©Copyright 2018 GEOSCIENCE RESEARCH INSTITUTE
11060 Campus Street • Loma Linda, California 92350 • 909-558-4548
All Galleries
All Images
Hills
Rwanda is known as the country of a thousand hills. These hills are mostly made of Precambrian rocks. However, the valleys between these hills are filled with alluvial deposits that provide a fertile soil for cultivation. Photo taken in the Kigali district.
Nonconformity
A "nonconformity" is a type of discontinuity surface where sedimentary rocks lie above crystalline rocks. The dashed line in the picture marks a nonconformity between the Mesoproterozoic Pikes Peak Granite (bottom/right) and the Pennsylvanian conglomerates of the Fountain Formation (top/left), near Denver, CO. The contact in this outcrop (near Red Rocks Amphitheater) is marked by a plaque.
Precambrian granites
Mwanza, located along the southern shore of Lake Victoria, is Tanzania’s second largest city and one of the fastest growing cities in the world. It is nicknamed “the Rock City” due to the numerous hills covered with granite boulders that stretch out into the lake, creating a unique landscape. Many of the city’s houses are built around the boulders and some use granite from the rocks. The granites of Mwanza are Precambrian and belong to the stable continental crust of the Tanzania Craton. Photo by Lance Pompe.
Geita gold mine
Large gold deposits are found in Precambrian Archean rocks of the Tanzania Craton. The Geita gold mine south of Mwanza and Lake Victoria is one of the largest in East Africa, being a type of deposit known as an orogenic gold deposit. Gold formed by a combination of geological events, in a region known as the Geita Greenstone Belt. As is often the case with mineral formation in rocks, hydrothermal fluid containing gold interacted with rocks in a fault zone, depositing the minerals. In Geita, sulphur present in the hydrothermal fluid acted as a ligand, allowing gold to dissolve in the fluid and be carried through the Nyankanga Fault Zone. Precipitation of gold occurred when the fluid interacted with a chemical trap, in this case the iron in the locally abundant banded iron formation rocks. The sulphur reacted with the iron, forming iron pyrite or fool’s gold, causing the dissolved gold to precipitate out of the fluid and be relatively concentrated and available for mining. Photo by Lance Pompe