Allandale Trachyte dike

What is a dike?
A dike is a vertical or near-vertical intrusion of magma cutting up through the surrounding rock. Because the magma is fluid and hot when it travels up through the rock, it will most commonly have a chilled glassy margin where it comes into contact with the colder rock and cooled quickly, relative to the inside. It will also often bake the surrounding rock due to it's heat. Crystals of minerals in the centre of the dike will grow bigger than towards the outside, as the dike retains it's heat longer in the middle, while it cools down and solidifies.

Trachyte, when the magma solidifies, is a lightish grey coloured rock. The major mineral component of trachyte is alkali feldspar (e.g. orthoclase), and it generally contains no quartz. Because of the higher silica content, trachyte magma and lava run more sluggishly and at a cooler temperature than basalt. It is therefore also more explosive in eruptions than basalt. Trachyte is the volcanic equivalent of the plutonic rock syenite. This dike is a late stage (relatively young) feature of the Lyttelton volcanic sequence, cutting across and through the older bedrock beneath.

Allandale is well known for it's rhyolite outcrops also. Allandale rhyolite is one of the older rock types (around 11-13 million years) on Banks Peninsula and underlie the main Lyttelton volcano (a strato-shield volcano composed primarily of low silica basalt with minor andesite and trachyte). Rhyolite is a very high silica igneous rock which means as molten magma it produces very explosive eruptions. This is because silica is one of the main causes of viscosity (stickyness) in magma and therefore makes it difficult for gas to escape easily. Pressure builds up underground often building short lava flows and domes which can explode violently creating a large column of ash and pyroclastic flows. The rocks in the Allandale region are made up of porphyritic (large crystals within a background of small, hard-to-see crystals) lava flows and domes, with some changing to dacite composition (less silica). The rhyolite is often flow-banded (layers formed in the rock while cooling when the lava was still trying to move), with breccias, tuffs and some obsidian (visible from the Summit Road above Gebbies Pass). Well formed columnar cooling joints can also be observed in road cuttings heading up the Summit Road from Gebbies Pass.
It's thought that perhaps these rocks record an early high silica phase of Lyttelton volcanism.

The swirling weathering patterns in the tracyte dike at this outcrop make for some interesting photographs!

Have a look carefully for the white feldspar crystals in the rock (seen best within the natural light grey colour of the rock between the orange iron oxide staining). Feldspar is normally a very hard mineral (Hardness 6 out of 10 on Moh's Hardness Scale). Note how crumbly and easily erodible this outcrop is - the reason for this is primarily due to the feldspar minerals! Do you know why this is? HINT: there is a chemical reaction going on here!

Trachyte is relatively easy to spot in the landscape due to it's very pale colour and distinctive orange/brown weathering. It is visible in a number of dikes across the Lyttelton and Akaroa volcanic complex, with some of the most spectacular examples seen here at Allandale, Pigeon Bay and Onawe Peninsula.