Monthly Archives: December 2018

Marie Tharp and the mid-Atlantic rift; a prelude to plate tectonics

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Map of North Atlantic mid-ocean ridge

The history of science is littered with the misplaced contributions by women, contributions that at best were pushed aside or ignored, and at worst thought of as shrill outbursts. Witness Rosalind Franklin’s fraught journey to DNA’s double helix, the recent unveiling of Eunice Foote’s experimental discovery of the greenhouse effect of CO2, and Bell Bernell’s discovery of pulsars, as corrections to a history where women found it difficult to escape the status of ‘footnote’. We can add Marie Tharp (1920-2006) to the growing list of corrections. In 1952 Tharp discovered the central rift system in the mid-Atlantic Ocean ridge (that later would become a critical component of sea floor spreading and plate tectonics) but for many years was regarded as a minor player in the burgeoning, post-war field of oceanography.

During the War, Tharp in her early twenties took advantage of opportunities to engage in university study, openings in science and engineering left by men who had gone to battle. She completed a Master’s degree in geology, but given that geology is a field-based discipline, and that women weren’t supposed to go into the field, she extended her studies to a Master’s in mathematics. In 1948 Lamont Geological Laboratory (now Lamont Doherty Earth Observatory) hired 28 year-old Tharp to draft maps of the Atlantic ocean floor, based on the growing database from SONAR and historical soundings. She worked with well-known geologist-oceanographer Bruce Heezen, who spent much of his time at sea. It must have been tedious work, but she counted herself lucky to have a position at all. This was a time when very few American universities (or anywhere else for that matter) offered science and engineering positions to women; a time of patriarchal condescension – “Mad Men” versus “Hidden Figures”.

 

description of SONAR
Tharp poured over depth and positional data for years, constructing 2-dimensional profiles of the Atlantic Ocean floor. She was aware, as other oceanographers were that an elevated region of sea floor apparently separated east and west Atlantic. This was initially mapped in 1854 by US Navy oceanographer, geologist and cartographer Matthew Maury, and later confirmed with depth soundings taken during the HMS Challenger expeditions (1873-1876 – Challenger had 291 km of hemp onboard to do this kind of thing; the ridge is generally deeper than 2000m). Tharp wasn’t surprised to find the Atlantic ridge on her profiles. What did catch her attention was the rift-like valley in the central part of the ridge; a geomorphic structure that was consistent through all her profiles. She immediately recognized the importance of this, because it implied significant extension, a pulling apart of Earth’s crust in the middle of the ocean. At the time, the general consensus was that ocean floors were relatively benign, featureless expanses. Her discovery indicated otherwise.

 

bathymetry profiles mid Atlantic

According to Tharp’s bio the response by Heezen and his colleagues was that she was being a typical woman – you know, “girl talk”. One can imagine the coffee room banter; ‘she’s great at drafting cross-sections but should leave the interpretation to the more learned’.

However, after some months and more profiles all showing the same rift- like structure, Heezen gradually accepted that this was real. A turning point for Heezen was the coincidence of several mid-ocean earthquake epicenters along the ridge. This was mid 1953. He understood its potential significance, particularly for those who thought that the hypothesis of continental drift had some credence (Heezen was not initially one of those people).

Ocean bathymetry studies in other basins in the early 1950s (Indian Ocean, Red Sea) revealed similar mid-ocean rifts. Tharp had by this time surmised that a rift valley coursed its way almost continuously the entire length of North and South Atlantic, a distance of 16,000 km; it was the largest continuous structure on Earth. The Lamont Doherty group gradually realized that the Atlantic structure, together with those discovered in other ocean basins, represented a gigantic Earth-encircling system of mid-ocean rifts, more than 64,000 km long.

Heezen presented their research to a 1956 American Geophysical Union conference in Toronto. Marie Tharp barely received a mention. She did co-author a few subsequent publications as an ‘et al.’, but it was a kind of ‘also ran’; the accolades and approbation went Heezen’s way.

Tharp was fired by the Laboratory, the victim of a spat between Heezen and Lamont boss Maurice Ewing, but she continued to develop the maps at home. Marie continued to work in the background, as the humble and grateful recipient of a job she considered to be fascinating; “I worked in the background for most of my career as a scientist, but I have absolutely no resentments. I thought I was lucky to have a job that was so interesting”.

Marie Tharp and Bruce Heezen

Marie Tharp was named one of the four great 20th century cartographers by the Library of Congress in 1997, was presented with the Woods Hole Oceanographic Institution Women Pioneer in oceanography Award in 1999, and the Lamont-Doherty Heritage Award in 2001.

