Surface of Lavaflows
The surface texture of lava depends on (i) the temperature of lava, (ii) the composition of lava, and (iii) the velocity of the flow during the eruption. The surface texture of newly consolidated lava-flows are of three types;
(1) Blocky Lava: Rhyolitic lavas, because of their high silica content, are usually very viscous. Their movement is often very slow. Such slow-moving lavas generally have a rough and clinker-like surface. Such lavas are called ”blocky lavas”.
(ii) Ropy Lava (Pahoehoe): Also called Pahoehoe. When a lava-flow has a smooth but twisted surface, it is called ”ropy lava”. Such a surface is commonly formed on fast-flowing fluid lava of basaltic composition. When the lava starts solidifying, a plastic skin is formed on its surface. The plastic skin is dragged into wrinkles as the still molten lava continues to advance beneath it.
(iii): Pillow Lava: When lava of basaltic composition flows over the seafloor, it solidifies with a surface, which exhibits pillow-like ellipsoids. The diameter of such ellipsoids vary from a few centimeters to a few meters. The lava having such a surface is called ”pillow lava” these pillows are commonly glassy outside and finely crystalline inside.
(iv): Domes and coulées: Lava domes and coulées are associated with felsic lava flows ranging from dacite to rhyolite. The very viscous nature of these lava cause them to not flow far from the vent, causing the lava to form a lava dome at the vent. When a dome forms on an inclined surface it can flow in short thick flows called coulées (dome flows). These flows often travel only a few kilometers from the vent.