Volcanic Activity

 Volcanic activity refers to the rising of molten material within the lithosphere or onto the Earth’s surface. Volcanic activity often occurs at convergent and divergent plate boundaries resulting in a wide array of features. Some features are formed on the surface and are known as extrusive volcanic features. Others are formed below the surface and are known as intrusive volcanic features.

Extrusive volcanic features

The CSEC / CXC syllabus requires you to know the following extrusive volcanic features:

Viscous lava domes

Viscous lava (also known as acidic lava) is very thick and does not flow very easily. It is very different from basic lava which is much more liquid and flows very easily. Basic lava is usually found near divergent plate boundaries while viscous lava is found near convergent boundaries. Viscous lava eruptions may result in the formation of very steep sided domes known as viscous lava domes. The Pitons in St. Lucia, seen in the photo below, are lava domes which experts believe were formed 200,000 to 300,000 years ago.


The Pitons

Ash cones

Eruptions of viscous lava may be explosive or violent. Violent eruptions may throw molten material miles into the air where it breaks up, cools and falls back to the earth as volcanic ash. After many eruptions over a long period of time, many layers of ash are built up forming a cone. The photo below shows an ash cone located in Arizona, USA.

Ash cone resulting from volcanic activity

Composite cones

Some volcanoes produce both ash and lava eruptions in succession. The result is a cone composed of successive layers of ash and lava. These are known as composite cones. Composite cones are found near convergent plate boundaries.

Click here to see a diagram of a composite cone!

Lava plateaux

Incredibly large amounts of basalt lava may erupt from fissures and cover many square miles of land. These flows may be several thousand meters thick and they usually “drown” the pre-existing landscape and form a wide flat plateau known as a basalt plateau. Basalt plateaux are very large features, covering hundreds of thousands of square kilometers.

Click here to see part of the Columbia River Plateau in North America!

Intrusive volcanic features

The CSEC / CXC syllabus requires you to know the following intrusive volcanic features:


A sill is formed when magma flows horizontally between rock layers, roughly parallel to the surface. This molten material cools forming a horizontal sheet of solid rock called a sill.


This is a vertical sheet of rock formed when magma moving  toward the surface (through layers of rock) cools and hardens.


This is a very large feature formed when an underground reservoir of molten rock cools and hardens.

Click here for a diagram showing intrusive volcanic features!

Learn about how volcanic features change over time!

Viscous Lava and Basic Lava

The islands of the Lesser Antilles exist near a convergent plate boundary. Many of these islands were formed by volcanic activity. Active or dormant volcanoes still exist on some of them and they have the ability to erupt viscous lava. Eruptions of viscous lava have a tendency to build steep sided cones. Viscous lava erupts very explosively and violently. Eruptions are often spectacular, throwing lava and ash many miles into the air. The video below shows an eruption of the Soufriere Hills Volcano in Montserrat in 2010.