When it comes to cave formation, I used to be in the dark as much as anyone else. Now, after all the research I've done into West Australian caves, even my daughter knows more about caves than you can learn in Harry Potter.
Oh, and here's a hint: there's more to stalagmites than the M in them.
How Do Caves Form?
I can't answer for how caves form throughout the world, but I can give you an idea about how the caves in the south west of Western Australia were formed.
The granitic rocks of the Leeuwin Complex here are between 540 million and 1.5 billion years old. They're believed to have been part of India when it collided with Australia, but tectonic plates took India north…leaving the Leeuwin-Naturalist Ridge in Australia.
Over the last million years (fairly recently in geological time), the continual changes in sea level have resulted in the deposit of fragments from shells, coral and other marine life on and around the ridge. This formed huge dunes. Rainwater and dissolved organic matter percolated through the sand dunes, concreting the sand at the base into limestone. Because this limestone's origin is wind borne sand, it's called Aeolian limestone.
While I understand most cave systems elsewhere in the world are formed in ancient marine limestone, the relatively young caves in Western Australia are formed in this Aeolian limestone, often when the drifting sandhills covered existing streams on their way from the granitic ridge to the Indian Ocean.
Water dissolves limestone (calcium carbonate). Especially if it's acidic from dissolved organic matter, and the brown West Australian creek water has plenty of that. So the acidic groundwater slowly carved out caverns in the limestone. Some of these have been discovered when water levels drop and an internal collapse allows an opening to the surface, but many of the caves in Western Australia are unknown and unexplored because they're still hidden.
When the water levels drop enough for the caves to be filled with more air than water, calcium carbonate-laced waters leaching through the cave ceilings form various speleothems or cave decorations. And that's when you get stalactites, stalagmites and all sorts of other weird and wonderful cave formations.
Okay, we'll start with the easiest and most recognisable cave feature: stalactites. According to my daughter, these hug tight to the ceiling, so you'll remember what they're called.
I'm told they resemble icicles hanging off the cave roof (we don't get icicles here, just stalactites). They're usually hollow inside, and it's through the central tube that water flows, depositing calcium carbonate when the water evaporates. This precipitated calcium carbonate forms crystals that extend the stalactite in length or in girth.
Stalactites start out as straws – delicate tubes of around 5mm width that water trickles through, growing longer over time. One of the longest such straws in the world is found in Jewel Cave in Western Australia – it's over 5 m long.
Most straws never reach that length without getting blocked, which is when water starts trickling down the outside of the stalactite, giving most of them a cone-shaped appearance.
Stalagmites, my daughter insists, might reach the ceiling one day. It takes a long time, though…of water dripping on a stone. Quite literally, actually.
When water droplets continuously fall on the same spot on the cave floor, they deposit calcium carbonate crystals, which can form a stalagmite. The faster the water drips, the wider the stalagmite will be – and if the drip comes from a stalactite above, the stalagmite will always be wider that the stalactite.
When stalagmites and stalactites meet, you get columns.
Lake Cave has a particularly fine column…but the base is suspended in midair.
Calcite rafts form on the surface of cave pools that are rich in dissolved minerals, like the pool in Lake Cave. However, most such rafts don't get more than 1 mm thick…or larger than 15 cm across, or they sink beneath their own weight. The exception is when they crystallise to the wall or other stone – as the near-miraculous suspended table in Lake Cave has. I remember the table sitting at the surface of the lake water when I was a child, but twenty years later, I can see the huge drop in water levels that leaves it high and dry now.
These odd formations are called shawls, curtains or draperies because they look like draped fabric – a similarity heightened by the streaks of colour from dissolved minerals in the water. These form from inclined ceilings or occasionally walls and they start out straight, but time and the slow trickle of water, depositing more calcium carbonate crystals can give it a curve, up to and including making the stone look like a piece of folded fabric.
The rosy colour streaks on the Mammoth Cave shawl have led to it being likened to a piece of streaky bacon.
Flowstone looks like a frozen stone waterfall and, given their slow flow rate, that's precisely what they are. They're formed by thin films of water flowing over stone, depositing calcium carbonate. Over many years, this accretes until the formations can be more then 10 m thick.
I've heard of several names for rimstone dams. Fairy pools, gour pools…basically, they're the opposite of stalagmites. They're small bodies of water, with the crystallised calcium carbonate around them forming a dam or series of terraces, not unlike miniature rice paddies. They can create pools or even dam cave streams.
And why all the fuss about cave formation in Western Australia?
Because Hell's in a limestone cave in Western Australia…and when Lucifer decides to take Melody Angel on a romantic getaway…it turns into The Holiday From Hell, the fifth book in the Mel Goes to Hell series.