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.
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.
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.
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.
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.
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.