Lava that has cooled and hardened on the earth’s surface is called dry lava. When lava emerges from a volcano or fissure, it is a molten rock known as magma. This magma can be over 1000°C (1830°F). As the magma loses heat and volatile compounds like water and carbon dioxide, it begins to solidify into lava. The solidified lava is called dry lava. There are various names used to refer to dry lava based on its physical characteristics.
Pāhoehoe
One of the most common types of dry lava is called pāhoehoe. Pāhoehoe lava has a smooth, unbroken crusty surface. As the lava flows, the surface cools into a wrinkly, ropy, or ribbon-like texture. The name comes from the Hawaiian language and means “ropy lava.” Pāhoehoe lava typically contains low levels of silica and high levels of magnesium and iron. This chemical composition allows the lava to flow smoothly and create the characteristic ropy texture.
‘A‘ā
Another common type of dry lava is called ‘a‘ā (pronounced ah-ah). In the Hawaiian language, ‘a‘ā means “stony rough lava.” ‘A‘ā lava has a fragmented, rough surface made up of broken blocks of volcanic rock. As ‘a‘ā lava moves downslope, the outer margins cool into rubble. So the interior of the lava flow continues to move, breaking apart the cooled exterior blocks. This gives ‘a‘ā flows their clinkery, jagged appearance. ‘A‘ā lava is higher in silica than pāhoehoe lava, which causes it to have a thicker, pasty consistency that resists flow.
Block lava
Block lava, also known as ‘a‘ā, is characterized by its fragmental and short-transported appearance. It consists of partially melted rubble that breaks off the front and sides of ‘a‘ā lava flows. As the ‘a‘ā lava advances, cooled lava crusts called clinkers break off into blocks from the flow front and tumble down the sides. So block lava builds up along the edges of ‘a‘ā flows. The angular, fragmented debris has a jumbled, chaotic texture. Block lavas are made of various lava lithologies, reflecting their mixed origins from the lava flow interior and margins.
Pillow lava
Pillow lava forms when lava emerges underwater. As the molten rock comes into contact with water, it cools rapidly into bulbous or tubular pillow shapes. Pillow lavas are characterized by their rounded, pillow-like structures that form from the fracturing of continuous lava lobes. The glassy outer skin of pillow lava cools quickly in water, while the interior remains molten and breaks through to form another pillow lobe. This creates stacked sequences of pillow lavas that resemble piles of pillows. Pillow lavas are found on the seafloor at mid-ocean ridges and underwater volcanoes.
Ropy lava
Ropy lava is a type of pāhoehoe lava that has a distinct ropey texture. As ropy pāhoehoe lava moves slowly downstream, the surface begins to fold over itself. This creates lava ropes that look twisted or braided. The ropey texture reflects the high viscosity of the lava that allows the surface to wrinkle and drag. Ropy pāhoehoe is a signature feature of basaltic lava flows that are rich in magnesium and iron. Famous examples of ropy lava can be seen at volcanoes like Kilauea in Hawaii.
Shelly pāhoehoe
Shelly pāhoehoe is a transitional type of lava between the smooth pāhoehoe and rough ‘a‘ā varieties. Shelly pāhoehoe starts out with a typical pāhoehoe surface. But as it moves, the lava tears apart into angular, shell-like fragments. These broken lava plates give shelly pāhoehoe its characteristic shelly appearance. It occurs when partially-cooled crusts on the lava surface shatter as the lava’s interior continues flowing. Shelly pāhoehoe represents a transition as pāhoehoe lava begins transforming into ‘a‘ā lava.
Entablature lava
Entablature lava refers to extensive sheet-like lava flows that emerge from fissure vents. The lava spreads out laterally into a broad uniform layer with a flat top and steep sides. Entablature lava sheets can extend for many kilometers from the fissure while maintaining a relatively constant thickness of 10-20 meters. Their flat tops and open channels make entablature lava flows resemble stone colonnades or entablatures. The uniform layers of entablature lava build up over time from successive horizontal lava flows issuing from fissures.
Toes
Toes are stubby protrusions of lava that form on the edges of lava flows. As lava moves downslope, the cooler outer margins begin to solidify. The interior of the flow continues to move, squeezing out blobs of lava at the flow front. This creates bulbous toes that extend out from the front and sides of the lava flow. Toes can range from just a few centimeters to over a meter long. They often have a rounded or cylindrical shape from the viscous lava being squeezed through a solidifying crust. Toes are most common along ‘a‘ā lava flows.