There is no question that Tharp’s discovery influenced the promotion of Continental Drift in the geoscience community. Alfred Wegener’s bold hypothesis (1915) had one major problem – there was no known mechanism that could move oceanic crust and continents around, like some precursor shuffle to a jigsaw puzzle. In 1929 Arthur Holmes posited a mechanism that involved large convection cells in the mantle, but this too lacked an important degree of empirical verification. Discovery of the mid-Atlantic rift provided a solution to this vexing problem, and in 1962 Harry Hess proposed that new magma, via mantle convection cells, was erupted from mid-ocean rifts allowing oceanic crust to spread outwards. This was Sea Floor Spreading, a precursor to the grand theory of Plate Tectonics – the tectonic shift in geological thinking wherein oceanic crust is created at mid-ocean rifts and consumed down subduction zones, with the continents playing tag.

Marie Tharp’s doggedness in her belief and understanding of mid-ocean rifting is now celebrated. It’s taken a few decades, but she is no longer a footnote.

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Atlas of soils and weathering

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typical soil profile

Soil, dirt, mud

The stuff kids get covered in when they’re having fun.

A veneer on Earth’s crust that provides sustenance in one form or another for all land-air living creatures, including us.  Without soils there would be no food web. No us!

Homo sapiens has learned to use soil to her advantage; growing things to eat, to construct shelter, to decorate. We have learned to utilize soils to the hilt. In fact on a global basis we have  taken so little care of them that they have become an endangered species. In our haste to produce food, to irrigate, to scythe through forests, to clear land for some other use, we have damaged soils, in many cases beyond repair.  Humanity, in its ignorance, greed and hubris, has managed to seriously compromise the utility of soils – the very things that make life viable.

From a structural perspective soils are quite simple; there is topsoil that contains a mix of organic matter derived from plants, macro- and micro-organisms, plus minerals derived primarily from the underlying sediment or rock (parent material).  This structure is illustrated in the profile image above.

From a biological perspective, soils are complex. Apart from the obvious worms and small critters, there is a burgeoning microflora – fungi and bacteria; and it is the microflora that does most of the work to create a vital growing medium. The microflora breaks down fresh organic matter converting it to humus, and converts nutrients like nitrogen, potassium, phosphorous and many trace elements, into water-soluble forms that plants can metabolize. A vital topsoil requires a healthy microbiota. Soils that are regularly exposed to herbicides, bactericides and fungicides will, over time, become depauperate in useful microflora.

soil fungal threads

Classification of soils can be complicated. In this Atlas I use a simple textural classification, summarized in the diagram below. It is a US Dept. of Agriculture classification; most other countries use this or slightly modified versions.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other Atlas categories.

Click on the image for an expanded view, then the ‘back page’ arrow to return to the Atlas.

The images

soil profile soil on volcanic ash

Left: Silt-sand loams developed on Late Pleistocene fluvial pebbly volcaniclastics (Hinuera Formation, Waikato, NZ). The transition from modified sediment to parent material is irregular. Right: Topsoil developed on multiple ash layers; remnants of an older (fossil) soil is located above the pale band of ash (Waikato, NZ).

 

soil on ignimbrite soil on co-ignimbrite ash

Thin (pumice) loam on  airfall ash, that overlies partly welded ignimbrite (the orange ash layers may be co-ignimbrite). Iron oxide staining indicates groundwater seepage and mineral alteration. Mamaku, north of Rotorua N.Z.

 

soil on paleotopography

Three sets of small V-shaped paleovalleys and intervening ridges cut into welded Mamaku Ignimbrite (about 220,000 years old), draped by at least three ash fall deposits and thin paleosols (paleosoils). The entire outcrop is overlain by a more recent soil and modern vegetation. North of Rotorua, NZ

 

soil on coastal plain gravels

Thin sandy loam topsoil and iron-stained layer on coastal plain gravels. The upper part of the topsoil has been disturbed by cultivation. The normal position of the watertable is at the transition to gray gravel. Kaiua, Thames coast east of Auckland, NZ.

 

soil on sand dunes soil on sand dunes

Eroded sections of coastal dunes, Raglan, N.Z.. Spinifex roots penetrate up to 2m into the sand. Soils here are very low in organic matter, almost 100% sand with high permeability and little capacity for water retention.

 

Spinifex roots in sand sand dune vegetation

Coastal dune root systems (Spinifex and Lupin). Much of the organic matter is oxidized rapidly. Dune instability means that topsoil formation is meagre.

