Typical Structure of Lava Flows Baked Margin The bases and tops of lava flows are often rubbly (sometimes showing pahoehoe or aa texture). The interior is frequently characterized by columnar jointing, and vesicles are frequently caught in the top section of the flow. Volcanoes generally produce two kinds of lava flows: sheet flows and spatter flows.
Sheet flows are dense, homogenous masses of rock that result from the cooling and solidification of lava as it spills out of the vent. The term "lava sheet" is also used to describe the flat surface of an active volcano. Sheets can be many miles long and vary in thickness from a few hundred yards to more than a mile. They usually contain a small amount of material eroded from the parent rock during their formation. The majority of volcanic rocks are formed as sheets of lava cool below 600 degrees Celsius (1100 degrees Fahrenheit), at which point they harden into stone.
Spatter cones are tall structures with a conical shape caused by the rapid expansion of gas bubbles within the lava as it cools. The spatter may be very fine, granular, or coarse with sizes up to 2 meters (6 feet) across. It forms a thin layer on the surface of the sheet or near the side of the cone.
This type of lava flow has a smooth ropy surface. The texture of such a lava flow is similar to that of a brownie pan. The smooth surface of the lava flow indicates that the gases contained inside the lava have not been able to escape. Thus, the lava is very fluid and easy to move.
The reason why the surfaces of lava flows are often smooth instead of rough like rock surfaces is because the bubbles inside the lava prevent it from forming large crystals that would eventually fracture into small pieces.
Lava can also be smooth due to erosion caused by wind or water. Wind can wear away parts of the lava field while water can break up solidified lava flows under its weight.
Finally, lava can appear smooth if it has not yet fully solidified but is still liquid enough to flow. Lava that has not had time to harden yet is called pāhoehoe or "smooth" lava. Pāhoehoe is the most common form of lava that crosses U.S. roads today. It tends to flow in long, narrow strips rather than in a full circle like hot rock oil does when it spills out of a bottle.
The word "lava" comes from the Latin word for "bubbling," due to the appearance of the fluid metal when molten. Liquid iron turns red-hot metal bubbles trapped within it.
The lava flow is named Pahoehoe if the lava is highly hot and has a low viscosity (runny with a low gas and silica content). If the lava, on the other hand, has a high viscosity (thick and pasty with a high gas and silica content), it is referred to as Aa. The term'spumation lava' is also used for this type of flow.
Lava with a high silica content means that there are more silicon dioxide molecules in each gas molecule. Thus, it forms a thicker layer when it cools down and takes on its final shape. Lava with a high silica content can be found in areas where there is a lot of volcanic activity such as islands or parts of continents.
Highly viscous lava can only be seen at the surface after an eruption because it flows out so quickly that it creates a new surface before any solid rock can form around it. This type of lava can be found near active volcanoes. In fact, it is used by scientists to study how fast-moving gases like sulfur dioxide react with water to create steam which blows away from the volcano in large clouds called pyroclastic flows.
Lava with a low silica content means that there are more oxygen molecules in each gas molecule. Thus, it forms a thinner layer when it cools down and takes on its final shape.
Smooth, softly undulating, or extensively hummocky surfaces define Pahoehoe lava flows. The flowing molten lava beneath a thin, still-plastic crust tugs and twists it into tapestry-like folds and rolls like twisted rope. Pahoehoe lava flows are supplied nearly entirely by spattering vents. They are found almost everywhere that volcanic activity has occurred on Earth, except on Antarctica.
When lava is forced up through fissures in the ground rather than coming from an active vent, it is called "spontaneous." Spontaneous lavas often contain more silica (glass) than other elements because they do not have time to react with other materials in the earth's crust. For example, silica tends to accumulate in the leaves of trees which grow in forest areas where lava has flowed. When these trees are burned for timber or soil, the fire can burn out before all the wood becomes fuel, leaving only the silica residue behind. Or, if there is much iron in the rock, then the lava may turn red or orange when it solidifies.
The word "lava" comes from the Latin word for sea water. Lava is melted rock that has been forced up from deep within the planet's interior and cooled slowly at the surface. As it cools, lava hardens into rocks that are rich in silicon and oxygen - the same ingredients as sand.