Point Hunter - Norfolk Island
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Your task for this Earthcache is to correctly identify and classify the rock formation that you are observing at these coordinates. The following information will assist you with this task. There is a series of questions at the end of this description. Please send your answers via email to the Cache Owner (CO) for validation and permission to log as a successful find.
Norfolk Island is a 3855ha, 8km x 5km, located in the South Pacific Ocean, is approximately 1000 kilometres east of the Australian coast. Norfolk is on the Norfolk Ridge linking New Caledonia and New Zealand and is a deeply weathered erosional remnant of volcanoes consisting of olivine basalt lavas and tuff. The areas volcanoes were active in the late Pliocene. The subaerial portion of the Norfolk volcano was constructed during several volcanic episodes from about 3.05 million years to about 2.3 million years ago. Both were built when relative sea level stood at about its present level, indicating that the Norfolk Ridge has been very stable at this latitude for the past 3 million years. It is scientifically thought that Norfolk, Nepean and Phillip Islands were one land mass. During this time reefs formed over the shallow marine platform surrounding the island. It is thought the sea-level dropped well below its current level causing extensive reef to be exposed and eroded. Eroded fragments were blown by the prevailing winds into the vicinity of what we now know as Point Hunter. “Norfolk (3455 ha), Nepean (10 ha), and Philip (190 ha) islands are the much-restricted post-glacial sub-aerial sections of a much larger (320,000 ha, c 85 times the present extent) flat-topped edifice on the Norfolk Island Ridge (Jones & McDougall 1973). The area of sub aerial exposure has repeatedly expanded and contracted in concert with sea level changes associated with the Quaternary glacial events, and presumably the terrestrial biota will have gone through repeated cycles of restrictions in area (“bottleneck events”, with each amelioration) and expansions (with enhanced chance of over-water colonisation, at each glacial low sea level stand). Jones & McDougall (1973) emphasise the apparent tectonic stability of the Norfolk Island Ridge, on which Norfolk Island and its outliers is situated), with no evidence for significant uplift or subsidence, so that sea level changes associated with ice volume fluctuations have been the only determinants of land area apart from the relatively much smaller effects of erosion of the presently emergent islands. The rocks are primarily oceanic basalts of Pliocene to early Pleistocene age (Jones & McDougall 1973); some late Pleistocene superficial calcarenites are exposed at the southern end of Norfolk Island and form most of Nepean Island. The youngest basalts on Norfolk Island are about 2.3 million years (ma) old (Jones & McDougall 1973; McDougall 1973), which is midway between the ages of Oahu (2.6-3.0 ma) and Molokai (1.8 – 2.0 ma) islands in the Hawaiian chain (Vitousek 2004).” Common rock types of Norfolk Island A. Igneous rock Igneous rocks are formed from the solidification of molten rock material. There are two basic types: 1) intrusive igneous rocks such as diorite, gabbro, granite and pegmatite that solidify below Earth's surface; and 2) extrusive igneous rocks such as andesite, basalt, obsidian, pumice, rhyolite and scoria that solidify on or above Earth's surface. Of interest in relation to Norfolk Island. • Pumice is a light-coloured vesicular igneous rock. It forms through very rapid solidification of a melt. The vesicular texture is a result of gas trapped in the melt at the time of solidification. • Basalt is a fine-grained, dark-coloured extrusive igneous rock composed mainly of plagioclase and pyroxene. B. Sedimentary Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks: 1) classic sedimentary rocks such as breccia, conglomerate, sandstone and shale, that are formed from mechanical weathering debris; 2) chemical sedimentary rocks such as rock salt and some limestones, that form when dissolved materials precipitate from solution; and, 3) organic sedimentary rocks such as coal and some limestones which form from the accumulation of plant or animal debris. Of interest in relation to Norfolk Island. • Breccia is a classic sedimentary rock that is composed of large (over two millimetre diameter) angular fragments. The spaces between the large fragments can be filled with a matrix of smaller particles or a mineral cement which binds the rock together. Collapse Breccia: Broken rock that originates from a cavern or magma chamber collapse. Flow Breccia: A lava texture produced when the crust of a lava flow is broken and jumbled during movement. Pyroclastic Breccia: A term used for a deposit of igneous rock debris that was ejected by a volcanic blast or pyroclastic flow • Limestone is a rock that is composed primarily of calcium carbonate. It can form organically from the accumulation of shell, coral, algal and fecal debris. It can also form chemically from the precipitation of calcium carbonate from lake or ocean water. Chalk: A soft limestone with a very fine texture that is usually white or light gray in colour. It is formed mainly from the calcareous shell remains of microscopic marine organisms such as foraminifers or the calcareous remains from numerous types of marine algae. Calcarenite (or superficial Calcarenite) is a type of limestone that is composed predominantly, more than 50 percent, of detrital (transported) sand-size (0.0625 to 2 mm in diameter), carbonate grains. Calcarenite is a type of limestone that is composed predominantly, more than 50 percent, of detrital transported)sand-size (0.0625 to 2 mm in diameter), carbonate grains. The grains consist of sand-size grains of ither corals, shells,ooids, intraclasts, pellets, fragments of older limestones and dolomites, other carbonate grains, or some combination of these. Calcarenite is the carbonate equivalent of a sandstone. Fossiliferous Limestone: A limestone that contains obvious and abundant fossils. These are normally shell and skeletal fossils of the organisms that produced the limestone. Oolitic Limestone: A limestone composed mainly of calcium carbonate "oolites", small spheres formed by the concentric precipitation of calcium carbonate on a sand grain or shell fragment. Tufa: A limestone produced by precipitation of calcium-laden waters at a hot spring, lake shore or other location. • Sandstone is a clastic sedimentary rock made up mainly of sand-size (1/16 to 2 millimeter diameter) weathering debris. Environments where large amounts of sand can accumulate include beaches, deserts, flood plains and deltas. C. Metamorphic. Metamorphic rocks have been modified by heat, pressure and chemical process usually while buried deep below Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture and chemical composition of the rocks. There are two basic types of metamorphic rocks: 1) foliated metamorphic rocks such as gneiss, phyllite, schist and slate which have a layered or banded appearance that is produced by exposure to heat and directed pressure; and, 2) non-foliated metamorphic rocks such as marble and quartzite which do not have a layered or banded appearance. Of interest in relation to Norfolk Island. • Slate is a foliated metamorphic rock that is formed through the metamorphism of shale. It is a low grade metamorphic rock that splits into thin pieces. • Quartzite is a non-foliated metamorphic rock that is produced by the metamorphism of sandstone. It is composed primarily of quartz. Your Questions: Question 1: In which classification would this rock formation fall? A, B, or C. Question 2: Further classify this rock formation by choosing one type of rock from the following? Pumice, Basalt, Breccia, Limestone, Sandstone, Slate or Quartzite Question 3: Further to you answer to Question 2; if your chosen a classification that has a further break down in classification – Name the final classification that you believe is an accurate description of the rock formation that you are looking at. Choose Collapse Breccia, Flow Breccia, Pyroclastic Braccia, Chalk, Calcarenite, Fossilifrous Limestone, Oolitic Limestone, or Tufa, Question 4: Looking at the surface of these rocks – which surface type would best describe their appearance? (a) Smooth (b) Pitted (c) Conglomerate (d) Porous Question 5: Consider the square patterns that can be observed in these rock formations. Which of the following processes is the most likely for this appearance? (a) Made by natural erosion (b) Made by human intervention Please send your answers to the CO for validation and permission to log this cache. I hope you have enjoyed discovering a little more about the wonderful Norfolk Island.
Additional Hints
(Decrypt)
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