Our coastal sands contain mainly quartz, and some feldspar. Also small amounts of titanium minerals, namely black ilmenite (titanium and iron oxide) and red brown rutile (titanium oxide) occur in the sand particles. If one looks carefully at the sand mixture one can identify the small black particles.
During the swash zone turbulence, the heavier ilmenite and rutile can often be deposited onto the beach, and the lighter quartz gets washed back down. We then see a black deposit/residue being left behind on the beaches. In March 2007, with the severe coastal storms and turbulent swash, it washed away most of the sand (and lighter quartz), leaving the more heavier and dense black ilmenite and red rutile. A very dark stained beach was left, resembling oil pollution.
An interesting fact is that Ilmenite also naturally sticks to the oils that the body exudes.
Swash
Swash, in geography, is known as a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach materials up and down the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes depending on the type of beach. Greater swash generally occurs on flatter beaches. The swash motion plays the primary role in the formation of morphological features and their changes in the swash zone. The swash action also plays an important role as one of the instantaneous processes in wider coastal morphodynamics.
Cross-shore sediment transport
The cross-shore sediment exchange, between the subaerial and sub-aqueous zones of the beach, is primarily provided by the swash motion. The transport rates in the swash zone are much higher compared to the surf zone and suspended sediment concentrations can exceed 100 kg/m³ close to the bed. The onshore and offshore sediment transport by swash thus plays a significant role in accretion and erosion of the beach.
There are fundamental differences in sediment transport between the uprush and backwash of the swash flow. The uprush, which is mainly dominated by bore turbulence, especially on steep beaches, generally suspend sediments to transport. Flow velocities, suspended sediment concentrations and suspended fluxes are at greatest at the start of the uprush when the turbulence is maximum. Then the turbulence dissipates towards the end of the onshore flow, settling the suspended sediment to the bed. In contrast, the backwash is dominated by the sheet flow and bedload sediment transport. The flow velocity increases towards the end of the backwash causing more bed-generated turbulence, which results in sediment transport near the bed. The direction of the net sediment transport (onshore or offshore) is largely governed by the beachface gradient.
Source : Wikipedia.
In order to log this Earthcache and claim a find, you will need to complete the following tasks / questions.
Educational logging requirements:
- Grab a handful of sand, and describe particle size, and colours observed. Try and identify the different composition types.
- What are the densities of quartz, ilmenite and rutile respectively? (as per the WESSA info board near the published co-ordinates).
- With the above knowledge of the densities, describe in your own words what would happen to the minerals during a swash action.
- Apart from the swash action, what other process can allow for the accumulation of ilmenite on the beaches?
- Optional Task: Take off your shoes, let your creative side out and return to your childhood: build a sand castle or similar, and post a picture of it in your log.
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