Gold in Our Hills

San Diego Geology, part 3 of 3

Because human life is so very brief when compared with geologic processes, rocks seem to us stagnant, unchanging things. Like all else on earth, however, rocks are constantly undergoing cycles of formation, decomposition, and metamorphism. Part 1 of this series described how molten rock, or magma, rises into openings in the earth's crust where it cools and solidifies to form igneous rock such as granite. And Part 2 described how, as rocks on the earth's surface weather, the particles of erosion collect and bond together to form sedimentary rocks, such as sandstone.

There is yet a third type of rock which is formed when igneous or sedimentary rocks are exposed to conditions of high temperature and pressure. If the increased temperature and pressure are too extreme, the rock will melt and become magma. But if conditions are moderate enough for the rock to remain solid, yet severe enough that the rock is stressed, the mineral structure of the solid rock will actually change in response to this stress. These changes in sold rock occur very, very slowly, and are called metamorphism. The new rock formed in this way is called metamorphic rock.

Conditions which can bring about metamorphism are found in regions where movements of the earth's crustal plates are deforming the land on a large scale. The rocks adjacent to such major crustal upheavals are subjected to high pressures and temperatures, and crystals within these solid rocks tend to re-orient their direction of growth so as to minimize the stress.

As magma flows into fractures in the area of deformation, hot liquids and gasses come into contact with the older, solid rock. As these very hot fluids flow through the region of metamorphism, they may draw off some minerals from the surrounding rocks and deposit others, further changing the rocks' mineral structure.

The reddish-brown outcrops visible in the road cuts along Banner Grade are schist, a metamorphic rock. If you examine these outcrops, you'll be able to see the layered, platy structure typical of schist. This layering of the rock reflects a parallel orientation of crystal growth within the rock. During metamorphism, crystals in these rocks tended to form at right angles to the direction of pressure, essentially lying down and stretching themselves out, to minimize the total strain on the rock.

Julian's gold deposits are also due to processes of metamorphism. Hot fluids from adjacent, molten rock flowing through the regions of metamorphism carried off various minerals, including gold, from the surrounding rocks. These minerals were then concentrated together as they dropped out of solution according to their weight.

Robin Hewitt, 1990