Quantitative Revolution -  A change in the focus of academic inquiry when measurement became the central focus of research. In Geography and Earth Science, this change took hold in the 1950s and 1960s. Quantitative measurements were used primarily for hypothesis testing. With measurement came mapping, models, statistics, and mathematics. Researchers began investigating the process rather than merely describing the environment. 

Quarrying - See plucking.

Quartz - A rock-forming mineral with the chemical formula SiO₂. Quartz is common in continental crust but rare in oceanic crust. 

Quartzite - A metamorphic rock rich in quartz created by the recrystallization of sandstone.

Quasi-Equilibrium - An equilibrium where the average condition of a system, as measured through one of its elements or attributes, does not fluctuate much over time because of the internal balance between cause and effect processes.

Quaternary - A geologic period that occurred roughly 2.588 million years ago to today. During much of this period, continental glaciers in the Northern Hemisphere covered large areas of the land surface in the high and middle latitudes. Homo sapiens appeared about 200,000 years BP (before present) and became the first species to alter the Earth's environment on a grand scale. This period contains two epochs: Pleistocene and the Holocene. See the International Commission on Stratigraphy (stratigraphy.org) for the most recent version of the geologic time scale.

Quick Clay - A type of clay that, when saturated, will allow heavy things to sink into it.

Quickflow - See direct runoff.

Quicksand - A deposit of water-saturated sand that appears solid but cannot support objects above a threshold level of weight. Objects with significant weight will sink into the quicksand.