What is Scoria?
Scoria is a highly vesicular, dark coloured volcanic rock that may or may not contain crystals (phenocrysts). It is typically dark in colour (generally dark brown, black or purplish red), and basaltic or andesitic in composition. Scoria is relatively low in density as a result of its numerous macroscopic ellipsoidal vesicles, but in contrast to pumice, all scoria has a specific gravity greater than 1, and sinks in water. The holes or vesicles form when gases that were dissolved in the magma come out of solution as it erupts, creating bubbles in the molten rock, some of which are frozen in place as the rock cools and solidifies. Scoria may form as part of a lava flow, typically near its surface, or as fragmental ejecta (lapilli, blocks and bombs), for instance in Strombolian eruptions that form steep-sided scoria cones. Most scoria is composed of glassy fragments, and may contain phenocrysts. The word scoria comes from the Greek σκωρία, skōria, rust. An old name for scoria is cinder.
Formation of scoria
Scoria is formed of the explosive from the volcano when it explodes, excessive gases and ash goes out of the volcano. These gases are dissolved in the magma under pressure of the overburden. The magma from the explosion of the volcano goes into air where pressure is released and the magma solidifies as the temperature is dropped than that of the subsurface. As the magma solidifies, the gases in the melt are not released from the melt before it solidifies. These gases produces round or elongated pores. These pore are vesicles of the gases zones which are evident of the melt solidification rapidly else gases would not have been trapped.
The scoria as formed from the explosion will be found near the mouth of the volcano and heavier rock would fall down the hill of the volcano.
Cinder cones
Cinder cone are the steep hills formed by the brief eruption of a volcano where scoria is deposited at the mouth and stack at on another. These have vertical relief with less than few thousand feet. These hill are often formed at intervals. More than just one volcano erupts near to another to form from few cinder cones to hundreds of individual cones that forms a cluster.
Most of the scoria falls to the ground near the vent to build up a cone-shaped hill called a "cinder cone." Cinder cones are generally small volcanoes produced by brief eruptions with a total vertical relief of less than a few thousand feet. They are usually very steep because scoria has an angle of repose of 30 to 40 degrees. In some parts of the world, cinder cones occur in clusters of a few to hundreds of individual cones. These areas are called "volcano fields."
An example is Maungarei in New Zealand, which like Te Tatua-a-Riukiuta in the south of the same city has been extensively quarried. Quincan, a unique form of Scoria, is quarried at Mount Quincan in Far North Queensland, Australia.
Vesicular basalts
When volcano erupts the melt has dissolved gases in it which due to release of pressure moves upward in the flow for its low weight. These gases attempt to escape the melt that's why it moves upward. As temperature and pressure is reduced the melt solidifies which in turn traps some of the gases. These gases produces vesicles in the rock body and the rock is called scoria or vesicular basalt.
Not to be Confused with Pumice
A vesicular igneous rock that is very similar to scoria is pumice. There are a few differences that can be used to distinguish them. First is their colour. Scoria is almost always black or dark gray to reddish brown, while pumice is almost always white to light gray to light tan. This colour difference is a result of their composition. Scoria forms from basaltic magma, while pumice forms from rhyolitic magmas which usually contain more gas.
Pumice has a much higher concentration of trapped bubbles - so many that the walls between them are very thin. The vesicles in pumice contain enough air that the rock will float on water. The thick walls of scoria make it heavy enough to sink.
Finally, when observed closely with a hand lens, you can often see tiny mineral crystals in scoria. However, close observation of pumice reveals a "glassy" texture similar to obsidian. Pumice consists mainly of glass materials rather than mineral crystals. A "glass" is a noncrystalline substance. In the case of pumice, it cooled so quickly that the atoms were unable to arrange themselves into ordered crystal structures.
Uses of Scoria
One of the main uses of scoria is in the production of lightweight aggregate. The scoria is crushed to desired sizes and sold for a variety of uses.
Concrete made with scoria typically weighs about 100 pounds per cubic foot. This is a weight savings compared to concrete made with typical sand and gravel that weighs about 150 pounds per cubic foot. This savings in weight allows buildings to be constructed with less structural steel. The air trapped in the scoria makes the lightweight concrete a better insulator. Buildings constructed with this lightweight concrete can have lower heating and cooling costs.
Crushed scoria is used as roofing granules, ground cover in landscape projects, and as a substrate in hydroponic gardening. Many dealers offer customers the option of choosing between black, brown, or red material. Scoria is also used as rip-rap, drainage stone, and low-quality road metal. Small amounts of scoria are used as sauna rock and as a heat sink in barbecue grills.
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