In most modern classifications, the deserts of the United States and northern Mexico are grouped into four distinct categories. These distinctions are made on the basis of floristic composition and distribution - the species of plants growing in a particular desert region. Plant communities, in turn, are determined by the geologic history of a region, the soil and mineral conditions, the elevation and the patterns of precipitation.
Chihuahuan Desert: A small area of southeastern New Mexico and extreme western Texas, extending south into a vast area of Mexico.
Great Basin Desert: The northern three-quarters of Nevada, western and southern Utah, to the southern third of Idaho and the southeastern corner of Oregon. According to some, it also includes small portions of western Colorado and southwestern Wyoming. Bordered on the south by the Mojave and Sonoran Deserts.
Mojave Desert: A portion of southern Nevada, extreme southwestern Utah and of eastern California, north of the Sonoran Desert.
Sonoran Desert: A relatively small region of extreme south-central California and most of the southern half of Arizona, east to almost the New Mexico line.
Three of these deserts - the Chihuahuan, the Sonoran and the Mojave - are called "hot deserts," because of their high temperatures during the long summer and because the evolutionary affinities of their plant life are largely with the subtropical plant communities to the south. The Great Basin Desert is called a "cold desert" because it is generally cooler and its dominant plant life is not subtropical in origin.