Re:Flat top volcanos. (Score 4, Interesting)

I'll hazard a guess; if the lava on the Big Island of Hawai'i was looked at from a great height then it too would look smooth. On the other hand, lava from Etna or Mt St Helen's wouldn't. The difference is between basic (Hawai'ia) and acidic (Etna) lava, or perhaps the ammount of water (of course, these two may be highly correlated). On Mars, the lack of plate tectonics means that little surface material (inc. water) is pushed back into the mantle (c.f. Etna) and only 'pure' mantle material errupts (c.f. Hawai'i). Into a thin atmosphere, which allows dissolved gasses to leave more rapidly, the erruption would tend to be quite gentle and lower (Martian) gravity means it can spread over larger areas than on Earth.

Result is wide smooth basalt flows.

(but I only ever did Geology in 1st year undergrad---1A NatSci---so this might be incorrect).