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A cinder is a pyroclastic material. Cinders are extrusive igneous rocks. Cinders are similar to pumice, which has so many cavities and is such low-density that it can float on water. Cinder is typically brown, black, or red depending on its chemical content. A more modern name for cinder is scoria.
The following geologic characteristics define a cinder:
- Having bubble-like cavities, called vesicles
- Measuring not less than 2.0 millimeters in at least one dimension
- Apparent specific gravity between 1.0 and 2.0
- Typical cinders are red or black in color.
- Contain numerous gas bubbles "frozen" into place as magma exploded into the air and then cooled quickly.
Cinders have been used on track surfaces and roads to provide additional traction in winter conditions. Cinders are also employed as inorganic mulch in xeriscaping, because of excellent drainage properties and erosion resistance. In this context, they are referred to frequently with the name "lava rock". This is why cinder is used in roads.