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Three Types of Lava Flow

Lava flow is first characterized by the mineral mixture, which is found within the hot magma that becomes lava when it emerges from a volcano or fissure in the ground. Basaltic, rhyolitic and andesitic lava flows are all determined by mineral content. Basaltic lava flows are further subdivided into three subgroups depending on whether the lava flow occurs underwater or on solid land. Underwater flows are called pillow lava while two Hawaiian terms, pahoehoe and a'a, are reserved for variations in terrestrial lava flows.

Magma
- Lava is somewhat of a dichotomous term. The word definitely describes molten magma once it has spilled out upon the earth's surface, but also lava or sometimes lava rock can be used to describe the solid strata, created as the molten liquid cools and solidifies. Lava rocks, created from lava flows, can be divided into three groups according to mineral content. These groups are classified as basaltic, andesitic and rhylolitic, with each one being determined by measuring the amounts of silicon, iron, potassium, calcium and sodium in each sample.
Underwater Lava Flows
- Lava that erupts from undersea volcanic activity sometimes flows downhill along the sea bottom. These are called pillow lava flows. In underwater locations, basaltic lava commonly flows from fissures and the tops of sea cones, or underwater volcanoes. Pillow lava flow gets its name from the large elliptical or circular lobes that are formed when the lava cools. Fresh pillow lava flows are easy to identify when located near an active underwater site. Old deposits now located above sea level can be correctly identified by the presence of streaks of sediments amidst the basaltic rock.
Pahoehoe
- Pahoehoe is one of the two Hawaiian terms used to describe terrestrial lava flows. The differences in appearance of this type of flow compared with the a'a variety are noticeable and are related to the cooling rate and speed of downhill descent. In general, pahoehoe flows produce smooth, tendon-like rock formations because the lava moves slowly and takes a long time to cool down.
A'a
- The other type of land-based lava flow is called a'a. These flows are faster moving, cool rapidly and leave behind rocks that are rough and jagged and sometimes contain many knife-like edges that protrude upward. It is possible for a'a and pahoehoe lava flows to exist side by side.
Block Lava Flow
- Block lava flows occur in the more viscous types of lava such as andesitic and rhyolitic. Because of their mineral content, this type of lava flows more slowly than the basaltic type. Though the surface may be rough, the base of the flow often forms large polygonal masses within the rock.

Rocks & Minerals