How do volcanoes form at divergent boundaries? Volcanoes help you have a glimpse of how active our planet’s tectonic processes are.
Also, volcanoes help build new crust, but these formations do not happen at random. The movement of our planet’s outer shell has a role to play.
And one important connection between volcanism and tectonics is at divergent boundaries.
But, how do volcanoes form at divergent plate boundaries?
Volcanoes form with magma rising through plate boundaries that develop as a result of tectonic plates moving apart.
Divergent Plate Boundaries and Their Connection with Volcanic Activities
Divergent plate boundaries form as the tectonic plates that make up the Earth’s crust move away from each other.
The boundary is effectively a crack on the surface of the Earth. It is where earthquakes happen but the boundary also offers a route for magma to reach the surface.
An Example to Understand
The Mid-Atlantic ridge is a divergent boundary. It is where the North American Plate and the Eurasian Plate are actively moving away from one another.
The space between is filled by the mantle, which lies between the planet’s superheated core and the crust of the plates we exist on.
It is mostly solid rock but it has hot convection currents, so the rocks are more plastic than solid or liquid. The currents drive the movement of the plates.
The rock melts to liquid magma when it meets the lower pressures closer to the surface.
Fact: The soils, weathered volcanic deposits, are rich in magnesium and potassium making them extremely fertile and high yielding.
How Do Volcanoes Form at Divergent Boundaries?
Many people have some understanding of volcanoes forming at divergent boundaries, but they have little information about how it actually forms.
Also, they wonder, “how do volcanoes form at transform boundaries?”
Here is a bit more information about the steps volcanoes form at divergent boundaries:
Step #1: The Process of Plate Separation at Divergent Boundaries
The mantle under the plates is hot, as high as 3700C near the core, where the rocks are molten in the high density and temperature.
However, the rock in the magma’s outer layers remains semi-solid because of the high pressure, about 136 GPa (1.4 million atm).
The mantle is around 1800 miles thick, divided into layers, including:
- The upper mantle
- The transition zones
- Lower mantle
- The D double prime where it meets the core
Its viscosity varies but because of the weight of everything in the world pushing on it, the pressure is highest closest to the core.
How It All Happens?
Because of the pressure gradient between the core and the surface, the molten rock of the mantle is pushed up through fissures.
It happens through the semi-molten layers into the lithosphere (crust).
It breaks the surface at the tectonic boundaries forming ridges such as the one in the mid-Atlantic, mantle plumes and geological hotspots.
Step #2: Magma Generation Due to Decompression Melting
The volcanic processes form the landscape of the earth as heat from the core is transferred through the mantle layers to the lithosphere, the top layer of the crust.
At the tectonic boundaries, the pressure gradient forces the mantle up, around and through the edges of plates. It forces the fissures and cracks further apart.
On the surface, this movement manifests as earthquakes but down below, with the falling pressure, the magma decompresses and expands.
At divergent boundaries, land masses are separated as the lithosphere stretches to accommodate the increasing volumes of the now liquid magma and pull apart.
An Important Consideration
It is this process that broke up the ancient supercontinents and left scientists puzzled as to why South America and Africa seemed to fit together.
This also prompted the first theories of continental drift although lacking knowledge of the parallel ridges out of sight in the deep ocean, they couldn’t work out how it happened.
Step #3: The Upward Movement of Magma and Solidification on the Crust
How the tectonic plates move apart was discovered accidentally during the first deep water dive down to the Mid-Atlantic ridge in a submersible.
What the parallel ridges and troughs found there were was a mystery then. But now we know they are the result of the sea floor spreading and mantle convection.
In short, it is the churning of the mantle that recycles the lithosphere. This crust becomes more plastic and less dense as the temperature beneath it increases.
The magma uses its heat to eat its way through the thinner edges of the crusts at the plate boundaries.
And it bubbles up to fill the gaps forming an elevation on the sea floor as it is rapidly cooled and solidifies as igneous rock.
