I spotted these attractive columns on this former bank building on a recent trip to Liverpool which I stopped to have a look at. The columns contained several xenoliths, which give an insight into how the rocks that were used to make the columns were formed.
1) Describe the columns - mention the crystal size, and colours you can see
2) Can you spot any xenoliths? Describe one (there's a good example on the left side of the left column). Why are they so noticeable compared to the rock surrounding them?
3) As xenoltihs are not proof that the rocks making up this column were formed underground, what other evidence from the columns would prove this? Use your answer to the first question and the information below to help.
4) Take a photo of yourself at GZ, or an identifying item, being careful not to reveal any of the answers to the questions.
Please submit your answers via message though the Geocaching website or via email - there is no need to wait for a response before logging your find.
Igneous rocks - formationIgneous rocks form from the cooling and solidification of molten rock material, and their formation location determines their characteristics. When igneous rocks form underground, they are known as intrusive or plutonic rocks. These rocks cool slowly because they are insulated by the surrounding earth, allowing large crystals to develop. Granite is a common example of an intrusive igneous rock, characterised by its coarse-grained texture and visible minerals.
The picture below shows an example of an intrusive igneous rock - granite
In contrast, igneous rocks that form above ground are called extrusive or volcanic rocks. These rocks cool quickly upon exposure to the atmosphere or water, resulting in small or no crystal formation. Basalt is a typical example of an extrusive igneous rock, noted for its fine-grained texture and sometimes glassy appearance. The rapid cooling process gives extrusive rocks a more uniform and smoother texture compared to their intrusive counterparts.
The picture below shows an example of an extrusive igneous rock - basalt
The differences in cooling rates between underground and above-ground formations also affect the overall appearance and mineral composition of igneous rocks. Intrusive rocks, with their slower cooling process, tend to have a more varied and visibly crystalline structure. Extrusive rocks, however, often exhibit a more homogeneous appearance and may contain volcanic glass due to the rapid cooling. These differences make it possible for geologists to determine the formation environment of igneous rocks by examining their textures and mineral content.
Xenoliths
Xenoliths are fragments of rock that are enveloped by a larger body of igneous rock. They originate from the surrounding rock material (country rock) that gets incorporated into the magma before it solidifies. While xenoliths are more commonly associated with intrusive igneous rocks due to the slower cooling process, they can also be found in extrusive rocks.
When a volcano erupts, the magma can carry pieces of the surrounding rock with it as it ascends to the surface. These pieces, or xenoliths, are then trapped within the extrusive rock as it cools and solidifies. The presence of xenoliths in extrusive rocks provides valuable information about the composition and conditions of the earth's interior, as they are essentially pieces of the deep crust or upper mantle brought to the surface.
Therefore, while less common in extrusive rocks compared to intrusive ones, xenoliths can still be present and provide significant geological insights. They are an important aspect of studying volcanic processes and the material from different depths within the earth that can be exposed during volcanic activity.