My brother and I were on the big island of Hawaii over winter break, and we got in to an argument on how the cooled lava becomes, over time, capable of supporting plant life.
He said that the major factor in changing lava into soil was the cumulative eroding effects of raindrops bashing the lava into tiny pieces.
I argued that while that certainly happens, the resulting medium – a layer of tiny larva particles – cannot provide the nutrients needed by the lush vegetation that we saw growing there.
I said that a far more important factor was the tiny soil particles and organic matter in those raindrops originating in other parts of the world that are deposited on the lava when they fall.
While you were on the Big Island, did you walk down Devastation Trail? If you did, you saw various stages of life gradually emerging from a lava flow. It doesn’t start with “lush vegetation,” but with tiny spores finding their way onto the lava . . . with probably only a small fraction, with the help of rain water, resulting in tiny mosses and ferns. As these die, they become part of the new soil, and over time, hardier plants can begin to emerge . . . each, after its death, becoming new soil.
Good dirt is a balanced mix of sand, silt and clay. Each is a product of erosion and chemical breakdown, with clay being the the most weathered and sand the least. Some plants like rocky soil. You will notice an abundance of ferns growing in lava rock fields. These plants roots help break up the rocks and when they die, they impart their nutrients into the soil, making it suitable for other plants…
When I took a holiday on Lanzarote, I was told that the soil is very fertile, because the volcanic particles are porous and hold water. I don’t know if that’s right, or if its also true about Hawaii, but It’s what I was told, FWIW.
Nobody has mentioned bird poop yet. Especially on volcanic islands, this is a major source of the organic nutrients. I’m not sure how the proportions of nutrients compare to chemical weathering, but (having owned a bird) I’m inclined to favor a Poop Theory of Vegetation.
As far as inorganic nutrients, I read somewhere somewhen that the volcanic ash, which can blow quite a distance is the main source to keep uncultivated soil fertile. Of course, mostly the minerals are recycled in situ, but there are always some losses. As for organic material, doubtless some comes from birds, probably some is blown in and perhaps microorganisms that can themselves grow without organic inputs provide some. When I visited Mt St Helens maybe 20 years ago, there was an amazing amount of growth on land that had been utterly devastated less eight years earlier.
Yabbut, which comes first? Without berries to eat, there will be no bird poop. So soil must already be in place to grow vegetation with berries before the pooping birds can provide their input.
The same goes for insect-eating birds … no food supply for them until there’s insects munching on vegetation.
(I know your reply was somewhat tongue in cheek, but the pendant in me just *had *to reply.)
Thanks for all the good input everyone. I’m glad to have this information, even though it indicates that my brother was more correct than I was.