Fact: The rock, basalt, is the most common rock on the planet and is a brand new area of crust.
Examples of Volcanoes at Divergent Plate Boundaries
Once you have an understanding of the process through which volcanoes form at divergent boundaries, it is important to get a better clarification through real-life examples.
Here are a few to help you out:
The East African Rift Valley
Seafloor spread happens on land as well as in the ocean and the process creates wide flat rift valleys.
There are several around the planet, many at triple junctions, where three tectonic plates meet. It has taken millions of years for the East African Rift Valley System to develop.
And it has several volcanoes, including:
- Mount Kenya
- Mount Kilimanjaro
Kilimanjaro is Africa’s tallest and largest free standing mountain at 19,340 feet above sea level.
It is located in Tanzania and made up of three cones, Kibo, Mawenzi and Shira. Kibo is dormant, the other two are extinct.
The Mid-Atlantic Ridge
The volcanic mountain range of the Mid-Atlantic Ridge rises from the Atlantic abyssal plain created by seafloor spread.
It crosses the hemisphere running from the Arctic to the Antarctic as a complex and dynamic system.
The Tristan da Cunha volcano sits on an active hotspot and first erupted some 3 million years ago.
Its main cone is now almost 30 miles across but there are other active cones on its flanks. It erupts violently, vesicular rock similar to pumice.
| Aspect | Description |
| Example | Mid-Atlantic Ridge |
| Plate interaction | Divergent plate boundaries |
| Tectonic plates involved | Eurasian plate and North American plate |
| Mantle | Between the Earth’s superheated core and crust, largely solid rock with hot convection currents |
| Plate movement | Mantle convection currents shift tectonic plates. |
Fact: The 1961 eruption of the Tristan da Cunha triggered earthquakes and landslides and opened up new fissures forcing the island’s occupants to evacuate.
Impacts of Volcanoes Formed at Divergent Plate Boundaries
Volcanoes forming at divergent plate boundaries have their impact on geography as well as the climate of the region where they are.
For instance:
Impact on Geographical Features
The East African Rift is an excellent example of how divergent boundaries shape the geological landscape.
Here the boundary is still developing and sits below a thick crust. The upwelling magma does not have the heat or the force to pull it apart in one single action.
Instead, the crust is forced upwards by the convection currents like a bubble, then pulled thin.
Since the development of the rift is incomplete, not all of it sits below sea level. Where it does, water has collected in a series of long linear lakes.
An Important Consideration
In the Red Sea, another divergent zone, the rift is complete.
The landmasses have fully separated dropping to sea level where the rift’s surface was flooded by an arm of the sea bringing early man’s migration out of Africa to an end.
Impact on Climate and Environment
The convection currents of the mantle form new land at divergent boundaries, sometimes suddenly and violently as a volcanic eruption.
Although a natural process, eruptions interfere with life cycles and earth processes destabilizing the atmosphere which impacts the climate globally.
More about the Ice Age
Between 1300 -1850 western North America and northern Europe experienced a ‘little ice age’.
A perfect storm of natural events with no singular cause but it included explosive volcanism.
The result was an atmosphere burdened with ash particles, so very little of already reduced sunlight could get through. The prolonged cold led to widespread famine and starvation.
An Important Consideration
Although divergent zones are dangerous places to live, people do live and farm on the slopes of volcanoes.
Fact: In Iceland in 2020, Eyjafjallajökull erupted and its ultra fine ash grounded the aviation industry throughout Europe creating logistical chaos worldwide.
Takeaway
How do volcanoes form at divergent boundaries? It is actually a dynamic and highly fascinating process.
It all begins with tectonic plates moving apart and giving the underlying mantle a chance to put magma to rise and turn into a volcano.
Interestingly, people live on these divergent boundaries and they farm on these lands as well.
Learning more about these landscapes is important because it helps to comprehend the complex makeup of the earth’s geology.