Black Rocks lies near the township of Breamea, approximately 17km's south of Geelong.
This is a location that Basalt meets the ocean, only a few places like it in Victoria.
Mt Duneed is a small hill in the south of Geelong. The name derives from the Gaelic word for a circular mound. The Mount Duneed recreation reserve now encloses the mount which is a volcanic cone. It's the highest point in the vicinity.
Many people wouldn't know that such a small hill is infact a volcano that had quite an impact on the Geelong area. Once there was a large, shallow embayment on the coast to the east and south of Mt Duneed, and lava from the volcano flowed into and filled the bay. Thompsons Creek now flows along the southern edge of the basalt and out to sea at Breamlea.
Basalt is a common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava exposed at or very near the surface of a planet. By definition, basalt is an aphanitic igneous rock with less than 20% quartz and less than 10% feldspathoid by volume, and where at least 65% of the feldspar is in the form of plagioclase. Basalt features a glassy matrix interspersed with minerals. The average density is 3.0 gm/cm3.
Basalt is defined by its mineral content and texture, and physical descriptions without mineralogical context may be unreliable in some circumstances. Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red due to oxidation of its mafic (iron-rich) minerals into rust. Although usually characterized as "dark", basaltic rocks exhibit a wide range of shading due to regional geochemical processes. Due to weathering or high concentrations of plagioclase, some basalts are quite light coloured, superficially resembling rhyolite to untrained eyes. Basalt has a fine-grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow, although it is often porphyritic, containing the larger crystals formed prior to the extrusion that brought the lava to the surface, embedded in a finer-grained matrix.
Black Basalt
Basalt with a vesicular or frothy texture is called scoria, and forms when dissolved gases are forced out of solution and form vesicles as the lava decompresses as it reaches the surface.
The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarse) groundmass are generally referred to as diabase (also called dolerite) or gabbro.
Q1 Describe the rocks in detail. Search among them, do you see any differences in the rocks, size and texture?
Q2 The rocks here are black but what other colour would you find in when it rapidly weathers and due to what? (read notes).
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