Kariotahi Black Sand Beach
This Earth Cache takes you to Kariotahi, southwest of Auckland.
Please bring a magnet to answer one of the questions. If you don't have a magnet with you, just take a small amount of black sand home and do the little experiment there.
Where does the black sand come from?
The black sand found here at Kariotahi beach on the west coast of the North Island, is made of titanomagnetite – a mixture of titanium and iron. This soft material comes from Mount Taranaki and is swept up the coast as far as North Cape – a distance of about 400 km – ground into rounder and finer grains as it goes.
The main bulk of the island was built up by coalesced volcanoes during the Pliocene (4.0-2.0 million years). During the Pleistocene (<2 million years) a number of young volcanic centers have been building over the Pliocene rocks and are separated from them by an extensive saprolite-laterite soil horizon caused by a long period of tropical weathering that formed about 2 million years ago. The latest eruptions of the northern North Island have been mainly basalt.
Black Sand
There are two types of black sand beaches; one where the grains are very sharp and one where the edges have been rounded over. It's how the lava enters the water that causes the different beach types. When it flows directly into the water it is still very hot (anywhere from 750 to 1,250 degrees Celsius). Contact with the cold water shatters it into small shards. These pieces are either deposited right there, or are carried, for a short distance, along shore by the currents and then deposited. In this case the grains will be very sharp.
If a stream cuts through an older lava bed it will break down the lava into smaller pieces but these will be more rounded since the water action is bumping them around against each other and the ground. That action is like sandpaper grinding off the sharp corners. Also, if the lava pieces are transported by the ocean currents for a long distance (kilometers) any sharp edges will be worn off in the same manner. So you have lava, hot or otherwise, entering the water, either being beaten up by the currents or being immediately deposited to form a beach.
The iron oxide compounds have originated from rocks deposited from volcanic activity, both coastal and inland, followed by subsequent erosion by wind, rain and waves. The magnetite eroded from the volcanic rock has been distributed along the coastline by the prevailing wind and sea currents.
The iron oxide compounds, including titano magnetite, found in the sand at Kariotahi beach do not originate in the local area, but have been carried here by the sea currents, which along with the prevailing wind, have deposited the minerals along this area of coast line.
CLASSIFICATION OF SANDS
Sands and Sedimentary Environments
The sands are formed from rocks. They consist of broken debris sizes of between 0.063 and 2 millimeters.
Wentworth scale
The Wentworth Scale consists of a logarithmic scale of size classification (larger diameter) of clastic sediments fragments (or detrital), the thinnest to the thickest, created in 1922 by CK Wentworth. This scale is often used by geologists and their classification usually differs slightly from those made by the engineers.
Rocks and sand
The finer grain, for the larger size, are: clay (<4 mm), silt (> 4 � <64 mM), sand (> 64 uM <2mm), bead (> 2 mm - <4mm), Pebble (> 4mm - <64mm), block or pebble (> 64mm - <256mm) and the boulder (> 256mm).
The colors in the sand
The color that the sands have relates much with its mineralogical composition. Thus, the silica sands are white when pure, as well as calcareous sands. The basaltic sand are black as well as those that are rich in organic matter or magnesium compounds. Iron compounds give the sands yellowish or greenish. The sand mainly composed of quartz grains, due to the hardness given by this mineral, scratching the glass and steel. They are unassailable by acids and are practically insoluble in water. Sand: Dune Environment The grains making up these sands are lightweight (are transported by wind), well calibrated (all have the same size) and rolled. They have scores and dull surface due to clashes with other grains. Quartz grains present since they are easily transported by wind.
Sources:
https://teara.govt.nz/en/1966/geology-new-zealands-geological-history/page-13
https://en.wikipedia.org/wiki/Volcanism_of_New_Zealand
Logging Requirements:
1. Feel and touch: what is the grain size of the sand at this location? Are the sand particles sharp or rounded?
2. What does this tell you (the texture of the sand) about how the beach was formed. Describe it with your own words.
3. Find a spot in the sand where the sand is dry and hold your magnet in the sand. What can you observe? (A little help with that: What colors are the particles on the magnet and what could they be?)
4. Please take a photo of yourself or your GPS at Kariotahi beach and upload it to your log.
You can log immediately. You don't have to wait for an answer. If something is unclear, I'll let you know. Your answers must be received within 10 - 14 days, or the log may be deleted.
Please send the answers in english or german language through the message center.
Have fun and enjoy the beach.