7.5 PRECAMBRIAN CRYSTALLINE & TERTIARY IGNEOUS ROCK AQUIFERS
Colorado’s crystalline rocks represent a unique and expansive aquifer system. The crystalline rocks are Precambrian aged (950 to 1800 million years old) igneous and metamorphic rocks; composed mostly of granites, gneisses, and schists; and geologically recent (Tertiary age) volcanic and igneous intrusive rocks. These rock types occupy approximately 19 percent of the state’s total surface area, and represent the fractured, crystalline-rock aquifers that supply much of the domestic water supply needs in the mountainous portion of Colorado. Unlike sedimentary rock aquifers, igneous and metamorphic crystalline rocks have no primary porosity; water is stored in fractures within the rocks.
Crystalline rocks are exposed at the surface throughout the mountainous, central portion of the state. The general physiographic features of this area include high peaks, great relief, rugged terrain, steep slopes, shallow soils, and extensive areas of exposed bedrock.
The extent and distribution of Precambrian-aged crystalline rocks is shown
in Figure 7.5-1.
The younger Tertiary age intrusive and extrusive (volcanic) igneous rocks generally lie west of and between the outcrops of Precambrian rocks (Figure 7.5-2). Widespread volcanic activity broke out about 36 million years ago and continued for 10 million years, depositing volcanic ash and lava flows over large areas. The San Juan Mountains represent one of the larger volcanic areas that contained several calderas.
The Precambrian igneous and metamorphic rocks and the Tertiary igneous rocks are discussed together because of their proximity in the mountainous area of Colorado and their hydrologic similarities. These rocks are exposed at the surface to some extent in all seven Water Divisions.