⇐ ⇑ ⇒ HI-RES Volcanoes National Park, Thursday 24th February Lava flow obstructing the road to Pu`u Huluhulu (left) and Mauna Ulu (3480', right). This is `a`a, lava with a rough rubbly surface composed of broken blocks. The transition from pahoehoe to `a`a is favoured by high viscosity from cooling or outgassing; the chemical composition is the same.