 

Pleistocene paleosols Pleistocene soil profile

Pleistocene dune sands are overlain by peat and leaf litter from an ancient coastal, Podocarp forest. Left: Old dunes cut by an uplifted marine terrace, capped by peat.  Right: Cross-bedded dune sands overlain by thin woody peat (O layer). The B layer here is sand enriched in iron oxides precipitated by ancient fluctuating water tables. Great Exhibition Bay, northern NZ

 

Pleistocene peat Pleistocene peat profile Pleistocene peat profile

Pleistocene woody peat and leaf litter at top (O layer), underlain by thick, blocky weathered, silt loams with abundant roots and buried logs. Great Exhibition Bay, northern NZ

 

reduction spots in sand dune reduction spots in dune sand

Semiconsolidated, cross-bedded Pleistocene dune sands beneath a peat. Here, the parent sands have reduction spots derived from leaching of iron oxides. Great Exhibition Bay, northern NZ

Liesegang rings

Liesegang rings are a common manifestation of shallow weathering in permeable sandstone.  The ring-like patterns form as iron oxides precipitate in concert with migrating groundwater. Mokau Sandstone, north Taranaki, NZ

 

soil iron pan in sand dunesinterdune lake deposits

Left: Remnants of a soil formed over stabilized Pleistocene dune sands. The abrupt steep-dipping contact with a younger set of dune deposits is delineated by resistant limonite pans. Right: The muddy loam indicated as ‘P’ represents a small Pleistocene interdune pond or lake; here overlain by multiple dune crossbeds. Kariotahi, west Auckland, NZ

 

spheroidal weathering in basalt spheroidal weathering in sandstone spheroidal weathering iron pan

Chemical weathering of bedrock. Left: Spheroidal weathering in an andesite lava flow promoted by mineral dissolution in shallow percolating groundwater. Late Pliocene Karioi volcano. Center and Right: A mix of spheroidal weathering and iron oxide precipitation in indurated Triassic sandstone-shale, where alteration patterns are strongly influenced by bedding and fractures; Kiritehere, NZ

 

muddy loam profile silt clay loam profile

Left: Profile of muddy loam developed on weathered turbidite sandstone-mudstone. Near Wellsford, north Auckland NZ. Right: Silty clay loam developed on Carboniferous shale, Cliffs of Moher, west Ireland

 

Alpine clay gravel loam Alpine clay gravel loam profile Alpine clay gravel loam profile

Gravelly clay loams and clays developed on steep alpine mountain slopes, Mt. Garibaldi, British Columbia. The organic layer is either very thin or absent. On the left, the parent material is probably lacustrine clay.

 

stoney silt clay loam Tuscany stoney silt clay loam Tuscany stoney silt clay loam Montefioralle

Typical stoney silt-clay loams of Tuscany; the location of Chianti Classico and olives. Bedrock parent material consists of Cretaceous-Paleogene marls, sandstones and shales. Left and Center: The 12th century fortification is Monteriggioni. Right: The hilltop village of Montefioralle.

 

permafrost soil patterned ground in permafrost

Arctic soils riven with permafrost. Left: Frozen muddy loam undergoing summer melt. Right: typical patterned ground in Arctic tundra.

 

weathered Atacama alluvial fan Atacama desert varnish

Arid soil in the Chilean Atacama commonly lack any organic component. Left: exposed surface of an inactive alluvial fan contains a mix of stoney material, sand, silt and mud. Calcite cements are common in some deposits. Right: Desert varnish, a common feature of prolonged, arid climate weathering. The varnish consists of silica, iron and manganese slowly leached from the original rock, and re-precipitated as oxides.

 

weathered lapilli soil

Weathering of basaltic lapilli below a very thin, incipient topsoil, shows gradual redistribution of iron oxides. The volcanic deposits are very young – about 800 years. Rangitoto, Auckland, NZ

 

soil creep Kansas soil creep and clay gravel loam soil creep and clay gravel loam

Left: Soil creep in sub-vertical shale, east Kansas, beneath a stoney clay loam. Center and Right: Old Red Sandstone bedrock extends into the low cliff where it is broken and gradually incorporated into gravelly colluvium by soil creep. The overlying thin sandy loam is formed predominantly on wind-blown sand. Red Strand, south Ireland.

soil creep County Cork

Soil creep in vertically dipping Devonian shale acts to incorporate shale slivers into the overlying clay-silt loam. Kinsale, south Ireland.

 

Burrens clints and grykes Miocene karst Takaka

Karst, common weathered landforms in carbonate rocks. Left: Clints and grykes formed by dissolution of carbonate along fractures in Carboniferous limestone, Burrens, Ireland. Right: Pinnacle dissolution structures in jointed Oligocene – Early Miocene limestone, Takaka, NZ (Image by Kyle Bland, GNS).

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