A conglomerate (pronounced /kɒnˈglɒmərət/) is a rock consisting of individual stones that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts.[1] Both conglomerates and breccias are characterized by clasts larger than sand (>2 mm).
Paraconglomerates consist of a matrix-supported rock that contains at least 15% sand-sized or smaller grains (<2 mm), the rest being larger grains of varying sizes.[2]
Orthoconglomerates are defined by texture. They are a grain-supported rock that consists primarily of gravel-sized grains (~256 mm), with less than 15% matrix of sand and finer particles.[3]
In rock types such as paraconglomerates and orthoconglomerates, were the matrix to be removed, the rock would collapse. This is because the larger grains are supported by the matrix and, without it, there is nothing to hold the grains together. Therefore, the higher the percentage of matrix, the more unstable the boulder. Breccias are a particular class of conglomerate that differ only in that the clasts are angular.
A spectacular example of conglomerate can be seen at Montserrat, near Barcelona. Here erosion has created vertical channels giving the characteristic jagged shapes for which the mountain is named. (Montserrat literally means "jagged mountain.") The rock is strong enough to be used as a building material - see Montserrat abbey front at full resolution for detail of the rock structure.
Another spectacular example of conglomerate, the Crestone Conglomerate may be viewed in and near the town of Crestone, at the foot of the Sangre de Cristo Range in Colorado's San Luis Valley. The Crestone Conglomerate is a metamorphic rock stratum and consists of tiny to quite large rocks that appear to have been tumbled in an ancient river. Some of the rocks have hues of red and green.
Conglomerate may also be seen in the domed hills of Kata Tjuta, in Australia's Northern Territory.
In the nineteenth century a thick layer of Pottsville conglomerate was recognized to underlie anthracite coal measures in Pennsylvania.[4]
When a series of conglomerates accumulates into an alluvial fan, in rapidly eroding (e.g. desert) environments, the resulting rock unit is often called a fanglomerate. These form the basis of a number of large oil fields, e.g. the Tiffany and Brae fields in the North Sea. These fanglomerates were actually deposited into a deep marine environment but against a rapidly moving fault line, which supplied an intermittent stream of debris into the conglomerate pile. The sediment fans are several kilometers deep at the fault line and the sedimentation moved focus repeatedly, as different sectors of the fault moved.
Metamorphic alteration transforms conglomerate into metaconglomerate.
