Need help identifying this rock!?

Question

ive been finding a lot of interesting little rocks on the beach (southern maine usa). their size ranges from penny-size to the size of a roll of scotch tape. i can always fit them in my hand. theyre always black. the rocks are shiny/glassy; sometimes theyve been worn down by the beach and are a little coarse, but if i break them open its always very shiny. there are no hard edges, theyre generally smooth. very light and soft: i go to hit one with a hammer and its smashes into powder. no holes, no grains or crystals, no layers, no smell: its just black, glassy rocks. at first i thought they were some kind of burnt wood, charcoal, but theyre too hard. then i thought it might be some kind of space rock: it looks like shiny space-rock, but its much too light and soft, no metal inside. ive been looking it up on the net, following these charts and it keeps coming up as (and looks like) obsidian, but its way too light and soft! i have no idea what it is! ask for specific details!

Answer 1

Southern Maine has lots of metamorphic rock so it almost has to be a metamorphic rock and not an extrusive igneous rock like obsidian, and even less likely a sedimentary type of rock like coal.

My best guess:

Graphite: this is black, light in weight, soft, weathers smooth and round, has no obvious cleavage, and will turn to powder when hit with a hammer. It can look almost metallic and can be scratched with a finger nail. It is also a metamorphic rock that fits with the geology of southern Maine. Graphite is found in many locations in Maine.

http://www.galleries.com/minerals/elements/graphite/graphite.htm

Near Portland you have Precambrian metamorphic rocks of gneiss and schist, and another zone of Ordovician age mixed volcanic rocks which are less metamorphosed. Obsidian is not listed as a known mineral in Maine. Here is the Maine Geological Survey's list of Maine's minerals:
http://www.maine.gov/doc/nrimc/mgs/explore/minerals/guide/guide.htm


A comment about tar balls (denatured sludge?) on the beach and oil rigs:
It is very UNLIKELY that tar comes from the oil rigs, but it is very LIKELY that oil rigs will be found where there are NATURAL sources of tar. One answerer seems to have this relationship confused. Most offshore oil rigs are located in areas where tar outcrops and oil seeps naturally out of the ground. Southern California is a perfect example of this, with thousands of natural oil seeps offshore, and natural outcrops of tar along the beaches, as well as offshore. When wave action breaks up these undersea tar deposits they float to the surface and end up on the beach. This is WHY they drilled for oil offshore.

EDIT: Last time I checked there was no offshore drilling in Maine, and no coal. I might not be right about graphite, but I'm pretty sure what rocks don't exist in a metamorphic terrain.

Answer 2

I agree with tigerceratops.....you've probably been picking up lumps of anthracite coal off the beach. Obsidian would be a bit too heavy to float on water and from the way you describe how it turns to powder when you hit it with a hammer, that is not the way obsidian fractures.....it's much more like coal.

That fact that they're soft sort of gives it away. Obsidian is quite hard and very sharp on edge. Coal is brittle and soft.

Answer 3

Tigerceratops and Ozzie may be on to something. It does indeed sound like coal...but the location bugs me a little. I've walked a LOT of south coast beaches, and have found blobs similar to what you're describing, which turned out to be denatured sludge from off-shore drilling rigs.
Even if the rigs are miles and miles off-shore, these little blobs can wash up onto the beaches. I suspect that's what you've got.

Answer 4

At first, I thought you were describing obsidian. But the fact that you hit it with a hammer and it turned to dust rather than fracturing leads me to think that it might be coal. There are two types of coal, bituminous and anthracite. Anthracite is the harder of the two and quite shiny, but still much softer than obsidian.

Answer 5

I wonder if it could be some kind of coal, since that is shiny, black, and sometimes soft and crumbly. Try burning a bit of the powder.

Answer 6

I'm siding with coal on this one. Coal is much lighter than it looks and I'm surprised there's no smell but then it's been in seawater. And if there's offshore drilling they might be what broke off when they're drilling which makes sense given coal-natural gas-oil-graphite relationship.

Answer 7

Sounds like obsidian (volcanic glass).

Answer 8

The way you describe it it sounds like volcanic deposits.

Answer 9

probably lava rock

Answer 10

i need more help to got it in mind