Earth, Our Environment - Class Notes
Chapter 16, Earth's Resources
Earth, Our Environment - Class Notes
Although we may not be aware of our dependence on natural materials, our society strongly depends on the availability of minerals. People have gone to war over mineral wealth and the economic well-being of many countries is a direct result of their mineral wealth.
An ore is a concentration of a mineral or mineral aggregate that has achieved a concentration of the valuable constituent that makes it profitable to mine and/or extract.
Early in human history, we exploited the natural materials to sustain and improve the quality of our lives. As a result, human population grew and we eventually became dependent on our ingenuity and our ability to find, mine, and use natural materials. The early history of humans is often divided by the types of natural materials that we exploited:
Mineral Deposits
The crust is primarily composed of oxygen, silicon, aluminum, iron, calcium, potassium, sodium, and magnesium. These elements make up 99% of crustal material. The remaining 1% includes many valuable materials. Unless minerals are somehow concentrated, it is usually too expensive to crush rocks and sift through the powder to find the materials we need.
Plate-tectonic processes often concentrate minerals in ore deposits or veins. This is an important reason for science of geology and an important reason for unraveling the details of how Earth works. If we know how a material is concentrated and we know the type of geologic environment conducive to that concentration, we can use that information to help find the mineral.
Recall that during the cooling of magma, minerals crystallize at different temperatures and the concentration of different minerals changes as the magma solidifies. This cooling "series" is one Earth concentrates materials. Chromite for example is a dense mineral that settles out of mafic and ultramafic magmas. We extract chromium from this material.
Diamond is a mineral that is "concentrated" by magmatic processes. Recall that diamond is a high-pressure polymorph of carbon that can only form at very high pressure. Thus diamonds are associated with features called kimberlite pipes, which are explosive volcanic vents that bring material up from 100-200 kilometers deep in Earth. Most dating investigations of diamonds suggest that they are nearly 3 billion years old and most kimberlite vents are found in cratonic (old) provinces of the continents.
Many ores are associated with fluid flow induced by heat from magmatic intrusions; veins of gold, silver, and copper are examples. Magma intrudes into the shallow crust and begins to cool. The heat of the magma drives a water convection system. Water in contact with the magma becomes heated and less dense and flows upward being and is replaced by cooler water. In this way, heat is carried away from the intrusion by water flowing through cracks. When the water is hot, it can dissolve minerals to form what is called a brine (a mixture of water and dissolved minerals). As the brine flows upward and cools, its ability to keep the minerals in solution decreases and minerals precipitate from the brine. The precipitated minerals fill in the cracks that the water is flowing through producing what we call a mineral vein.
Sedimentation also can naturally concentrate material. Two processes operate during sedimentation - precipitation and sorting. A good example of mineral concentration by precipitation is salt which we discussed in Chapter 4. A more complicated precipitation process is associated with Mississippi Valley-type ore deposits. Brines rising from deep and compacting sedimentary basins flows through porous carbonate rocks. As a result of chemical reactions with the carbon, zinc and lead sulfides are precipitated within the limestone.
One of the most important iron-producing deposits are the "Banded Iron Formations (BIFs)". The BIFs consist of inter-layered iron oxide and chert (silica). The exact process that created the BIFs is unknown. The BIFs are found on all the continents and formed between 2 and 3 billion years ago. Their formation may be related unusual weathering processes that were the result of a different atmospheric chemistry at the time of deposition.
Sedimentary sorting is another important process of mineral concentration. Placer deposits are the result of weathering and gravity-induced density sorting. Panning for gold is a process that relies on placers. The gold is eroded from a hydrothermal deposit and flows down stream.
Nonmetallic Resources
We have focused on metallic minerals, but we also make great use of nonmetallic minerals such as clays and stone. Limestone, for example, is very common in this area and is used as a building stone, to make concrete, and as aggregate for construction. Metamorphosed limestone and dolomite, marble is also used as building and sculpture stones. Granite has been used as building and a decorative stone.
We also rely on Earth to provide fuels. From Uranium to natural gas, the fuels we burn are natural products. Perhaps we make most use of fossil fuels such as coal, oil, and natural gas (wood is another organic fuel, but it's not fossil). Interesting enough, we owe a debt to previous life for much of our fuel resources. Peat and coal are resources formed from decayed plants. Oil is produced by the decay of marine micro-organisms.
