Ignimbrite - Betws-y-Coed
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

Ignimbrite - Betws-y-Coed

This ignimbrite rock was formed by an explosive volcanic eruption in the Ordovician period. Ignimbrites (literally 'fiery rock dust cloud') are the result of avalanches of superheated ash and dust that speed down erupting volcanoes during an explosive volcanic eruption. One of the strongest pieces of evidence that this particular ignimbrite formed from an explosive volcanic eruption are the angular shaped 'fiamme'. Fiamme are glass fragments with 'flame-like' shapes, which are the remains of frothing lava that exploded yielding small fragments of broken bubble wall with shapes said to resemble flames. The glass and crystal fragments travel as part of the avalanche and are still sufficiently hot when they fall to the ground to deform and flatten under the weight of the overlying deposit. This sample was collected west of the village of Betws-y-Coed, Caernarvonshire, Wales.

The thin section contains a fine-grained matrix that was originally glass but became altered during low grade metamorphism. There are also grains of altered orthoclase feldspar and quartz, and occasional lithic fragments (grains of rock and soil that were ripped up during the eruption and incorporated in the ignimbrite).

Additional images
  • ignimbrite - width 7 cm
  • ignimbrite (wet) - width 3.8 cm
53.098, -3.8123
Betws-y-Coed, Caernarvonshire, Wales
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
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: