Let's take a look! Taking into account the density of basalt (3000 kg/m3), this will be an object a bit less than a kilometer across. read more
"Basalt is a common gray to black volcanic rock. It is usually fine-grained due to rapid cooling of lava on the Earth's surface. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. read more