This short walk has it all, an almost perfect caldera, fossils and a chance to see garnets and sapphires in the wild as it were
A Fistful of Garnets
This short walk has it all, an almost perfect caldera, good views, fossils and an excellent chance of finding garnets and sapphires in the wild as it were.
Crater del Hoyazo Coral Rim
Garnets have been the gem of choice for the rich and famous for thousands of years. Red garnet necklaces have been found in the tombs of Egyptian Pharaohs and ancient Romans used garnets in signet rings to stamp wax seals on important documents.
Garnets form in metamorphic rocks and in igneous rocks, that is rocks that have formed from magma, from deep in the earth’s crust. Normally garnets require extremely high temperatures and most importantly, extreme pressure, and form in metamorphic rocks; rocks altered by high pressure after they have formed such as schist, gneiss, quartzite and marble. Typical metamorphic rocks are severely folded in places where tectonic plates meet.
More rarely, garnets form in igneous rocks such as rhyolite. Why and how they form in such rocks is, believe it or not in this modern scientific age, not clearly known. What is known is that in order to form in igneous rocks there has to be a presence of manganese. So, what do we know about garnets that form in rhyolite, which is a silica rich volcanic rock.
Carter del Hoyaza Caldera
Start with a volcano erupting rhyolite, either as a flow or a very hot ash that formed dense deposits. These materials would have started at around 700-900 °C, and contained a lot of gasses. Those gasses, plus heated groundwater, would be looking for a way out. They would seek out joints and fractures and find their way into hollow cavities. Water, hydrogen, and carbon monoxide would continue on their journey, while some metals like manganese and aluminium, plus other condensable elements, would begin condensing into crystals. The temperatures would still be relatively high: we know they must have been around 600°C. Snuggled into the flows, where cooling was slower, crystals grew. Our garnets are red and formed of manganese, aluminium and silicon oxide. The Greeks had a name for them: pyrope, meaning ‘fire eyed’.
And this is how we find them today, tucked within voids inside the ryolite, which we never would have expected to contain such a thing.
Rhyolite and Garnets
Sapphires also form in igneous rocks and the process is much the same as described above. They are made of aluminium and oxygen and are the second hardest mineral after diamond. Normally coloured blue due to the presence of iron and titanium, ours are a rarer species, colourless, with no impurities whatsoever, known as white sapphires. The size of the sapphires (small) also tells us that the rhyolite in which they are found cooled relatively rapidly.
View of the Coast
So much for the glittery bits. Now we can look at the rhyolite that has an equally interesting history. Before I go any further, I will tell you that we are in the caldera of a volcano near Nijar which is on the northern edge of the Cabo de Gata, a famously volcanic landscape.
Rhyolite, Fossil Coral, Garnet and Sapphires
The geological history of the Cabo de Gata is a fascinating subject but hugely complicated. Briefly, 15 million years ago, the whole area was beneath the sea. The African tectonic plate was and is pushing against the Eurasian plate. In the Cabo de Gata area, they meet in a more or less straight line that runs southwest to northeast along a line called the Carboneras fault. The fault cuts deep into the earth’s crust, the source of the magma. About 10 million years ago, eruptions beneath the sea formed volcanoes, just the same way as if the eruptions had been above the water except, due to the water, the lava cooled more rapidly than it would in air, hence the small crystals mentioned above.
To confirm the fact that the volcanoes were beneath the sea, the caldera of the Nijar volcano, that only measures something short of a kilometre in diameter, has a rim that is formed of fossilised coral, 250 metres above sea level.
About 7.5 million years ago, the pressure of the two tectonic plates pushed the Nijar volcano into the air, along with the other Sierras in this part of Almeria. The seawater escaped through a weak part of the rim on the south side of the caldera, forming a neat entrance into the caldera itself.
The geology and chemistry is the hard part. The walk is easy and starts at a track on a road that runs parallel to the A7 at Nijar. Leave the A7 at the Nijar junction and head towards the village. After 200 metres you will see a low, yellow building to your right and an unmetalled track leading off to your right. Follow this track for about 1 kilometre. The path you want is an obvious feature leading to a cleft in a ridge. It follows the line of the Barranco Hoyazo.
Walk up this path, which is quite steep and eroded with water channels in places until you reach the cleft. You are now standing on the southern edge of the caldera where the sea escaped so many years ago. The caldera is almost circular and the far rim is just less than 1 kilometre away.
Follow the obvious path down into the caldera. The four humps in the centre are the remains of the granite extrusions where the lava erupted from the earth, they are called plugs. If you look up at the rim you will see that there is a band of sandy coloured, fragmented, coarse rock. This is the remains of the coral reef formed when the caldera was beneath the surface of a shallow sea. You will find pieces of this fossilised coral lying around. Some contain fossils. The one we found was that of a cockle like shellfish but, being at least 7.5 million years old, it is unlikely to be a species living today.
Beneath the coral rim the pale grey rock is the rhyolite. Rhyolite is similar to granite in its chemical composition, both are igneous rocks; the differ because granite crystalises out slowly at great depth, sometimes 10s of kilometres beneath the earth’s surface whilst rhyolite crystalises rapidly on the surface. The result is coarse grains in granite and fine grains in rhyolite. Granite is known as an intrusive rock, rhyolite is a volcanic rock. Mid way down the caldera, in patches, the rhyiolite is exposed. In some of those exposures you will find small red crystals sticking out of the rock ; garnets.
In addition to being fine grained, this rhyolite is also fairly soft. Over the millennia it has eroded and been washed down to the bottom of the caldera where you will see deep water channels carved into the soft rock. This is where the garnets gather, as well as the sapphires if you are lucky.
You can return the way you came or continue by following the Barranco Hoyazo path up the north east side of the caldera to the rim. An easy path takes you northeast down to the Barranco del Algarrobo. Turn right until you hit an unmetalled track and turn right again to return to your vehicle for a total distance of 4 kilometres.
Please note. It is illegal to remove rocks or minerals from the confines of a Natural Park.