Types of Lava

Pahoehoe

Pahoehoe lava is a common type of balastic lava that cools down forming typical smooth, billowy, or ropy surfaces. Often, it forms by many small breakouts of lava from an over-crusted inflating flat, irregular flow. It is typical for shield volcanoes such as on Hawaii, where the name (meaning something like "good to walk") comes from. Pahoehoe lava forms from hot basaltic lava when the viscosity of the lava (e.g. because of low gas bubbles content and high temperatures) and/or the strain rate of the flow (related mainly to eruption rate and steepness of the ground) are low. When these factors change, the same original lava can produce the other end-member known as a’a lava, which has a fragmented, rough, sometimes spiny, or blocky surface. In addition, transitional types of lava between both a'a and pahoehoe lava can be found.

Pahoehoe flows are smooth down to a scale of a few mm. Instead of consisting of only 1-2 large flow units, a pahoehoe flow consists of thousands on thousands of small flow units called toes. Each toe is usually <30 cm thick, 1-2 m long, and 30-50 cm wide

Close-up photo of an active pahoehoe toe. This toe is about 30 cm wide at its widest. Note how it has erupted out of a crack in a previous toe and is flowing over yet another previous toe (with the ropy texture). Note also that with the sun shining on it, one side of the active toe doesn't look all that different from the surfaces of the older inactive toes; late afternoon and early morning (and night) are the best times for observing lava flows.

Pahoehoe flows are associated with low-effusion rate eruptions and are emplaced at low volumetric flow rates (2-5 cubic meters per second) and slow flow front velocities (1-10 m/hour) [See the A'a page for a velocity comparison chart]. Pahoehoe flows can be just as long as 'a'a flows. The longest post-contact flow was also erupted from Mauna Loa in 1859 (forming the second half of the "paired flow"; Rowland & Walker 1990), and is 47 km long. This strongly contradicts the notion that flow length is directly determined by effusion rate.

http://volcano.oregonstate.edu/pahoehoe

The low velocity of pahoehoe flows means that the skin that forms by air-cooling is not disrupted during flow and can maintain its smooth, unbroken, well-insulating surface. Thus the temperature and viscosity of lava do not change very much even tens of kilometers from the vent The advancing front of a pahoehoe flow consists of hundreds or thousands of active toes. Each stops flowing after a few minutes and becomes inflated (with lava) as the eruption continues. Eventually the cooled skin fractures, often at the seam between two toes, and a new toe forms.

A’a Lava

A'a' lava is the most common appearance type of lava flows that cool down forming fragmented, rough, sometimes spiny, or blocky surfaces. A'a' lava forms when the viscosity of the lava (e.g. because of high gas bubbles content and relatively low temperatures) and/or the strain rate of the flow (related mainly to eruption rate and steepness of the ground) are high. When these factors change, the same original lava can sometimes produce the other end-member known as pahoehoe lava which has a smooth, often twisted surface. In addition, transitional types of lava between both a'a and pahoehoe lava can be found. A’a flows move slowly – you could easily outrun one – but they move fast enough to tear down buildings, cover roads, and destroy vegetation.

Pillow Lava

Pillow lava is typically found erupting from underwater volcano vents. As soon as the lava contacts the water, it’s cooled down and forms a hardened shell. As more lava issues from the vent, the shell of lava cracks and more “pillows” come out of these cracks.

Blocky lava

A surface flow of hot, molten lava covered in a carapace of crystalline, angular blocks which tend to be smoothly faceted and may have dimensions up to several metres. The blocks, which have the same composition as the flow interior, are formed by fragmentation of the chilled flow surface as lava continues to move within the flow interior. Blocky lava morphology is usually confined to lavas of high viscosity and intermediate to high silica contents.

https://www.universetoday.com/29655/aa-lava/

http://www.decadevolcano.net/photos/keywords/a_a_lava.htm