Basalt - Giant's Causeway
Click the microscope button to view a thin section for this sample.
Click the microscope button to view a thin section for this sample.

Fact sheet

Basalt - Giant's Causeway

This basalt sample was part of a flow from the Antrim plateau basalt in Northern Ireland. The basalt flows emerged from fissure eruptions during the eruption of the North Atlantic volcanic province around 55 million years ago, during the Paleogene period. Our sample is the same basalt that forms the Giant's Causeway, a famous columnar basalt exposure on the Antrim coast. The site is a UNESCO world heritage site so this sample comes from basalt outcrops a few hundred metres to the east. The Giant's Causeway is just one of many columnar basalt exposures around the UK, such as Fingal's Cave on the Isle of Staffa. Columns of basalt form when a thick lava flow cools and contracts, allowing vertical cracks to open up and migrate down through the rock. These 'cooling' cracks generally create hexagonal shapes, but other polygonal shapes also occur.

The sample is a medium-grained basalt in thin section, consisting of a framework of plagioclase with pyroxene forming a poikilitic texture. There are patches of alteration that may once have been olivine or glass but are now entirely transformed to clay minerals and chlorite.

Additional images
  • basalt - width 3.7 cm
55.233313, -6.5162
Giant's Causeway, Co. Antrim, N. Ireland
About this collection

The United Kingdom Virtual Microscope (UKVM) collection consists of igneous, sedimentary and metamorphic rocks from around the UK.

It is intended as a teaching resource, helping to tell the story of the common rock types and how they form, and reflecting the history of the UK at the margins of the continent of Europe. The collection is a series of teaching sets, for example igneous rocks from the North Atlantic Igneous Province and SW England; high-temperature metamorphic rocks from Scotland and low-temperature metamorphic rocks from Wales; and sedimentary rocks, including English limestones and sandstones.

Sample details

Rock-forming mineral
Accessory minerals
iron oxide
Category guide  
Category Guide
Refers to any word or phrase that appears in the individual rock names. Names are generally descriptive; they allow users to search for broad terms like ‘granite’ as well as more specific names such as ‘breccia’. However, the adjacent descriptions of the specimens captures a wider range of general words and phrases and is a more powerful search tool.
Refers to any word or phrase that appears anywhere in the descriptions of the specimens
Accessory minerals
Minerals that occur in very low abundance in a rock. They are usually not visible with the naked eye and contribute perhapssver, they often dominate the rare elements such as platinum group metals.
Rock-forming minerals
Minerals that make up the bulk of all rock samples and are also the ones used in rock classi?cation.
Selecting one or more period, for example 'Jurassic'.
A term used to group together related samples that are not already gathered into a single Collection. For instance, there is a ‘SW England granites’ theme that includes such rock types as granite, hydrothermal breccia, skarn and vein samples.
A general term used to label a rock sample. It is a useful way of grouping similar samples throughout a collection. Category names are often, but not exclusively, common rock names (e.g. granite, basalt, dolerite, gabbro, greisen, skarn, gneiss, amphibolite, limestone, sandstone).
The owner of the sample that appears in the collection. For example, NASA owns all the samples that appear in the Moon Rocks collection
We would like to thank the following for the use of this sample: