As the name indicates, metamorphic (meta = change, morph = form)rocks are pre-existing igneous or sedimentary rocks that have been altered, or metamorphosed, deep within Earth’s crust. The rocks changed form in response to intense fluctuations in temperature, pressure, shearing, stress, or chemical environment. During Colorado’s mountain building events, the intrusion of igneous bodies increased the temperature to result in contact and regional metamorphism. The dominant metamorphic rock types in Colorado are gneiss, schist, amphibolite, and quartzite. Pictured right:High-grade metamorphic rocks near Georgetown.
Contact metamorphism occurs when hot magma intrudes into cooler rock. The intrusion heats the surrounding rock, making the low-temperature minerals unstable. These minerals change to minerals that are stable at the new, higher temperatures, such as in Yule Marble (left).
Regional metamorphism occurs because pressure and temperature change over distance. Different pressure andtemperature combinations createa variety of minerals. In the course of a drive through Big Thompson Canyon from Loveland to Estes Park, you traverse all of the mineral zones of regional metamorphism. The presence of biotite near the mouth of the canyon signals an area of low-grade metamorphic rocks. Farther up the canyon are the garnet and staurolite zones indicative of formations subjected to higher temperatures. Within five miles, you reach the community of Drake in the highest (sillimanite) temperature and pressure zone of regional metamorphism. Exposed in the hills above Drake are coarse-grained pegmatites that are vein-like offshoots of the batholith. The high-grade rocks and pegmatites are indicators that you are approaching the 600-square-mile granitic batholith that surrounds Estes Park.
Around Estes Park and Rocky Mountain National Park, the metamorphic rocks were raised to temperatures and pressures at or near their melting point. This gave rise to migmatites, an intimate mixture of igneous and metamorphic rocks. Migmatites are found throughout much of the Front Range, (pictured right).
The map below shows the distribution of metamorphic rocks through Colorado.