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Pyroclastic material
Any solid material ejected from a volcano.
Q
Quartz
The mineral silicon dioxide. Quartz is easy to recognize because it has a glassy appearance. It is colorless when pure, but impurities can give it a variety
of colors, especially brown. Quartz almost always forms irregular grains, rather than crystals, with well-defined faces. That is because quartz has a relatively low melting point and is the last mineral to crystallize in an igneous rock. It fills all
the remaining gaps between the crystals that have already formed. Quartz is quite hard, and its compact structure makes it extremely resistant to erosion.
 Refraction—Seismic waves travel through the crust and are changed
in speed and direction
(refracted) by the materials
Rhyolite
of the Earth. While earthquake waves travel right through to the other side of the Earth, some regions fall into “shadow zones” and never record an earthquake at all.
Focus
R
Rayleigh wave
Refraction
Shadow zones
A type of surface wave produced by an earthquake. Named after Lord Rayleigh (1842-1919), the English physicist who predicted its existence.
Rayleigh waves move forward with an oval motion similar to
the waves caused when a stone
is dropped into a pond. They are the slowest, but often the largest and most destructive, of earthquake waves. Rayleigh waves are usually felt as a rolling or rocking motion. During major earthquakes they can even be seen as they approach by the way the ground humps up as each wave crest passes. (See also: Seismic waves.)
Reflection
The rebounding of a wave. In
the case of earthquakes seismic waves are reflected from different kinds of rock inside the Earth. By studying the pattern of reflected waves, scientists can better understand the composition of the inside of the Earth.
The bending of seismic waves as they move between materials with different properties. In particular, the pattern of refracted waves leaves a shadow zone on the
side of the Earth opposite the source of the earthquake. This has helped locate the boundaries between crust, mantle, and core.
Reversed fault
A fault in which one slab of the Earth’s crust rides up over another. Reversed faults are only common during plate collision. (See also: Normal fault.)
Rhyolite
An acid lava whose mineral content is similar to granite.
It is a very sticky lava (see: Viscous, viscosity) associated with explosive eruptions. It does not form extensive flows and quickly solidifies, often before extensive crystals can form. It has a number of glassy variants that form on the surface, including obsidian and pumice.
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