Normally, when an organism dies, bacteria set in and decompose the resulting material. However, if the material gets buried, the bacteria cannot operate as efficiently and the decay process is slowed or halted. Peat is a valuable source of fertilizer and fuel. In areas where plant material from mosses to trees is produced faster than the material can decay, peat forms.
With deeper burial, the material that made peat can be transformed into lignite and eventually coal. Coal is a sedimentary/metamorphic rock with a range of compositions. The higher the carbon content, the higher the grade of coal. With increasing burial, volatiles are driven from the decaying material and the pressure transforms the material into coal. The highest grade of coal is anthracite, which has been subjected to deep burial and metamorphosed by structural deformation.
Naturally, since coal depends on plants for its existence, is primarily limited to the Phanerozoic sedimentary deposits. Much of Earth's discovered coal was produced during a peak period around 300 Ma. Coal is an important fuel still used extensively. This was the fuel that powered the Industrial Revolution. However, coal burning produces pollution and acid rain and has been more recently replaced by the "cleaner" burning petroleum products.
Petroleum refers to all hydrocarbons (liquid, solid, r gas) that are found in rocks. Petroleum thus includes natural gas and crude oil. Most oil forms from organic matter trapped in marine sediments. The main contributor are plankton, which thrive in coastal waters. Provided that the material is kept in a low-oxygen environment (which inhibits decay) and buried to a depth of several km (which drives off water), the organic material will form petroleum. Most of Earth's oil formed after 200 Ma, as plankton became more abundant. Another factor may be that older oil was metamorphosed during its long history. The search for petroleum has been a great impetus for understanding plate tectonics and sedimentary basin formation and evolution.
Oil-Reserve Formation:
We use indirect imaging methods such as seismic prospecting coupled with an understanding of the development and geologic structures of basins to find subsurface oil.
Earth is an ideal place for us to live:
Earth is rich in water - 71% of the surface is covered with oceans and much more water remains locked in the minerals deep in Earth. For humans, perhaps our most precious resource is fresh water. We cannot survive without it and it makes up about 0.65% of the available water on Earth.
Most of the potable, or drinking-quality water is stored in pores fractures cracks in rocks. Porous and permeable rocks are usually limited to shallow part of Earth because the pressure at depth generally causes the pores and fractures to close. We call a rock that acts as a water reservoir an aquifer. Such rocks are porous (they have space between their mineral grains that can store water) and permeable (the pores are connected and water can flow through the rocks). An impermeable rock impedes fluid flow. Clays and shales are examples of these aquicludes.
For the most part, the flow of water through rocks is governed by gravitational forces. The rate at which the water moves depends on the rock type and texture and can vary from meters per second to cm per century.
The water table is underground "boundary" below which all the pores spaces and fractures are saturated with water. Sometimes, in swamps and near rivers, etc. the water table reaches the surface and produces springs. Other times, we must drill down to the water table to access the water. In essence, this is mining for water. If the groundwater is capped by an aquiclude, it may be under pressure. Then simply tapping the water will produce an artesian well (water maintains a constant level equal to the height of the water table).
The depth to the water table can be seasonal - high during times of heavy precipitation and low during times of drought. We can also impact on the depth of the water table through modifications of rain runoff (such as those caused by urbanization). We can over-extract water and cause a drop in the water table or subsidence.
Sustaining our large world population depends on minerals and materials from Earth. Geoscience is the branch of study we use to understand and find mineral resources. Igneous, sedimentary, and metamorphic processes concentrate minerals into ore that we can exploit to satisfy our needs. These processes are a result of interactions among the tectonic plates and between the plates and the hydrosphere. To find our needed resources, we study these natural interactions and geologic processes to correlate the dynamic activity with the distribution of natural resources. With this information, we examine rocks to decipher the geologic history of a region. From the history we assess the potential of a region for mineral wealth. This is the economic basis for the geosciences and why we pay people to study Earth.
Many scientists, particularly those at universities, have a different reason for studying nature. Perhaps Cicero (106 BC-43 BC) said it best:
We just want to know how Earth works and how we fit into the picture.