The dirt on metamorphic rocks
Add some heat and lots of pressure and voila, everyday rocks like limestone and sandstone turn into their much more glamorous metamorphic cousins.
Marble is best known for what it can be made into — incredible artworks or tacky ashtrays. But before it was that prized white-grey rock, marble was something a lot more pedestrian — limestone.
For all its 'you've made it' cache, marble is just a chunk of garden-variety limestone that's had an extreme makeover. In geology-speak, it's the metamorphic version of limestone.
The classiest marble is from Carrara in northern Italy. Its smooth crystals and whiter-than-white colour have made it the rock of choice for sculptors and architects from the Romans on. But like all marble, Carrara started life as seafood.
Seashells are made of calcium carbonate, or lime. Calcium carbonate is not soluble in water, so shells sink to the seafloor when their residents die. With enough weight on top, the water molecules between the minerals get squeezed out, and the calcium carbonate becomes cemented together to form limestone, a type of sedimentary rock. And that's how the bit of seafloor now known as northern Italy formed over hundreds of millions of years.
From marinara to Michelangelo
That limestone seafloor would be still sitting there today if the Earth wasn't covered in plates that keep smashing into one another. About 40 million years ago, the particular plate this seafloor sat on was involved in some serious mountain building. It smashed into a neighbouring plate, thrusting the seafloor upwards and scrunching it up to form the Apennine mountain range. It's hard to imagine the massive forces involved all that earthmoving, but the weight from above and heat from molten rocks below were enough to cook the limestone underneath and change it into marble.
Heat and pressure rearranges minerals in limestone to form a much more complex crystal structure. The crystals are jammed together so tightly that the marble version of the rock is far more compact than the original limestone.
But while metamorphosis gives marble and limestone different looks and strength, chemically they're still peas in a pod. You can check for yourself by putting a few drops of acid on a bit of each rock (vinegar will do, but hydrochloric acid really gets things going). The acid reacts with the calcium carbonate in both types of rock to form bubbles of carbon dioxide. (Note: don't try this in Florence; they're kinda funny about their marble antiquities.)
Metamorphosis isn't confined to limestone. Every rock has got a metamorphic version that it will turn into, given enough heat and pressure. The metamorphic versions of other sedimentary rocks like sandstone and shale are well known, at least to home renovators. Quartzite kitchen benches were once sandstone, and slate tiles are the pressure-cooked version of shale.
Igneous rocks like granite and basalt are no strangers to heat and pressure — they're cooled down versions of magma (molten rock) — but they metamorphose into other rocks too. Cook up some granite under pressure and you'll get gneiss (pronounced nice), and metamorphosed basalt gives you amphibolite, a real personality rock.
The rocks they are a changin'
Metamorphosis is certainly a handy way to make new rocks from old, but you don't need smashing plates and rising mountains to give rocks a makeover. You just need lots of time and weather.
It might take millions of years, but a granite mountain range can be transformed into sandstone. Just ask Uluru.
An icon of ancient worlds and timelessness, the sediments that make Uluru started life as huge granite mountains about 550 million years ago. But even the toughest igneous rocks fall to pieces when they're bombarded with water, wind and heat. Weathering broke the granite down into crumbs of quartz which were transported hundreds of kilometres and eventually compressed to form the best-known block of sandstone in the world. But the story won't end there.
Weathering created Uluru, but it will destroy it too. The exposed parts of Uluru sandstone will be reduced back to sediment, although its lack of cracks and joins makes it a tough nut to flatten. Like all surface rocks, Uluru's future will be an ongoing cycle of wearing down to sediment, washing or blowing away somewhere, getting compressed into rock and wearing down all over again. If it's lucky, it might just end up near a bit of mountain-making action and metamorphose into a dashing quartzite feature wall or kitchen benchtop.
Thanks to Dr Paul Willis from the Royal Institution of Australia.
Published 03 March 2010