Welcome to the Scotchman's Lead Reserve, a riverside crown land reserve. the land of the Wadawurrung people. You are invited to stroll along the paths and take in the serenity of the area.
To make your way to the posted coordinates, after parking in the parking area, enter the reserve via the marked trailhead and proceed toward the river which is to your right as you enter the pedestrian gate. Stay on the path that swings around to your left past the frog ponds and continue through the pines, past the windmill and continue on to GZ.
The strange thing about Basalt.
It’s the fine grained igneous rock that makes up the sea floor and sometimes spews forth from volcanoes on the continents. End of story right? Wrong!
Basalt is the most common rock type in the Earth's crust (the outer 10 to 50 km). In fact, most of the ocean floor is made of basalt. As well as some huge areas of some of the continents. It is a dark fine grained rock made up of the minerals olivine, pyroxene and plagioclase feldspars (notice…no quartz). Almost always these minerals are too fine grained to be seen with the naked eye. However sometimes Olivine can be seen as small sand-sized grains of olive-green glass in the specimen.
But on an active basaltic volcano, basalt can look very different from what most of us have seen. Basalt can be glassy black with amazing irredescence. It can be bright red. It can also be rough chocolate brown. Basalt can be solid or full of gass bubbles.
All this variation is due to the way the basalt has been erupted onto the Earth’s surface. So while it is all the same rock type chemically, it’s appearance can be so varied.
What is Basalt?
Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as in a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a composition similar to gabbro. The difference between basalt and gabbro is that basalt is a fine-grained rock while gabbro is a coarse-grained rock.
Earth's Most Abundant Bedrock
Basalt underlies more of Earth's surface than any other rock type. Most areas within Earth's ocean basins are underlain by basalt. Although basalt is much less common on continents, lava flows and flood basalts underlie several percent of Earth's land surface. Basalt is a very important rock. Basalt often contains vesicles, formed when dissolved gases bubble out of the magma as it decompresses during its approach to the surface, and the erupted lava then solidifies before the gases can escape.
Basalt-Forming Environments
Most of the basalt found on Earth was produced in just three rock-forming environments:
1) oceanic divergent boundaries,
2) oceanic hotspots, and
3) mantle plumes and hotspots beneath continents. The images on this page feature some of these basalt-forming environments.
Basalts at Oceanic Divergent Boundaries
Most of Earth's basalt is produced at divergent plate boundaries on the mid-ocean ridge system. Here convection currents deliver hot rock from deep in the mantle. This hot rock melts as the divergent boundary pulls apart, and the molten rock erupts onto the sea floor. These submarine fissure eruptions often produce pillow basalts.
The active mid-ocean ridges host repeated fissure eruptions. Most of this activity is unnoticed because these boundaries are under great depths of water. At these deep locations, any steam, ash, or gas produced is absorbed by the water column and does not reach the surface. Earthquake activity is the only signal to humans that many of these deep ocean ridge eruptions provide. However, Iceland is a location where a mid-ocean ridge has been lifted above sea level. There, people can directly observe this volcanic activity.
Oceanic Hotspots
Another location where significant amounts of basalt are produced is above oceanic hotspots. These are locations where a small plume of hot rock rises up through the mantle from a hotspot on Earth's core. The Hawaiian Islands are an example of where basaltic volcanoes have been built above an oceanic hotspot.
Basalt production at these locations begins with an eruption on the ocean floor. If the hotspot is sustained, repeated eruptions can build the volcanic cone larger and larger until it becomes high enough to become an island. All of the islands in the Hawaiian Island chain were built up from basalt eruptions on the sea floor.
Plumes & Hotspots Below Continents
The third basalt-forming environment is a continental environment where a mantle plume or hotspot delivers enormous amounts of basaltic lava through the continental crust and up to Earth's surface. These eruptions can be from either vents or fissures. They have produced the largest basalt flows on the continents. The eruptions can occur repeatedly over millions of years, producing layer after layer of basalt stacked in a vertical sequence.
The Newer Volcanics Province is a geological area which is a volcanic field, formed by the East Australia hotspot across south-eastern Australia. It covers an area of 15,000 square kilometres, with over 400 small shield volcanoes and volcanic vents. The area contains the youngest volcanoes in Australia.
The volcanoes date from the Late-Pleistocene to Holocene ages. The area is characterised by flat lava flows, forming a plain above which rise numerous small scoria cones, tuff rings, and maars. The most recent eruptions in the region took place at Mount Schank and Mount Gambier, estimated about 5000 years ago, when several maars were formed and associated lava flows spread around the cones.
Prominent volcanoes within the province include:
Mount Schank, Mount Napier, Mount Gambier (including Blue Lake), Tower Hill, Mount Elephant, Mount Eccles (Budj Bim), with associated Tyrendarra lava flow, Mount Leura, Mount Noorat, Mount Buninyong, Lake Bullen Merri, Lake Purrumbete and Red Rock.
The Earthcache Logging Task
Depending on the time of year that you visit, you may not see the feature at GZ due to raised water levels in the Yarrowee River. If that is the case, then proceed to Reflective Point (S37 40.492 E143 51.050) about 60 metres further on, where you will find examples of basalt with alternate questions to answer to complete the task.
1. GZ will have you on the banks of the Yarrowee River within the confines of the Scotchman's Lead Reserve. Observe the rock outcropping in the river and describe what you see, if the outcrop is visible. Describe the size, shape, colour and determine if these relate to what you would expect if the rock was basalt.
Alternate questions if GZ is under water.
2. Reflective Point
Examine the basalt rocks at this location, can you see vesicles in the rocks and how may the vesicles form? During your examination of the rocks, did you see any olivine? (olive green sand sized particles visible with the naked eye)
3. Attach a photo at Scotchman's Lead Reserve with yourself or your team name in it.
Hope you enjoyed this EarthCache & please feel free to upload a photo too
Once you complete the Earthcache requirements you can post your find without delay, as per the guidelines. You will also need to verify your find by sending me an email providing answers to the questions. This can be through the message centre or via email linked here