Deep Pacific Upside Down Waterfall

This passage from Helen Czerski's Blue Machine: How The Ocean Shapes Our World gives us a glimpse of the wondrous undersea universe we are just beginning to explore.

"We see upside-down waterfalls, she says. I don't understand what she means at first, and it takes me a few seconds to process the video as Deb keep talking. In those vertical chimneys, the walls crack and hydrothermal fluids come leaking out and  you get something that looks like half a toadstool growing out of a tree in an old growth forest. And suddenly I see it. This is a gigantic hydrothermal chimney looming out of the darkness, and hot water is indeed leaking out of its side. But because hot water is less dense than cold water, the hot water keeps flowing rapidly upwards. When it first hits cold water, its clearly dumped some minerals and made a ledge that sticks out- that's the toadstool shape that Deb is referring to. The water flowing upwards has had to flow outwards underneath the ledge before it can carry on upwards. But the ledge has developed a hollow on its underside like an upside-down bowl, so there is a pool of hot water there, held in the hollow as if it were filling up the inside of an umbrella. The boundary between hot and cold water shimmers like a mirror. And then the hot water is spilling out of its hollow and continuing upwards into the gloom. It really is an upside down waterfall".

Helen Czerski is watching this footage captured by a remotely operated vehicle exploring the area around the Juan de  Fuca Ridge, an undersea mountain chain a few hundred kilometers west of Seattle. Here, the Pacific and the Juan de Fuca tectonic plates diverge. Scientists are closely monitoring this ridge for seismic and volcanic activity, using a network of sensors  called the Regional Cabled Array. Deb Kelly is the Director of this project. Hydrothermal chimneys are sulfide and carbonate mineral deposits that form when hot mineral saturated sea water emerges through cracks in the ocean crust. The are common near mid oceanic ridges where the interaction of sea water and rock heated up by magma generates vigorous hydrothermal systems.

I'm only a quarter into this book and am enjoying every page of it. Highly recommended!

Articles: Earthquake Temperatures, Marshes, Ocean Drilling

Sharing some good stuff I came across recently.

1) Taking the temperature of faults. From AGU News. Rocks get hot as they slide past each other during faulting. Chemical changes in organic molecules and helium content in zircon crystals are sensitive to temperature changes. Scientists have used these to estimate short lived temperature rise during faulting. Understanding patterns and magnitude of earthquake generated heat informs us about earthquake intensity, heat dissipation, and fault movement history. 

2) Why a Marsh: "Neither land nor water, maybe both, a marsh is a balancing act, a collaboration between changing elements, uncertain by its very nature, in flux"

A beautiful long essay by Daniel Wolff and Dorothy Peteet on marshes and wetlands and the vital role they play in ecosystem functioning. Especially fascinating is the description of Piermont Marsh, along the banks of the Hudson river, north of New York City. Sediment cores going back a thousand years preserve a record of climate change and human modification of the landscape. The marsh was first exploited by Native Americans and then more extensively by European settlers. It is a detailed look at the wealth of information a wetland can yield about climate, ecology, and human disturbance of the environment.

3) Drilling Into the Sea Bed: This is a really interesting summary of scientific ocean drilling by Neena Notman . They are attempts going on to reach the earth's mantle by drilling into the sea bed. Although drilling in the deep ocean seems daunting, this makes more sense than trying to reach the mantle from land. The crust making up the continents is much thicker. Ocean bed drilling offers a short cut to the earth's mantle. We know about the mantle mostly through geophysical data. Actual samples of the mantle are rare. Recovering pristine pieces of the mantle will allow us to validate what we've gleaned from geophysics. Drilling is also going on along a subduction zone to understand the nature of fault interface and rock properties.

 

Oceanic Crustal Thickness Since The Breakup Of Pangea

Of interest:

Decrease in oceanic crustal thickness since the breakup of Pangaea - Harm J. A. Van Avendonk, Joshua K. Davis, Jennifer L. Harding and Lawrence A. Lawver

Earth’s mantle has cooled by 6–11 °C every 100 million years since the Archaean, 2.5 billion years ago. In more recent times, the surface heat loss that led to this temperature drop may have been enhanced by plate-tectonic processes, such as continental breakup, the continuous creation of oceanic lithosphere at mid-ocean ridges and subduction at deep-sea trenches. Here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time. We find that oceanic crust formed in the mid-Jurassic, about 170 million years ago, is 1.7 km thicker on average than crust produced along the present-day mid-ocean ridge system. If a higher mantle temperature is the cause of thicker Jurassic ocean crust, the upper mantle may have cooled by 15–20 °C per 100 million years over this time period. The difference between this and the long-term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss. We also find that the increase of ocean crustal thickness with plate age is stronger in the Indian and Atlantic oceans compared with the Pacific Ocean. This observation supports the idea that upper mantle temperature in the Jurassic was higher in the wake of the fragmented supercontinent Pangaea due to the effect of continental insulation.

Continental insulation refers to the idea that an unbroken continental crust such as that provided by a supercontinent may act as a blanket resulting in a slow build up of heat over tens to hundreds of millions of years in the underlying mantle. Eventual continental breakup will lead to enhanced magmatism and thicker ocean crust along these previously insulated regions.

