Tides

The truth of the matter is that local topography is all-important in determining the features that our minds make 'the tide'

- Rachel Carson “The Sea Around Us”

With each ebb and flow the shoreline changes. Rocks and sand are exposed only to be hidden again a few hours later. Tides are rhythmic and predictable – a look at a tide chart will tell you when a beach will be exposed. What makes tides predictable is how they are created, yet how the tide manifests at a specific location is shaped by the local topography.

The sun and moon's gravities pull at the earth, and since the oceans are a fluid they are free to respond to this pull in a very observable way. Tides start out uniform with depth since the pull from the sun and moon act on the entire water column at once. The wave created is thousands of kilometres long – the true tidal wave, but it won't look like a wave to an observer like lapping wind driven waves do instead tides raise and lower the sea level, exposing more or less of the beach. Since tides are a type of wave, they take time to propagate. This means that low tide (or high tide) occurs at different times at different locations. How the tides propagate is determined by the shape of shorelines and roughness of the ocean floor. As tides flow onto the shallower continental shelf, they slow down and their energy builds up in a smaller volume, amplifying the rise and fall of the water.

Anyone living near a sea shore can observe tides on a daily basis and there are tidal records extending back to antiquity. To build our modern understanding, Lord Kelvin led the first systematic effort to resolve the fundamental frequencies in 1867 and much of this work still holds today. There are semi-diurnal (twice-a-day), diurnal (once-a-day) and longer period components measured in days, months, and up to just over 18 years. To add complexity, these constituents can interact with each other, amplifying or damping local tides, however, the semi-diurnal lunar tides dominate other tidal frequencies at most locations.

The maximum height a tide reaches varies greatly depending on location. On Hawaii tides are tiny, reaching only 30 cm, while in the Bay of Fundy tides reach up to 12 m. The Bay of Fundy is often cited as having the highest tide in the world, however it has a competitor; Ungava Bay in northern Quebec, and it is still not determined which has the higher tides.

Strong tidal currents are typically the result of a constriction on the flow. Consider a garden hose: if you turn it on the water comes out at a certain rate, if you cover part of the opening with your finger, water will come out faster. The same amount of water is coming out, just the opening has changed. Even though the Islands of Hawaii have tiny tides, there are huge currents in the area. The islands are actually part of a mountain chain with a height comparable to the Himalayas. Tidal waters are pushed up the slopes and between the islands, forcing them to speed up.

Tides are an important factor in ocean mixing, which is a ongoing area of investigation for physical oceanographers. In a coastal environment, tides keep the water mixed, bringing up nutrient- rich water from the depths and removing waste.

Tippy fish

I had a little aquarium on my desk before we moved. Now that we have space I'm setting up a much larger tank but I haven't transfered over my fish yet. So right now my little aquarium is sitting on a dresser in front of a window. It's a west-facing window, so in the evening the sunlight comes pouring in the side of the tank. I looked in my little aquarium a couple of evenings ago and my cardinal tetras were swimming on their sides. Something had to be wrong! A terrible tetra plague? Inner ear infections for all? I immediately called in my fish biologist spouse. He took one look at the tank and laughed. It turns out my fish were just confused.

As I've mentioned in one of my other blog posts, cardinal tetras are small fish with vivid iridescent blue and deep scarlet stripes. Cardinal tetras come from shallow tributaries of the Negro and Orinoco Rivers in South America. Their habitat changes as wet and dry seasons cycle each year, and they move from flooded forest areas to crowded streams when the water is low. The middle Negro is the primary fishing area for these fish, where about 20 million of them are captured annually. About 90 % of fresh water aquarium fish are bred in captivity, but some, like cardinal tetras, don't cooperate and breed easily in captivity and so are caught in the wild. It has been demonstrated that well-managed fisheries for some aquarium fish (like the quick-to-mature, and naturally prolific cardinal tetra) can actually provide a good living for folks living in rural tropical areas – in fact it can pay better than farming and fishing for food, is way safer than mining for gold, and better is for the environment than cutting down the trees. The tough part is determining if a specific fish came from a well-managed fishery – which I don't have an answer for.

