Tectonic Plate Movement: Volcanoes

From magma to a volcano

A volcano forms where molten rock reaches the Earth’s surface through an opening called a vent. Below the surface, molten rock is called magma; once it erupts, it is called lava. Magma may collect beneath a volcano in a magma chamber before rising through the crust. Lava cools and solidifies, and repeated eruptions can build up a volcanic mountain.

Most volcanoes occur at plate boundaries. Plate movement helps create the conditions for magma to form and provides pathways through which it can reach the surface. However, not every type of plate boundary produces volcanoes.

Constructive boundaries: plates move apart

At a constructive plate boundary, two plates move away from one another. As the crust pulls apart, magma rises through cracks and erupts as lava. The lava cools and solidifies, creating new crust. Much of this activity takes place along mid-ocean ridges.

The lava erupted here is commonly basaltic: it has low viscosity, meaning that it flows easily. It can spread a long distance before cooling, so repeated flows can build broad volcanoes with gentle slopes. These are shield volcanoes.

Iceland lies on the Mid-Atlantic Ridge, where the North American and Eurasian plates move apart. Its volcanic activity is also strengthened by a hotspot beneath it, so it is an example of two volcanic settings acting together.

Destructive boundaries: subduction produces magma

At a destructive plate boundary, plates move towards one another. Where an oceanic plate descends beneath another plate, the process is called subduction. This can happen beneath a continental plate or another oceanic plate.

As the descending oceanic plate heats up, water is released from minerals and sediments carried down with it. This water lowers the melting temperature of the mantle above the descending plate, allowing some mantle rock to melt and form magma. The magma rises through the overlying plate and may erupt at the surface.

Volcanoes at these boundaries commonly erupt more viscous, sticky lava. This does not travel as far from the vent, so it builds up steeper slopes. Together with ash deposited by explosive eruptions, it forms a composite volcano.

Why shield and composite volcanoes differ

The shape of a volcano reflects how its erupted material behaves. Imagine pouring a runny liquid onto a surface: it spreads widely. A much thicker material stays closer to where it is placed. Lava behaves in a similar way, although it also cools and solidifies.

Shield volcanoes are built mainly from successive flows of runny, basaltic lava. These spread out to form a wide base and gently sloping sides. Eruptions are usually less explosive, with lava flowing from vents or cracks. Mauna Loa in Hawai‘i is an example of a shield volcano.

Composite volcanoes, also called stratovolcanoes, have steeper, more cone-shaped profiles. Their sticky lava accumulates near the vent, while successive eruptions build layers of lava and ash. Gas can become trapped in viscous magma, allowing pressure to build and making explosive eruptions more likely.

These are typical relationships, not fixed rules for every eruption. Less explosive activity is also not the same as harmless activity: lava flows can still threaten land and buildings.

Hotspots: volcanoes away from plate boundaries

Volcanoes can also form above hotspots, where a rising mantle plume supplies heat and magma. Magma reaches the surface through the crust, often producing shield volcanoes with runny lava.

The Hawaiian Islands formed as the Pacific Plate moved over a hotspot. Over millions of years, movement carried earlier volcanoes away while new volcanoes formed above the hotspot, producing a chain of volcanic islands.

By contrast, conservative boundaries do not normally produce volcanoes because little or no magma is generated there. Collision between two continental plates mainly builds mountains rather than producing the subduction-related volcanism described above.

Where volcanoes form

• Constructive boundaries: plates separate → magma rises through cracks → lava cools to form new crust. Runny basaltic lava commonly builds shield volcanoes.
• Destructive boundaries: an oceanic plate subducts → releases water → mantle above it partly melts → magma rises. Composite volcanoes commonly form.
• Hotspots: magma rises above a mantle plume. Plate movement can produce a chain of volcanoes, as in Hawai‘i.
• Conservative boundaries and continental collision: volcanoes do not normally form.

Compare the two types

Runny lava spreads far; sticky lava builds up near the vent and can trap gas.