The surface of our planet is always changing. The continents we know today are temporary. They are comprised of many smaller fragments that are currently glued together in the configuration we are so familiar with, and have carved up with imaginary political boundaries. But in a short stretch of geologic time the outlines of the continental shapes that have such emotive power for us will change, breaking apart only to reform in new patterns that would make earth wholly unrecognisable. This phenomenon called plate tectonics is a dynamic process that profoundly affects the climate and can even change the course of life itself. Plate tectonics is also responsible for the formation of mountains. But as soon as mountains are formed they begin to be washed away, forming a continuous cycle. This rock cycle has very important implications for those interested in mineral deposits and their mining. Let's begin with the two main ways that mountains are created:
- Mechanical buckling, crumpling and shearing as two plates impact each other upon collision. This process is responsible for the North American Rockies, The European Alps and the Himalayas.
- Chains of mountain forming volcanoes. In this situation, instead of the two colliding plates crashing together and buckling, in the collision one plate "loses" and is forced underneath the "winner". The denser plate is the one forced back into the earth. This process causes melting of the overlying plate resulting in volcanic eruptions that parallel the line of collision. The whole process is called subduction and is the cause of mountain chains like the Andes and the Cascades in the Pacific Northwest of the United States.
After the building of mountains by tectonic plate interactions and volcanic eruptions, the natural processes of chemical weathering and physical erosion immediately begin to rapidly level the new peaks to plains. Erosion is the loosening, removal and downhill transport of rock and soil from the landscape by wind, water or ice. Weathering is a rock destructive process in which physical disintegration and chemical decomposition break down rock exposed at the earth’s surface. This happens because rocks are made up of minerals that chemically break down when they exposed to air and water.
Geologists have long shown that over the past 500 Million years the atmosphere had up to 25 times the CO2 in it that we have in ours today. This gradual chemical weathering of rocks has helped create an atmosphere that we can live in. Weathering begins with water. Rain picks up CO2 from the atmosphere to form a weak acid. Through this process, independent of any human input, all rainwater is naturally slightly acidic. This weak acid then attacks exposed basic rock-forming minerals like feldspar to form softer clay minerals, consuming CO2 in the process.
Feldspar + Water + CO2 = Clay + HCO3 + SiO2
Additionally, the magnesium and calcium silicate minerals that basaltic volcanic rocks are made up of, weather to form calcite (CaCO3) a mineral that is a carbon sink.
The consumption of CO2 in the rock weathering process is sped up when more rock is exposed to the environment. Obviously this happens on a planetary scale when mountains are pushed up.
In a lesser way human activity like mining can expose rock to the atmosphere as well. Given the current view condemning CO2 as a harmful emission, rock piles resulting from mining can be considered societal assets that increase the natural removal of CO2 from the atmosphere.
Rocks weather at different rates but all rocks eventually break down to softer clay and carbonate minerals such as calcite that are easier to erode. In this way the processes of erosion and chemical weathering work together: as erosion removes weathered rock, it exposes fresh rock for weathering.
Eventually even Mountains are Washed Away!
Mountains are the cause of their own demise. Mountain building sets up new weather patterns and the forces of gravity immediately take over to rapidly level mountains by washing them away as sediment where they are deposited in low areas and the oceans.
For example the highest mountain chain in the world, the Himalayas formed as a result of the Indian plate colliding with the Eurasian continent.
New mountains create new climate patterns. The Himalayas created the monsoon rain season, which ravages the exposed weathering rock and washes the debris away in the resulting river systems the monsoon begets. The higher mountains are, the quicker they erode away. The Himalayas are crumbling at the high rate of up to 1 kilometer every million years, which is an extremely short period of time geologically. The Himalayas are still actively being pushed up as the plate collision hasn't yet ceased. But when it does, these majestic mountains will rapidly wash away into the mists of time just as all mountains in the past have been flattened by the same ravages of our atmosphere.
This is why the oldest parts of the planet like Australia and Northern Canada are generally flat. The landscapes we connect with, like us, are not permanent and, in geologic terms, are changing rapidly.
Weathering and Mineral Deposits
Simple weathering of rocks can actually make mineral deposits by concentrating metal-minerals in oxidized soil residue made from weathered rock. In tropical environments rocks that do not themselves have sufficient metal to be mined are weathered to a soil called a regolith or laterite.
Aluminum and nickel oxides can be concentrated because other elements have been washed away as part of the weathering process.
Tropical countries reap the rewards of their climate by providing the world with their weathered aluminum rich rock called bauxite, the world’s source of aluminum. Nickel Laterite Deposits are formed by weathering in the same way and provide much of the world’s nickel from basic tropical red dirt.
Weathering also has a significant effect on the economics of mineral deposits by removing sulphur from metal sulphide minerals. Metals rarely occur in their pure but are often formed in nature combined with sulphur as minerals called sulphides. Metals are not usable as sulphides and after mining the sulphide minerals, sulphur needs to separated and removed to produce a pure metal.
This process takes human energy but if we are lucky, nature has already done the work for us. Take for example the iron sulphide pyrite which is the most common sulphide mineral. When sulphide minerals are exposed to air and water they corrode. When exposed to air and rainwater, sulphur is washed away as acid leaving iron hydroxide, a rusty mineral, behind.
pyrite + oxygen + water = RUST (iron hydroxide) + sulphate + ACID
The resulting acidic waters are more capable of dissolving metals which are also washed away out of the rock. Where sulphides have been exposed to weathering for a long time, in places like Nevada or Chile for example, nature has removed all the sulphur for us.
Rusty red and orange stained rock exposures called gossans are left behind when sulphides are broken down by weathering. Most gossans are formed from weathered pyrite, a non-economic mineral and are therefore not of interest. But some gossans mark the former presence of valuable metal sulphides so geologists and prospectors look for these colourful rock exposures in their search for economic mineral deposits. Prospectors also look for naturally acidic streams with dissolved metals contents. Such conditions point to the presence metal-bearing sulphides being eroded upstream.
Removing sulphur through weathering is often helpful
for gold and copper, as these metals can be more cheaply removed from weathered
rock than from fresh sulphides. The reverse is often true for silver, lead and
zinc which are generally more inexpensive to refine into metal as sulphides
than from oxides. To learn more about the weathering of sulphides and its effects on the economics of minerals deposits as well as the environment please visit our discussion of ACID ROCK DRAINAGE.