The Earth Story

As the ice melts, future rocks are being born

One thing that the gradual melting of the cryosphere is going to contribute to the geological record is a pulse of sediments that will take a variety of forms, from stones dropped by melting icebergs (and originally named dropstones) to the fine grained rock flour that is being carried into the sea by these braided rivers and waterfalls at the edge of Iceland's icecap. We can even see from way up in space that some braids of the lower streams are draining an area containing more sediment than the others. The plumes of sediment diffract the light nicely as they diffuse, giving the water its eerie bluish colour.

These sediments will reach the sea floor in time and will slowly turn into rock as further layers are piled above and the resulting heat and pressure expels the water and induces chemical reactions such those producing the common calcite or silica cements that bind many sedimentary particles together into rock. They will then sit for an indefinite time, until either some kind of tectonic uplift (such as a buoyant granite intruding below or a rise in the crust induced by a particularly hot bit of mantle passing under the plate) or a continental collision and attendant mountain building event exhume it back under the sky into the world of moving air and water again..

The particles of sediment were of course eroded by ice, wind and rain from the volcanic rocks that form the island (due to its position both on the Mid Atlantic ridge and above a probable mantle plume (see http://bit.ly/1FibMgN for an explanation)). Every rock has been through at least one, and often various iterations of this cycle in various forms. Some old high grade metamorphic rocks in cratons (see http://bit.ly/2n8hXhu) have been repeatedly overprinted by geological adventures, the information only teased out with complex and time consuming analysis.

As the world continues to warm with likely difficult consequences for us and much of the higher life with which we are (not)sharing the planet with over the next centuries followed by the millions years recovery time from a mass extinction, it's comforting to think that whatever happens, the rock cycle will endure for a good old time to come...

Loz

Image credit: Digital Globe_ _

Check out those braided stream and fjord views

Rocky landscape and braided stream, mountains of Kyrgyzstan

Braided stream in the mountains, Mount Rainier national Park.

Rugged peaks around Mount Rainier National Park

Braided stream choked with sediment, Mt. Rainier National Park.

Exit Glacier, one of the sights in Kenai Fjords National Park, Alaska. Melt water drains down the Moraine River, an excellent example of a braided stream.

Rakaia and her braids

The Rakaia River flows from New Zealand’s Southern Alps eastward into the Pacific Ocean, just south of Christchurch. It is a textbook example of a braided river: formed of multiple small channels separated by braid bars, flowing over a flood plane defined by stable banks. In the case of the Rakaia, the river bed in the highland section was formed by earlier glaciation, and is composed of wide shingle sediments up to 100m thick. It is fed by melt water from the Lyell and Ramsay Glaciers and flows 150km from its source to the ocean. High supply of sediment off the Southern Alps leads to a braided system that is up to 2km wide in places. The sinuous channels are constantly snaking and changing across the riverbed, which is wide enough to flood safely, allowing farming up to the banks. As it approaches the sea, the Rakaia runs into a lagoon separated from the ocean by a boulder bank, with the exit from the lagoon shifting with time.

~SATR

Image: DigitalGlobe satellite image of the mouth of the Rakaia river, passing through New Zealand’s lush farmland.

http://www.bbc.co.uk/learningzone/clips/the-rakaia-river-a-braided-river-valley/3075.html

Dimensions in biodiversity of a braided river: http://tinyurl.com/btagx9s

Flooding on the Irawaddy

These 2 images from the Landsat 8 satellite show the Irawaddy River in Myanmar; the upper image was taken during a normal August and the lower was taken in August of 2015.

In the upper image, the Irawaddy is a classic braided stream, wandering back and forth between a number of channels with bars and islands of sediment in the gaps in-between. Heavy rains and Tropical Cylone Komen struck the country in July, causing significant flooding on this river and bringing it up to the level where it covers virtually all of the bars in the channel. You can also see that the smaller tributary channels are swollen as well.

More than 100 people reportedly died in the flooding and large amounts of farmland have been lost for the year.

Floods like these are natural features; in fact the bars probably testify to previous floods. When a river like this reaches flood stage, it submerges the piles of sediment in the channel and begins mobilizing that sediment. Floods help take sediment out to sea and if they are not anchored by vegetation, the bars and channels can be moved around and reconstructed in the process.

-JBB

Image credit: NASA/USGS/Landsat http://earthobservatory.nasa.gov/IOTD/view.php?id=86394

The kinds of neat things geologists can do in a laboratory. Here’s a flume tank, set up to run water down a slope, loaded with lots of sediment so that we can watch how it is redistributed. Various river processes like growth of bars, avulsions, channel migration, and channel expansion all make appearances.

Braided Streams

Here’s one of my favorite fluvial features: braided streams.

The intricate network of joining, separating, and rejoining channels is what makes a stream braided. The braided channels shown here are part of the Brahmaputra River, which runs through Tibet.

Most braided streams form when a stream is “incompetent” or carries too much sediment. The excess sediment is deposited in sand bars that separate the channels, thus creating the intertwined look. Braided streams are especially common in glacial areas (where they often carry glacial melt laden with sediment) and in areas with variable seasonal flow. This particular river carries a lot of glacial flour, or very fine sediment that was powdered by glaciers—nature’s icy version of a mortar and pestle.

-CM

Photo credit and more information: NASA http://1.usa.gov/1M2nQ3n