This from Wikipedia: ...Clays are distinguished from other small particles present in soils such as silt by their small size, flake or layered shape, affinity for water and high plasticity index."
I thought clays were also characterized by ionic polarity (like water, thus the affinity for water), while silt was not. Both, being very small particle size, settle slowly in water, and thus take a lot longer to accumulate than larger granular sediment (like sand (or shells)) when they have to settle out of the water column.
quote:Clay deposits are mostly composed of clay minerals (phyllosilicate minerals), minerals which impart plasticity and harden when fired or dried, and variable amounts of water trapped in the mineral structure by polar attraction.
Or am I mixing definitions:
quote:The distinction between silt and clay varies by discipline. Geologists and soil scientists usually consider the separation to occur at a particle size of 2 Âµm (clays being finer, silts being coarser), sedimentologists often use 4-5 Î¼m, and colloid chemists use 1 Î¼m. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg Limits.
Defining it by size alone seems a little arbitrary to me, especially when you can have particles above and below that limit that exhibit plasticity and particles above and below that limit that don't exhibit plasticity (or should we have 'plastic clays' and 'non-plastic clays'?).
quote:In the Udden-Wentworth scale (due to Krumbein), silt particles range between 1â„256 and 1â„16 mm (3.9 to 62.5 Î¼m), larger than clay but smaller than a sand. In actuality, silt is chemically distinct from clay, and unlike clay, grains of silt are approximately the same size in all dimensions; furthermore, their size ranges overlap. According to the USDA Soil Texture Classification system, the sand-silt distinction is made at the 0.05 mm particle size.
quote:Clay minerals are typically formed over long periods of time by the gradual chemical weathering of rocks (usually silicate-bearing) by low concentrations of carbonic acid and other diluted solvents. These solvents (usually acidic) migrate through the weathering rock after leaching through upper weathered layers. In addition to the weathering process, some clay minerals are formed by hydrothermal activity.
This implies that all clays take a long time to make, but I also thought that clays were formed by (mixing water with) volcanic ash?
quote:Volcanic ash consists of very fine rock and mineral particles less than 2 mm in diameter that are ejected from a volcanic vent. Ash is created when solid rock shatters and magma separates into minute particles during explosive volcanic activity. The usually violent nature of an eruption involving steam (phreatic eruption) results in the magma and perhaps solid rock surrounding the vent, being torn into particles of clay to sand size.
Which would be a fairly rapid process for making clay (mixed with larger particles) layers, and the different size particles could be separated by erosion and different settling rates ...
The chemistry of ash is very diagnostic and can be chemically characterized by looking at grain sizes, mineralogic compositions, and trace-element compositions. Each volcanic eruption, even if it's from the same volcano, will be slightly different, and so by studying these differences and mapping out the extents of the ash layers themselves, we can often trace them back to their source.
When deposited on land, these ash layers can remain relatively fresh retaining their primary mineralogy, though glass will devitrify (to clay) over time in the presence of water. When the ash falls on water, however, like the ocean or an inland sea, and is subsequently deposited in a subaqueous settings, the glass shards quickly devitrify into clay particles. (Note: devitrification means that the amorphous glass structure, which is unstable at surface conditions and free of water, will change into a more stable, orderly, and water-bearing form, and so in the case of volcanic glass, the glass converts to clay - typically bentonite.)
Some of the larger ash particles become clay in water?
Precambrian from their stratigraphic position and complete lack of fossils. That makes them some of the oldest sedimentary rocks in the world. It's unusual to find supracrustal rocks (rocks that are deposited on the earth's surface like sediments or lava flows) this old that havn't been metamorphosed (deeply burried and altered mineralogically).
Do we have a sedimentary layer without (recognizable) signs of life near the bottom of the canyon, and a layer above that ...
Message 143 (Jar): Now for the big question. It seems that this last layer is the first to actually show signs of live, but only of cyanobacteria. Is this correct?
Message 144 (Rox): I believe the first signs of life in the Grand Canyon are visible in the Bass Limestone. I recall reading about stromatolites in the Bass.
I thought the Bass (3a?) was above the Vishnu Schist (1a) and the schist was metamorphic, and that only leaves group 2 below (did I miss something between the Vishnu and the Bass? (and is this where the layer(s) got "pinched"?)
Message 1: I'd like for us to discuss the Grand Canyon, beginning with the lowest, oldest layers and then working up to the top, layer by layer. I'd like to see explanations for each layer, it's composition, the environment when it was created, and get questions about that layer answered before we move up to the next layer.
If possible, can we begin with the Vishnu Schist?
This is a great thread that I would like to see continued.
Exposed by a rockfall, the oddly angled tracks offer a rare glimpse into early animal behavior.
Rowland learned of the tracks by chance back in 2016, when a paleontologist friend of his was hiking the Grand Canyon with students. As they hiked Bright Angel Trail, the group saw a rock that had fallen out of the cliffside and cracked open.
The busted boulder had split along an inner seam, revealing a natural mold and cast of 28 footprints stretched out in a line more than a yard long. The friend let Rowland and park officials know, and a field crew pulled the rock off to the side. In May 2017, Rowland visited the prints, and in March 2018, he returned with San Diego State University geologist Mario Caputo to study them.
Exposed by a rockfall, the footprints are now the oldest ever discovered in the Grand Canyon, scientists reported this month at the Society of Vertebrate Paleontology's annual meeting in Albuquerque, New Mexico. The prints are also noteworthy for their strangeness: Each one is angled 40 degrees from the direction of walking, as if the animal had a diagonal gait.
Tantalizingly, these prints were made right around the time that reptiles' oldest ancestors started to diversify, and they resemble 299-million-year-old footprints found in Scotland made by early reptiles or amphibians with reptile-like proportions. If similar animals made both trackways, the Grand Canyon tracks could be the oldest footprints of their kind by more than 10 million years.
The team is also debating why the footprints are angled so weirdly. Was the animal struggling to walk into a strong wind? Or did it angle its gait as it walked down a sand dune, so it didn't slip? Caputo's study of the sandstone will help tease out these scenarios.
“If the exposure allows it, it will enable me to say, okay, this organism was walking near the crest or the summit of this dune ... or perhaps in an area in between dunes,” he says. Such specifics will let researchers reconstruct a single moment in the life of an animal that died more than 300 million years ago—an impressively intimate scientific feat.
Dry sand dunes ... footprints ... too bad it doesn't say which sandstone layer the boulder came from.