Scoria is extrusive igneous rock which is vesicular and is of dark colour. Colour ranges from black or grey to reddish brown. Composition of scoria is similar to basalt and can be that of andesite.
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 gaseszones 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 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.
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.
The uses of scoria is to make light weight blocks and for that is used as a crushed aggregate. Due to its vesicles it traps gases which makes the concrete block of light weight. It is also used in the plantation where the drainage rocks are scoria.