Slabby pāhoehoe
Slabby pāhoehoe is a variety of pāhoehoe lava with a jagged, slab-like surface. It forms when the crust of a pāhoehoe flow partially solidifies and breaks up into angular fragments. As the lava’s interior continues advancing, it pushes or drags the broken crusty slabs along the surface. This gives slabby pāhoehoe lava a rough, fragmented appearance while maintaining the general morphology of a pāhoehoe flow. Slabby pāhoehoe represents a transition from smooth pāhoehoe to more fragmented ‘a‘ā lava.
Hummocky lava
Hummocky lava has an irregular, bumpy surface created by draining lava. When lava solidifies, its interior can continue flowing and drain out from beneath a solidified crust. This causes the lava crust to collapse into small hills and depressions known as hummocks. The resulting terrain resembles a bumpy, hummocky landscape. Hummocky lava is thought to form when inflation of the lava crust ceases, allowing the still-molten interior to drain out. It is found on many pāhoehoe lava flows.
Lava rise pits
Lava rise pits are circular depressions that form in lava crust when dense magma rises up from below. As lava moves downhill, lighter bubbles and crust get pushed to the top. Denser magma underneath then pushes up in places, causing surface crust to collapse into pits. The resulting holes range from centimeters to meters across and can be quite deep. They have steep, circular walls formed by the upwelling magma. Lava rise pits are seen on pāhoehoe flows, where they give the surface a pockmarked appearance.
Lava tubes
Lava tubes form when the surface of a lava flow solidifies and insulates the still-molten interior. The lava inside continues flowing downstream, creating a tunnel called a lava tube. Lava tubes have arched roofs formed from the solidified surface crust. They drain lava from inside the flow efficiently. Tubes can be meters to tens of meters across and extend for kilometers. After eruption ends and a lava tube drains, a cave-like conduit remains as dry lava where the tube once existed.
Tumuli
Tumuli are mounds on lava flows formed by the upwelling of viscous lava below the surface. As lava moves, interior pressure can push up against the crust. This produces a rounded hump on the flow called a tumulus, ranging from 1-10 meters tall. Tumuli often form in lines or clusters reflecting the pathways of lava flowing under the crust. Their shape is influenced by the lava viscosity – highly viscous lava forms steep-sided tumuli. Finding tumuli helps identify features like lava tubes buried below the surface.
Pressure ridges
Pressure ridges are linear mounds formed by compressive forces within a lava flow. As lava moves downslope, sections of the flow can push and rub against slower-moving sections. This compression causes crust wrinkling and thrusting that piles up ridges of lava. Pressure ridges resemble wrinkles in the lava crust ranging from centimeters to meters tall. They run parallel to the direction of lava flow and reflect differential movement of lava underneath the crust.
Squeezes
Squeezes form when semi-molten lava beneath a solid crust gets forced out at fractures. As lava inside a flow continues flowing, liquid material can get squeezed out through cracks in the upper crust. This produces smooth mounds called squeezes emerging from a fractured surface. Squeezes often form aligned rows following fractures. They represent places where fluid pressure in the lava flow exceeded the strength of the already cooling crust.
Volcanic rock
Once lava has fully crystallized and solidified, it forms an igneous volcanic rock. Dry lava becomes volcanic rock after all its molten components have completely cooled and hardened. The physical characteristics of the lava shape the texture and mineralogy of the rock. Smooth pāhoehoe lava develops into a fine-grained basalt, while blocky ‘a‘ā lava becomes a coarse-grained basaltic rock. Over time, weathering and erosion modify the original dry lava into new rock forms and volcanic soils.
Conclusion
Dry lava encompasses the many varieties of lava flows that have cooled, hardened, and crystallized after a volcanic eruption. The surface textures and shapes of lava reflect its chemistry and the conditions under which it was emplaced. Names like pāhoehoe, ‘a‘ā, entablature lava, pillow lava, and slabby pāhoehoe capture the diversity of volcanic landscapes created by flowing magma. Studying dry lava gives insights into the fascinating behavior of lava during active eruptions. The next time you are near a volcano, look for evidence of the dry lava varieties that make each volcano unique.
| Lava Type | Characteristics |
|---|---|
| Pāhoehoe | Smooth, ropy surface |
| ‘A‘ā | Rough, fragmented blocks |
| Pillow lava | Rounded pillow shapes |
| Block lava | Angular, broken rubble |
| Shelly pāhoehoe | Plate-like fragments |
| Entablature lava | Broad flat sheets |
| Toes | Knobs protruding from flow edges |
| Slabby pāhoehoe | Fragmented, ragged crust |
| Hummocky lava | Irregular mounds and hills |
| Lava rise pits | Circular collapse depressions |
| Lava tubes | Drained conduits under surface |
| Tumuli | Mounds from upwelling lava |
| Pressure ridges | Linear mounds on crust |
| Squeezes | Extruded mounds at fractures |
