In this lesson, we learn why a large portion of Earth’s valuable mineral deposits are located along current and ancient tectonic plate boundaries. We’ll also review the types of plate boundaries and how mineral resources are formed at each type.
Plate Tectonics and Mineral Resources
The boundaries between tectonic plates are extremely dynamic places. Here, you’ll find volcanoes, earthquakes, mountains, and even important mineral resources.
As we’ll find out in this lesson, the mineral deposits formed at tectonic plate boundaries are heavily dependent on the volcanic activity in these areas.Let’s first review plate tectonics. The theory of plate tectonics states that the Earth’s crust is broken into rigid plates that slowly move around the surface of the planet. Imagine trying to wrap a dinner plate around a basketball. The plate would have to be broken in several places to cover the surface of the ball.
Because the Earth’s crust is rigid, it is broken into several pieces to cover the Earth. The plates move around because they are riding on the slightly molten mantle.There are three types of plate boundaries: convergent, divergent, and transform. Convergent boundaries are where two plates are moving toward one another. Plate convergence normally results in subduction, which is where one plate dives beneath the other. Divergent boundaries are where two plates are moving away from each other and are mostly located under oceans.
Transform boundaries are where two plates are moving laterally past one another, like the famous San Andreas fault in California.Areas that are not on a plate boundary are called intraplate settings. Each type of plate boundary has distinctive mineral deposits associated with it. In this lesson, we’re going to cover the mineral resources that we find at each of these plate environments.
Let’s start with divergent boundaries. Remember that divergent boundaries are where two plates are moving away from one another. The two plates moving away from each other creates a low-pressure zone where magma can rise to the surface.
The hot magma already contains high concentrations of potentially valuable metals. Water percolating through the rocks leeches the metals and further concentrates them in valuable deposits. These magmas heat the water already circulating through the rock, creating hot hydrothermal fluids.Hydrothermal fluids circulating at divergent boundaries form dramatic black smokers on the seafloor that are abundant in sulfur-containing minerals, such as pyrite, which is an iron sulfide, and galena, which is a lead sulfide.
Both iron and lead are important metals in many industries. Within the past decade, mining companies have begun extracting these resources from under two miles deep of ocean water.While some operations may be trying to directly mine the seafloor, there is an easier way to get at some of these deposits. Ophiolites are chunks of oceanic crust that have been emplaced onto continents. This happens during subduction events when the seafloor gets scraped off the down-going plate. But most importantly, it allows geologists to easily access some of these mineral zones that would otherwise be only on the seafloor.
Although there are very few ophiolites on Earth, they have been extensively sought after for their rich chromium resources.Divergent boundaries on continental crust can create mineral deposits in a similar way to those under the sea. The separating crust allows magma, enriched in heavy metals, to travel toward the surface, where it interacts with groundwater. This metal-bearing groundwater will later deposit adjacent mineral resources, such as native copper, which is the mineral that contains only copper ions.
Copper mines throughout northern Minnesota and Wisconsin were formed from a one-billion-year-old rifting event. These mines were some of the most productive mines in the country in the mid-19th century. Because geologists recognize the economic potential of ancient rift zones, they are often targeted when looking for a new mining opportunity.Another deposit formed at divergent boundaries on continental crusts are evaporites.
Evaporites are minerals that are left behind when briny water evaporates. The Basin and Range province of the United States, which includes the Great Salt Lake in Utah and Death Valley in California, contains large evaporite deposits left behind from evaporated lakes. These deposits include salt, potash, and gypsum, all of which are valuable commodities.
Subduction zones are notable for the dramatic stratovolcanoes along their borders, such as Mt. Rainier outside Seattle or Mt. Fuji in Japan. These volcanic zones are also a hotbed of valuable mineralization. Similar to divergent boundaries, water percolates through the hot volcanic rock and magma beneath volcanoes at subduction zones.
This groundwater picks up metals as it travels within the crust, eventually forming enriched deposits of mineral resources. This process is responsible for forming over half the world’s copper deposits. These magmatic fluids can also concentrate gold deposits, such as the Mother Lode of the Sierra Nevada, which was the source of the famous California Gold Rush.
Unlike subduction zones and divergent plate boundaries, transform boundaries do not produce similar mineral resources to subduction and divergent boundaries. Why do you think this is? As we have learned, valuable mineral resources at divergent and subduction zones are mostly volcanic in origin. So, why isn’t this happening at transform boundaries? Because transform boundaries rarely have associated volcanic activity.
Here, plates move past one another but do not see the dramatic temperature or pressure changes that can cause volcanic activity.However, at transform boundaries small pull-apart basins often form in the offsets of strike-slip faults. These low-lying basins accumulate sediment and clastic rocks, which are a great medium to store oil and gas deposits.
Oil does not exist as great underground lakes within the Earth, but rather is stored within the pore spaces of underground rocks. The oil and gas are mobile until they meet some structural barrier that keeps them from flowing further. The complex faulting and structures at transform boundaries provide ample opportunity to trap oil and gas deposits within the heavily faulted zones.
Let’s review the main points of this lesson. We learned that the majority of mineral resources tied to plate tectonics are either directly or indirectly related to volcanic activity. At both divergent boundaries and subduction zones, magma concentrates heavy metals, which are later further concentrated by hydrothermal fluids within the Earth. These processes are efficient at forming copper, chromium, and gold deposits.At divergent boundaries on the surface of the Earth, evaporating seas will form extensive evaporite deposits of salt, potash, and gypsum. At transform boundaries, complex structures and faulting can allow oil and gas to be trapped in some situations; otherwise, you will not find the magmatic mineral resources that are found at subduction and divergent boundaries.
When this lesson is completed you should be able to identify the three types of plate boundaries and describe how mineral deposits are formed at each.