N through R

N THROUGH R:

• Oolith – this is a small, round piece of calcium carbonate made through the rolling wave action of water in shallow seas.

• Oolitic limestone – this is limestone you see with small grains in it called ooliths.

It is made when calcium carbonate precipitates in warm seawater near the shallow coastline.

• Oxidation – a chemical process occurring between certain metal elements and oxygen. It is what happens to make rust-colored oxides and "rust" out of surface iron deposits.

• Pahoehoe – this is the Hawaiian name for the ropy surface lava formed in a skin that folds into twisty shapes while hotter lava flows beneath it.

• Pebbles – a rock piece that is usually round and less than 10 cm in total size (but usually larger than 1 cm).

• Pele's hair -this is a form of lava that forms a fine stringy pattern as it cools rapidly so it looks like hair.

• Physical weathering – this is the type of weathering that happens from mechanical breakdown of surface rocks.

• Pillow lava – this is lava that looks like pillows formed when it comes into contact with cold seawater. It comes up from the seafloor.

• Plate tectonics – this describes the very slow motion of the major ridge plates on the earth's lithosphere.

• Playa – this is a lake you might see in a desert that is only temporary. Usually these are seen dried out unless there has been recent rainfall.

• Plucking – this is a glacial term where rock at the base of a glacier gets eroded from a combination of icing and movement.

• Porous – this is a phenomenon where sedimentary rock (mostly) has spaces that have filled in with groundwater.

• Pumice – extremely lightweight igneous rock made from gas-rich magma that has erupted under explosive conditions.

• Pyroclastic and pyroclastic flow – this is when hot gases and ash with rock are erupted from a volcano, rushing down the hillside at more than 100 miles per hour. The term "pyroclastic" refers to this rock and ash.

• Quartz – this is white or gray glassy material sometimes seen within granite. It is made from silicon dioxide.

• Quartzite – this is extremely hard, nearly white sandstone that is actually quartz grains that have become completely cemented together.

• Re-crystallization – this is when a mineral changes form from something like clay to a form such as mica. This occurs because of a chemical change in the rock and not because of melting or dissolving.

• Regional metamorphism – large-scale metamorphic changes in rocks because of widespread heat and pressure changes (the kind that builds mountains are an example).

• Rhyolite – a type of igneous rock with fine grains made from extremely thick lava.

• Rock salt – this is sodium chloride that came from the sea at one time but evaporated and got incorporated into certain rocks.