So why are my fish tipping? It turns out to be a phenomenon called phototaxis: cardinal tetras orient themselves based on the direction light is hitting them. In their wild jungle habitat, the sunlight is always coming from above and provides a frame of reference that the fish use to align their own, internal, up/down direction. In my tank, the afternoon sun is coming through the side so they line themselves up as if the sun was coming straight down.

I would include a picture but the algae really likes the light and has grown on everything

Opals – pretty things

I have an opal pendant that a friend brought back for me a few years ago from her trip to Australia. It looks like cracked pale blue ice on top of purple and green lights. Colours shift as the viewing angle or lighting changes. Some believe that opals have healing powers and can help you find your true love -- I'm not so sure about that, but I do think opals are pretty examples of some neat optics.

The opal's popularity peaked with the flappers in the 1920's. The artisans of the time catered to art deco ideas, a style seen as glamorous and modern. They valued opals because of their subtle effect and how nice they looked with enamel, another popular material of the time. Ironically, my grandmother gave me some costume jewelry from that period that doesn't have a single opal. Since then, opals have been studied under electron-microscopes, made in labs and even found on Mars. The name opal likely comes from a Sanskrit word upala meaning 'valuable stone', a term adapted into Greek and later into Latin.

Opals have been mined dating back to Roman times, and probably longer than that. Currently almost 95% of all opals come from Australia, while other sources include Mexico, Brazil and the United States. New sites are still being found. Every opal site produces opals that are slightly different, being a different colour pattern, clarity or luminosity.

So what is so captivating about an opal? A single stone can display all the colours of the spectrum in moving patterns depending on the viewing angle – this movement across the opal face is called the 'play of colour'. These effects can even take on an iridescent quality that is impossible to reproduce (I tried to capture these effects with my camera and it didn't work). If you look through the stone and see red (a rare thing), all the other colours will also appear in that stone because red is at the long end of the colour spectrum (630-740 nanometers) and can be broken into all the other colours. Finally, opals can have a translucence ranging from transparent to a milky opacity. The colours found within an opal are formed through the interference and diffraction of the incident light as it passes through the internal structure.

If you examined the micro structure of an opal you would see silica spheres of 150 to 300 nanometers packed together in a not-quite regular lattice. An opal isn't a true crystal due to the its non-regular lattice, instead it is classified as a 'mineraloid'. The quality of an opal can be determined from its structure – the more regular it is, the more valuable it is. Another aspect to notice at the microscopic scale: namely, pockets that were once filled with water. These pockets affect the optics and are a remnant of how opals are formed.

Opals are formed from dissolved silica in water. Let's start with dissolved silica in a bucket sitting in the sun; over time the water will evaporate and if new water isn't added the silica molecules will bind together, forming small spheres. Eventually, if the bucket is left undisturbed for eons, the spheres will become tightly packed and will form layers. More water will evaporate as the layers compact and harden, leaving tiny spaces where the water once was.

Light waves range in size from about 400 to 760 nanometers – about twice the 150 to 300 nanometers of the silica balls. As light enters an opal, the the silica spheres and the water voids diffract or absorb some wavelengths, changing the composition of the light wave and hence the colour of the light reflected by the opal. By changing your viewing angle the light path through the opal to your eye also changes creating new patterns and colours. The most spectacular and likely most expensive opal is the fire opal. Here the silica spheres are lined up so well they act like a diffraction gate, breaking down the incident light into a full rainbow of colours.

Remember the water inside an opal? Well, it isn't completely gone, 2 % or more remains. That little bit of water is required to keep the opal from becoming brittle and paler. So to keep your opals pretty you have to wear them; the humidity in the air and on your skin will keep them looking their best.