Anson Bay Earthcache EarthCache
Hidden : 3/15/2015
Difficulty:
2 out of 5
Terrain:
3.5 out of 5

Size: Size: other (other)

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Geocache Description:


Norfolk Island is almost entirely volcanic. Basaltic lavas are predominant along with basaltic tuffs. Two basic types of lava flow that are found on Norfolk Island are sheet lava and pillow lava
 
Igneous Rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.

When the magma solidifies within the earth's crust (intrusive), it cools slowly forming coarse textured rocks, such as granite, gabbro, or diorite.

Extrusive igneous rocks, also known as volcanic rocks, are formed at the crust's surface as a result of the partial melting of rocks within the mantle and crust. Extrusive igneous rocks cool and solidify quicker than intrusive igneous rocks. They are formed by the cooling of molten magma on the earth's surface. The magma, which is brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate. Hence such rocks are smooth, crystalline and fine grained. Basalt is a common extrusive igneous rock and forms lava flows, lava sheets and lava plateaus. Some kinds of basalt solidify to form long polygonal columns.

Lava Flows
Sheet flows form broad, laterally extensive blankets of lava. They form during relatively high effusion rate eruptions, are very fluid. Some lava sheets have thick vertical columns (columnar basalt), others have thin columns of variable orientation and irregular jointing.

Pillow Lava are circular and elliptical shaped structures that are typically formed when lava emerges from an underwater volcanic vent.

Norfolk Island Geology

The earliest traceable volcanic event on Norfolk Island is the eruption of lavas from an unknown vent some 3 millions years ago. The basalts derived from this lave can be found in the vicinity of Ball Bay, with isolated occurrences at Cascade, and are referred to as Ball Bay Basalts. Apparently there was no more volcanic activity for some 200,000 years.

The next period of volcanic activity began about 2.8 millions again and is the period when most of the Norfolk Island complex was built. It appears that the Mount Pitt/Mount Bates summit area was the volcanic focus on Norfolk Island during this period. No vents can be identified but an agglomerate which cannot be far from its source is exposed on the north flank of Mount Bates. At some stage during the volcanic history of the Island the crater was breached on its southern side by basaltic lava flows that built the southern plateau of Norfolk Island.

The three types of basalt, which have resulted from the later volcanic activity are very similar and all appear to have come from the same eruptive centre. They are:
• Duncombe Bay Basalt, which is confined to a portion of the north coast below the summit area is dated between 2.66 and 2.69 million years;
• Cascade Basalt which occurs almost continuously in cliffs along the north and west coasts, with ages from 2.41 to 2.74 million years; Separating it from the Duncombe Bay Basalt it rests on a layer of yellow tuff up to 20m thick;
• Steeles Point Basalt, which occurs along most of the coast perimeter of the southern plateau and is dated between 2.33 and 2.39 million years old.

Each of these lava flows is separated by a conspicuous bed of tuff which represents the deposition of sediment during volcanic explosions.

Access
Pedestrian access to the beach and earthcache is by a winding unsealed walking track that zigzags down the steep cliff slopes. To log this earthcache you will need to walk 800m down to the bottom of Anson  Bay via the access track.
 
Tasks
Please email us the answers to the following with the cache name. We will contact you if more information is required. Logs that have not had the answers sent to us will be deleted. Please do not include your answers in the logs.
1. Which of the Igneous rock types are present here – Intrusive or Extrusive?
2. Which of the types of lava flow are you looking at above you – Sheet lava or Pillow lava?
3. What type of Basalt are you looking at above you – Duncombe Bay Basalt, Cascade Basalt or Steeles Point Basalt?
4. Optional – Post a photo of Anson Bay or the lava feature with your log
 
References
Sampson, Helen “Geology of Norfolk Island” (ISBN 0975713906)
Jones, J.G and McDougall “Geological history of Norfolk and Philip Islands, southwest Pacific Ocean” Journal of the Geological Society of Australia (1973)
Administration of Norfolk Island - Plan of Management, Anson Bay Reserve

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