By Sabine Stanley, Ph.D., Johns Hopkins University
The outer shell of the Earth is a series of large blocks called the tectonic plates. Though the surface of the Earth appears to be still, it is actually moving all the time. And, it is this movement of these plates that is responsible for earthquakes, volcanic eruptions, and the formation of mountains on the Earth’s surface.
(Image: USGS/Public domain)
The Earth has seven major tectonic plates and some smaller plates. The plates float on a weaker upper mantle or asthenosphere. They are named after the major continents and ocean floors that they encompass. Some major plates include the Pacific Plate, the North American Plate, the South American Plate, the Eurasian Plate, and the African Plate. The largest amongst these plates, the Pacific Plate, lies beneath the Pacific Ocean. About 250 million years ago, these plates were all arranged in such a way that they formed one gigantic supercontinent called ‘Pangaea’. However, over a period of time, the movement of plates led to the disintegration of the Pangaea and the formation of the current continental shapes.
Learn more about the Continental Drift.
Tectonic Plates Move at Snail’s Pace
The rigid plates of the Earth’s surface are in constant motion relative to each other. These plates are moving at a very slow pace of about a few centimeters a year. This is because solids move, flow, and deform very, very slowly. Materials such as water that have shorter timescales of flow have lower viscosities, while solids such as rocks have higher viscosities. Hence, the rate at which solids will move will be extremely slow. Similarly, the viscosities of various layers in the Earth’s mantle differ. The viscosity of the lithospheric plates is a hundred times more than that of the upper layer of the mantle, the asthenosphere. So, in comparison, the asthenosphere will flow much faster and is more deformable compared to the lithosphere.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
Plate Tectonic Boundaries: Transform, Divergent, and Convergent
Despite the slow movement of the tectonic plates, the boundaries between these plates could be geologically active. This is because these tectonic plates move relative to each other. The movement of the plate tectonic boundaries can be classified as follows:
- Transform Boundary—This occurs when two plates slide past one another. One example is the Pacific plate sliding northwest relative to the North American plate; this is marked by the famous San Andreas Fault. Earthquakes are common along these faults and the San Andreas fault causes some of the strong earthquakes in California.
- Divergent Boundary—This occurs when two tectonic plates move away from each other. When the plates move apart, a fissure opens up and the molten rock rushes from the mantle to the surface. The opening or fissure helps lower the pressure of the mantle layer and allows the molten material to the surface. The molten rock then solidifies to create a new surface crust. Instances of divergent boundary motions in the middle of the Atlantic Ocean include the African and South American Plates, as well as the Eurasian and North American Plates. The divergent plate movement in the mid-Atlantic has led to the formation of the Mid-Atlantic Ridge, a giant mountain range in the middle of the Atlantic Ocean. Spanning a length of about ten thousand miles and height of over a mile, it is the longest mountain range on Earth.
- Convergent Boundary—This occurs when two plates are moving towards each other. The new crust formed at the ridges cools down and starts moving towards another plate. The denser of the two plates will buckle beneath the other plate into the mantle. The zones where the plates sink back into the mantle are known as subduction zones and are geologically active. Strong earthquakes around the Pacific Plate are consequences of the subductions in these regions.
In addition, the subducting zones can also cause volcanic eruptions as the subducting plates experience higher temperatures and pressures deep inside the Earth. In fact, there are volcanoes all along the rim of the Pacific Plate from the west coast of North and South America to the east coast of Asia. The series of volcanoes associated with the plate is known as the ‘The Ring of Fire’.
Learn more about the volcanic hot spot near a mid-ocean ridge, formed by moving tectonic plates.
Mantle Convection
The outer surface of the Earth is colder compared to its hot interiors. In effect, a colder and denser plate from the Earth’s surface sinks at the subduction zone and continues to descend until it reaches the core-mantle boundary. Further descent to the core is not possible as the core is composed of iron, which is much denser than the mantle rocks. Over a period of about 200 million years, the subducted slab eventually reaches the bottom of the mantle. The slab attains the same temperature as the rocks in its surroundings and becomes a part of the mantle. Simultaneously, new crusts are formed at the mid-ocean ridges, and these new surface plates are exposed to the colder temperatures on the surface of the Earth. This entire process is called Mantle Convection. Thus, the Earth’s surface is being constantly recycled as new crusts are created at the ridges and old surfaces are being destroyed at the subduction zones.
Common Questions About the Mechanism of Plate Tectonics
It is a misconception that the rocks in the upper mantle are molten. The Earth’s mantle is made of solid rock. In spite of higher temperatures in the range of 1000° to 3500°C, they remain solid because they experience pressures up to around a million bars.
When the rocks in the mantle melt, the plume rises to the surface as hot-spot volcanoes. There are around 40 hot spots across the globe, the well-known manifestations of these being the islands of Iceland and Hawaii.
Yes, underwater mountains rise above the ocean. In places such as Iceland, there is more volcanic activity than other parts of the Mid-Atlantic range. This is because of the presence of a number of hot-spot volcanoes in the region. Iceland also has the unique distinction of having both hot-spot volcanoes and plate boundaries of the North American and Eurasian tectonic plates.
The Earth constantly recycles its surface and even the best-preserved parts of the Earth are subjected to continuous weathering processes, precipitation, and winds. This means that the old surfaces of the Earth are fast disappearing and very little of it is available for examination.
