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The Gibraltar Cross of Sacrifice is made of Cornish granite. The Royal Engineers built the memorial for the Imperial War Graves Commission after World War I. The Governor of Gibraltar, General Sir Horace Lockwood Smith-Dorrien, unveiled the memorial on Armistice Day in 1922.
Granite is an igneous rock which is formed through the cooling and solidification of molten rock called magma or lava. When the magma cools and solidifies, randomly arranged interlocking crystals form. The size of the crystals depends on how quickly the molten magma solidified. The more slowly the magma cools, the bigger the crystals are.
Occasionally, you will see shapes or impurities in granite that have a different colour and density than the surrounding rock.
Xenolith is a piece of rock trapped in another type of rock. Most of the time, a xenolith is a rock embedded in magma while the magma was cooling.
1. Formation of Xenoliths in Granite:
- When a magma body intrudes into the Earth's crust and solidifies to form granite, it may incorporate xenoliths from the surrounding country rock. These xenoliths can be of different origins, such as pieces of sedimentary, metamorphic, or older igneous rocks.
- Xenoliths can be transported along with the rising magma due to the force of convection currents, gravitational settling, or mechanical mixing processes. As the magma moves through the crust, it may engulf and entrain xenoliths, incorporating them into the growing granite body.
2. Types of Xenoliths in Granite:
- Xenoliths in granite can vary in size from microscopic inclusions to large blocks several meters in diameter. They can be of different shapes, such as angular or rounded, depending on the degree of erosion or fracturing they experienced before being incorporated into the magma.
- The composition of xenoliths can also be diverse, ranging from mafic (dark-colored) to felsic (light-colored) rocks, reflecting the different types of rock found in the Earth's crust.
3. Significance of Xenoliths in Granite:
- Xenoliths provide valuable information about the composition and characteristics of the Earth's crust at the time of granite formation. By studying the mineralogy, texture, and geochemistry of xenoliths, geologists can deduce the nature of the source rocks, their age, and the geological processes that influenced their formation.
4 The colours of Granite:
The amount of each mineral in the granite, and the rate at which it formed, determines the color. For example, granite with a lot of potassium feldspar will be pinker, while a lot of muscovite will make it appear green.
Granite's color comes from the minerals that make up the stone, including:
- Quartz: Usually milky white
- Feldspar: Typically off-white
- Potassium feldspar: Salmon pink
- Biotite: Dark brown or black
- Muscovite: Yellow or metallic gold
- Amphibole: Black or dark green
- Garnet: Deep red
Questions to answer.
1 - The Cross is made of granite but looking at it closely and using the information above what three minerals make up the majority to make its colour.
2 - Is the Cross of Sacrifice an igneous, sedimentary, or metamorphic rock?
3- Look closely at the panel to the left of 1914 – 1918. - Describe the Xenolith in the panel - length and width and rough shape and colour?
4- Is the Xenolith mafic or felsic?
5. Take a photo of yourself/mascot and/or your GPS with the memorial, post this with your log. Any logs without a photo will be deleted)