The Pangaean paleogeography of the Triassic (252 million to 201 million years ago) is depicted in the map below. The distribution of continents is lopsided covering the sites of the future Atlantic and Indian Oceans.

 Source: Paleobiology Navigator
 

Why Is There A "Lost Continent" Underneath Mauritius

Yes, the term "lost continent" brings up visions of a lost world full of fantastic creatures that once existed deep in the earth's past. Or, of a civilization that once was, but was swallowed up by rising seas and which now only remains on the margins of human memory.

The "lost continent" underneath Mauritius is making news. It is more accurate to say that there is continental crust underneath the oceanic lavas of Mauritius. And that continental crust is very old. Geologists found crystals of zircon in young lava that erupted on Mauritius about 5.7 million years ago. The age of the zircon is however Archean in age, between 2.5 billion - 3 billion years. That means the zircon crystal did not form in the young lava, but belongs to the older foundation of the island. They were extracted from this Archean crust by rising molten material and brought to the surface about 5. 7 million years ago.

The crust making up the earth continents is primarily made up of granitic and andesitic rocks and sedimentary cover. This crust is light and thick (30km-40 km) and it sticks above sea level.  On the other hand, the crust making  up the ocean basins is made up of basalt and is denser and thinner (~10 km). So, what is Archean continental crust doing in the middle of the Indian ocean, surrounded by Cretaceous-Cenozoic oceanic basaltic crust?

The answer lies in the way Gondwanaland broke up, or rather the way India broke away from Madagascar about 88 million years ago. This was a continuation of the progressive breakup of Gondwanaland that began in the late Jurassic about 150 million years ago. A large rigid continent need not break into two clean pieces. Very often, the edges splinter. Several smaller fragments of continental crust are left isolated near the edges of the two continents.

The map below shows these continental splinters scattered in the Indian ocean as Madagascar and India broke apart and drifted away from each other.

Source: Lewis D. Ashwal, Michael Wiedenbeck, and Trond H. Torsvik 2017

Mauritius is part of a series of splinters that collectively are called Mauritia. These splinters were part of the Archean continental nucleus that made up Madagascar and the western Dharwar craton in south India.

Here is the interesting part that many news reports haven't touched on. Look closely at the map above. Trace the Carlsberg Ridge southwards. The Indian plate, which is drifting northwards, lies to the east of this ridge and the African/Somali plate to the west. Today, Seychelles and Mauritius is on the African/Somali plate and Chagos and the Laccadives on the Indian plate. But, when the initial separation happened about 84 million years ago, Seychelles and most of Mauritia were on the northerly drifting Indian plate. This is because 84 million years ago, the plate boundary between the Indian and African plates was formed by sea floor spreading in the Mascarene basin.

This is depicted in the  paleo-geographic reconstruction below. At 65 million years, the CIR or Central Indian Ridge is where sea floor spreading is forming the Mascarene basin. Seychelles and Mauritia lie to the east of this ridge on the Indian plate.

Source: Shankar Chatterjee et. al. 2013

Later, beginning around 62 million years ago and continuing up to about 41 million years ago, the loci of sea floor spreading jumped eastwards. The result was the formation of new plate boundaries between the Seychelles and Laxmi Ridge (62 mya) and between Mauritius and Chagos/Laccadives (42 mya).  These "ridge jumps", as they are called, formed the Carlsberg Ridge and  transferred Seychelles and Mauritius on to the African/Somali plate. Continued northward drift of India coupled with sea floor spreading and the formation of new oceanic crust along the Carlsberg Ridge has formed the broad oceanic basin of the Arabian Sea/ Indian ocean.

The process of continental breakup involves extensional forces that stretch and thin the crust. Fault movements cause a subsidence of crustal blocks. Many of the splinters at the edge of major continental margins are such thinned downfaulted blocks. They thus often get submerged under the sea.

Open Access

Map: Thickness Of The Crust

A ten km contour interval? Well yes, if  you are mapping the thickness of the earth's crust!

This map brings out beautifully the distribution of the two distinct types of crust on earth. Crust making up the continents is granitic to andesitic in composition, buoyant and is old. Crust making up the ocean basins is mafic in composition, gravitationally unstable (it is heavier and it subducts) and is young.

The 30 km contour outlines roughly the continental crust:

I got this from - The Continental Record and the Generation of Continental Crust (open access)

How does the earth look in terms of its topography? There is a bimodal distribution of the surface elevations on earth. This is a consequence of the contrasting chemical-mechanical properties of the continental and oceanic crust. The figure below brings out the distribution.

Why is there some continent  below sea level? Plate tectonic configuration can be such that at different coastlines continents could be in the act of converging with an oceanic plate. Or, having long broken away from another continent, they posses a passive or divergent margin. At such passive margins, continental crust does not end at the present day coastline, but extends further out until the edge of the continental shelf where there is a sudden deepening of the sea floor. That is roughly where the ancient continent broke up. As it drifted away, new oceanic crust formed between its conjugate continent on the opposite side. Sea level rise after the last glaciation has flooded continents, thereby submerging portions of these passive margin low gradient shelves.

Source: The Continental Record and the Generation of Continental Crust.

Sometimes, a single map or a graph can bring out a fundamental truth about the making of the earth.