Welcome to Tabletop Mountain, located on the eastern side of Toowoomba, rising 613m above sea level. This cache will take you to 3 waypoints on the mountain, your task is to observe the rocks at each location. The hike is just under 2km return and is classified as a grade 5 with steep hills, cliff edges, loose rocks and some rock scrambling required. Please be careful and it is not advisable to climb Tabletop in the wet.
At the top, on a clear day you can enjoy great views of the Lockyer valley and beyond. Enjoy your walk!
Tabletop Mountain is an extinct volcano which erupted approximately 20-25 million years ago.
Below the ground, molten rock (magma) rose toward the surface, the intense heat boiling the groundwater. This caused a violent explosive eruption and the settling debris formed a crater of tuff (loose rocks of volcanic ash). Magma filled the crater and overflowed as lava. The cooled, solidified lava formed basalt. Over time the volcano has eroded. The softer tuff at the crater edges has eroded faster, undercutting the tougher basalt. The basalt has broken off in blocks falling as scree slopes.
Basalt columns are created by the cooling and contracting of lava—made of 90% basalt—which causes the ground to crack into long, geometric columns. This process is called columnar jointing. During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns. These structures are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed. The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, while slow cooling is more likely to produce large columns.
Vesicular basalt is a dark-colored volcanic rock that contains many small holes, more properly known as vesicles. A vesicle is a small cavity in a volcanic rock that was formed by the expansion of a bubble of gas that was trapped inside the lava.
To log this earthcache, please email the answers to the following questions. Please do not post answers or spoiler pics in your log or it will be deleted.
1 - Observe the large rocks on the way to and around WP 1, many are broken up but there are areas that are still intact (look to the south side, just off track). From what you have read in the description, do you think these rocks are an example of columnar jointing? Give reasons for your answer.
2 - Observe the rocks at WP 2, how do they differ to the rocks at WP 1? What is the difference between the rocks on the track and the rocks just to the south side of the track? Why do you think this difference exists?
3 - Talk a walk down the footpad around WP 3. You are walking over and around small rocks in the soil. What is the distinct feature of the rocks along this path? How does this feature form?
4 - Using the diagram in the description, identify where you think waypoint 1, 2 and 3 are located.
5 - At WP 4 take a photo of yourself, caching party, caching name or personal object with the rocky cliffs, that lead to the top of the mountain, in the background and include it in your log (see image in the gallery).