Given enough time, almost anything can happen, and even unusual things can get preserved in the rock record. Consider how uncommon geysers are on the earth today. Most of them are in Yellowstone National Park, and most of the rest are in Iceland and the Kamchatka Peninsula, with a very few in other places. If that sparse distribution is anything like it was in the past, then we would not expect to find the kinds of rocks deposited by geysers and other hot-water sources very often, and we don’t. But we do find some.
The stuff that makes most of the mounds around the geysers at Old Faithful is silica, silicon dioxide, essentially quartz, the most common mineral in the earth’s crust – or it might be amorphous opal, the same thing but with water in its molecular structure. It’s called siliceous sinter or geyserite. Note that the online Encyclopedia Britannica says Mammoth Hot Springs is composed of geyserite – but it’s not, it’s calcite.
So you could eventually get a big enough build-up of siliceous sinter to be retained in the rock record. And indeed there is one, from about 400 to 412 million years ago, early Devonian, at the village of Rhynie, Scotland, northwest of Aberdeen. The rock is called chert, which is the common term for very fine grained, cryptocrystalline (that means hidden crystals, they are so tiny) quartz.
The Rhynie Chert is quite remarkable simply because it has been preserved – and it is so well preserved that the actual geyser vents can be seen. They are the oldest geyser vents known.
But it’s even more significant than that. The Rhynie Chert is a lagerstatten – a collection of fossils that are simply amazing in their preservation and completeness. As the very fine silica was deposited, it coated and trapped plants and animals, preserving them with microscopic detail intact, right down to the cellular level. At Yellowstone you sometimes see plant stems encased in sinter. It’s like that, but with such detail that this is one of the most important localities in the world for understanding early Devonian life on land.
The Rhynie Chert is a small part of the complex system of rocks called the Old Red Sandstone that we talked about May 3. The location was well into the Caledonian Mountain belt, and the hot waters that made the chert were probably related to the mountain building and associated faulting.
The details of plant anatomy that are preserved in the Rhynie Chert are the best of this age anywhere. The plants are pretty simple – fungi, algae, and lichen, but also including some higher vascular plants with stems that are differentiated into xylem and phloem, the key structures in modern plants that allow them to transfer nutrients and water throughout their bodies. Some of them had scales extending from the body – not true leaves, but perhaps an early expression of structures that would evolve into leaves. In reconstructions, they look a little like small cacti with fleshy scales instead of spines.
Although animals are much less common in the Rhynie Chert than plants, it’s still the most diverse collection of terrestrial animals known from the Devonian or older. All the animals are arthropods, the group that includes arachnids, insects, crustaceans, eurypterids, and trilobites. One group of arachnids in the chert look a lot like spiders, and some that are about 6 or 8 millimeters long are so well preserved that things like their book-lungs, mouth parts, and muscle tendons can be examined. That simply does not happen with any kind of frequency. There are also small harvestmen – daddy-longlegs – and small shrimp, mites, centipedes, and other animals.
There’s an outstanding website with great information and photos of the Rhynie Chert and its fossils, provided by faculty at the University of Aberdeen, Scotland, and the National Museums of Scotland.
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On May 8, 1902, Mont Pelée erupted, destroying St. Pierre, Martinique, in the West Indies. All but two of the 30,000 inhabitants were killed by pyroclastic flows that came into the city within minutes of the eruption. Pyroclastic means “fire-borne broken pieces,” and a pyroclastic flow or nouée ardente in French, meaning “burning cloud,” is a mass of hot incandescent gas, ash, rock, and debris that is dense enough to hug the land surface rather than being erupted high into the atmosphere. Pyroclastic flows can reach temperatures of 1,000 °C (1,830 °F) and can flow as fast as 100 miles per hour or more. St. Pierre didn’t have a chance.
—Richard I. GibsonRhynie Chert website