Key Features:
XENOLITH (Ancient Greek: 'foreign rock'): A term given to a rock that is embedded within another type of rock. As magma travels along older rocks it tears off bits and pieces trapping them in the magma.
DIKE: An intrusion formed when either magma (molten rock) or sediments (sedimentary rock) fill a fracture in older rocks. They can be millimetres to hundreds of metres wide and long.
GRANITE: The most common intrusive igneous rock (from magma) on Earth. It is coarse-grained and composed mostly of quartz, alkali feldspar and plagioclase. It cools slowly and solidifies underground.
Left photo - xenolith, right photo - dike, both embedded within granite. Photographs taken Mount Tennent: June 2022.
Background:
We acknowledge the Ngunnawal people as traditional custodians of this region. Mt Tennent was originally referred to by its aboriginal name 'Tharwa' meaning 'thunder mountain'.
Early European pastoralist Allan Cunningham renamed this mountain to Mount Currie in the 1820s after a British explorer. In the late 1820s it was renamed Mount Tennent after a local bushranger, John Tennant.
Geology:
Mount Tennent is covered by a vast expanse of exposed weathered granite structures in the form of massive boulders (batholiths), expansive slabs, tall cliffs, weathered rocks, and soils as well as granite lined waterways and walking trails. Magma dikes and xenoliths are two common features seen within the granite, on and off track. These features are often overlooked as part of the same rock. However, xenoliths are much older, and the dikes much younger, than the granite they lie within.
Granite on Mt Tennent is coarse to medium grained with a combination of mottled pinks, white and/or grey blacks. Three key minerals are quartz, feldspar, and mica. During the Late Silurian to Middle Devonian Period (430-360 million years ago) Mount Tennent was impacted by several volcanic eruptions creating the 'Booroomba Leucogranite' and the 'Tharwa Adamellite' outcrops.
At Virtual Stages 1, 2 and 3 you are surrounded by 'Booroomba Leucogranite' as there is an abundance of xenoliths visible and the granite is coarsely crystalline and pale being low in iron and calcium. The 'Tharwa Adamellite' is easily seen around the summit. It is darker with pink colours as it contains more iron, and xenoliths are not as evident.
Dikes within granite can be found all over Mount Tennent. They are mostly light coloured containing large amounts of quartz with some more fine-grained than others. They are a harder rock than the surrounding granite that is weathering at a quicker rate.
Mount Tennent Summit: Granite near and far on distant mountain tops. Photograph taken: June 2022.
Access:
Parking is available at the Namadgi Visitor Information Centre. In case you are delayed, consider parking in the section that is open 24/7 hours, and not in the area that closes at 4:00pm. This hike is on defined trails that are mostly steep with sections of granite stairs to navigate. If you stay on track, it is a 15.5km return hike taking on average 6-7 hours to complete.
Logging Requirements:
To log this cache you will need to address the following 7 points (5 questions and 2 mandatory photos). Send your answers via 'Message Center' to the CO as the preferred method. You can log this cache with the required photos immediately. If there are issues with your answers or photos, you will be notified.
Questions / Photos:
Virtual Stage 01: Dike underfoot:
1. What is the average width in centimetres of the dike on the trail?
Virtual Stage 02: Xenolites abound:
2. Trackside there is an impressive example of a granite boulder containing numerous dark coloured xenoliths. Explain briefly how these rocks may have become trapped within the granite boulder millions of years ago?
3. In this boulder which rock type do you think is harder, the xenoliths OR the granite they are contained within? What is one piece of evidence that you can see to support your answer?
4. Approximately how many xenoliths can you see within this one boulder (count all sides - no need to lift the boulder to look under)? What is the the approximate width in centrimetres of the biggest xenolith?
Virtual Stage 03: Granite Slab: Can you see the large granite flake that looks like Australia?
5. Post a photo of yourself, or something that identifies you with either the fine quartz dikes, or the 'Australia' flake.
Virtual Stage 04: Summit Fractures.
6. Note the multiple evenly spaced fractures (running north-west) within the large granite slab you are standing on. What is the approximate length in metres of the longest fracture you can see? No need to drop over the edge to make your estimate.
7. Post a photo of yourself, or something that identifies you with the great views to the west in the background.
Consider posting additional photos to encourage others to hike on this beautiful granite trail to the summit.