costume. Simply take an orange t-shirt and add the symbol for pi. Added effect if the shirt has a pumpkin on it or there is a pumpkin costume available.