Mafic Dyke & Sills

MAFIC DYKE & SILLS

Stevens Island - NE Arnhem Land, Northern Territory Australia - Google Earth Image

The Stevens Island formation has all the elements of a ‘man-made’ stone fish trap - except that it is upside-down – instead of Horseshoe ‘Ʊ’ it is Omega ‘Ω’ shaped. It is therefore the result of natural processes.

The PastMasters Omega Proposition is that the lines of dark rock are Mafic Dykes - walls of solidified volcanic rock that filled cracks in the surface of the Earth and elsewhere. We are not claiming that Dyke Swarms explain every line of rocks everywhere - nor that everything is known about these extraordinary processes. You will see that the classification is not yet settled – even the three basic forms lack definition & the American variant ‘Dike’ does not help. These volcanic leads concentrate minerals - so are well known to prospecting geologists - as well as volcanologists and geologists in general. This is our attempt to bring these phenomena into the public domain, so that extraordinary claims may be tested empirically. Irrespective - to have lived on this planet & remained unaware of mother nature’s stretchmarks - would be a shame.

This page is an introduction to Mafic Dykes which are the result of volcanic activity. They range in size from seemingly isolated ribbons of rock through to Giant Radiating Swarms of which some 30 examples were recorded on Earth in 2011 – 16 on Mars and 118 Radiating Dyke Swarms on Venus (see Ernst et al below). In the intervening quarter of a century these numbers have grown significantly and now include the many hundreds of Mantle Plume Heads on Earth. The most recent Australian Dyke Swarm being discovered off Exmouth in WA. (see Magee & Jackson 2019 bottom of page)

Mafic Dykes Stevens Island. Google Earth

Stevens Is. NT - The Omega Trap - Google Earth Image

Stevens Island's Omega Trap is 47kms WSW of the 'Trap' in Australians Bay, at the SE corner of Marchinbar Island (aka Great Wessel Island). The trap shape is exacerbated by the angular cut of the foreshore whereby a circular diversion, in an otherwise straight wall, appears magnified.

The Dyke Swarm containing the Stevens Is. feature - belongs to the Galiwinku-Derim Derim (LIP) which is part of the North Australian Craton (NAC).
A Craton is a stable part of the Earth's crust at the centre of a continent. LIP is Large Igneous Province - an area of volcanic activity that brings iron & magnesium-rich rock to the surface.

There is a Large Igneous Provinces Commission within the International Association of Volcanology and Chemistry of the Earth's Interior. In 2020 their LIP of the Month was the Datong-Galiwinku Reconstructed Large Igneous Province which linked the LIPs of North China and North Australia.

The link is in Sources & Resources at the foot of this page. It has good diagrams and explains how Mafic Dyke Swarms are one of the visible features that spread out from a source volcano and its associated volcanos, plumes, vents etc. The majority of these traces are buried so they are only visible when the land is eroded, the harder areas are left as islands etched by the sea or when the land itself is deeply eroded by weathering that rivers cut cross-sections into the volcanic zone.

A map correlating Australia's lowest areas with volcanic activity and stone fish traps is presented below.

Diagram LIP Commission 2020

The NNW-trending Biberkine mafic dyke swarm, in the southwest of West Australia, is dated at 1390 ± 3 Ma which makes it co-existent with the Galiwin'ku dyke swarm. The extent of the dyke swarm is uncertain but aeromagnetic data suggest that the dykes are part of a linear swarm several hundred kilometers long. A link to the paper '1.39 Ga mafic dyke swarm in southwestern Yilgarn Craton marks Nuna to Rodinia transition in the West Australian Craton' 2018 by J. Camilla Stark et al in Precambrian Research Vol. 316, October 2018, Pages 291-304 - see link in Sources & Resources below.

Mafic Dyke Swarms

Today, the Continent of Australia is not volcanically active - it was not always thus - between 500mya and 2.7bya there was extensive volcanic activity both below ground and on the surface - which was subjected to burial and erosion. In the Top End, we are fortunate to have a landscape in the Wessel Islands that provides glimpses of the processes involved. Mission Beach at Elcho Island was a beach some 500mya - it was subsequently buried by some 8000ft (1.7 to 2.2km) of sandstone laid down in the familiar layers of silt brought down by massive rivers from an eroding mountain range to the NW. The majority of this soft sandstone has now been eroded away - except where a layer of harder material, such as bauxite, consolidates the surface leaving a range of hills which the rising seas turn into a chain of islands. Where the islands are cut by the sea we are offered a near vertical section through the landscape that reveals the dark volcanic rock that has filled cracks in the Earth's crust as it has risen under pressure from the molten rock below. Rivers carve similar sections through the landscape down the deepest valleys to the coast and provide a useful cross-section.

Dyke Swarms are vertical walls of rock that criss-cross through the Earth's crust. As volcanic activity pushes up the crust from below - the fractures fill with molten rock. Up to 2.2billion years of erosion, burial and erosion later - they appear in the landscape in three basic forms of Parallel, Radiating & Curving lines which are MaFic because they are dark - being volcanic rock, high in Magnesium [Ma] & Ferric Oxides - so magnetic.

How these features are formed is discussed in the papers at the foot of the web page. Parts of them are intelligible to laymen and contain diagrams which may be helpful once the basic idea has penetrated the brain's defences. It is counter-intuitive because the forces are pushing up from below to split the crust like a loaf of bread and fill the gaps with molten rock from the centre of the Earth. All this occurs over thousands of millions of years, with the results lying buried below your feet until exposed when the land surface is eroded by weathering or cut away by either the sea, a river or a bulldozer.

The Kildonan Dyke Swarm - South Coast of the Isle of Arran - Off the west coast of Scotland - inside the Mull of Kintyre. Google Earth

By way of an introduction and acclimatisation - this page shows examples, from all over the planet, to demonstrate that it is a fundamental process which is now visible globally to us through Google Earth and especially visible upon beaches where the land is sectioned by the sea or where rivers excavate an inspection trench through the lowest parts of the landscape and reveal the dykes as narrow rock bars.

Dykes Felsite in Shale Le Pulec Jersey Geo Channel Islands

Pan-African Damara Sequence Brandberg Cape Town University

Faial Is. Portugal Google Earth

Dyke Faial Is. Portugal - H. Sooslu image

GARY HINCKS / SCIENCE PHOTO LIBRARY
Caption Igneous intrusion. Cut-away illustration of the features produced when rising magma (molten rock) intrudes into existing rock. Clockwise from upper right: a volcano, formed when magma erupts through a conduit to the surface as lava; a stock or basolith, formed when rising magma replaces or forces aside existing rock; ring dykes, concentric vertical intrusions which cut through existing rock layers; sills, intrusions which run between the existing layers; and radial dykes, another form of vertical intrusion which may run for hundreds of miles. At bottom is a magma chamber.

Great Dyke volcanic intrusion - Zimbabwe - Science Photo Library

Great Dyke - Zimbabwe - NASA

NASA / GSFC / METI / JAPAN SPACE SYSTEMS / U.S.,JAPAN ASTER SCIENCE TEAM / SCIENCE PHOTO LIBRARY
Satellite image of part of the Great Dyke volcanic intrusion (down centre) in Zimbabwe. This intrusion is thought to be about 2.55 billion years old, and was created when molten lava filled and widened a fracture in the Earth's crust. It is over 500 kilometres long and has an average width of 10 kilometres. The area is rich in minerals, including platinum, chromium and gold. Image obtained by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite.

Alaskan Dyke S. Johnathan image Nat Geo

Minette Dyke New Mexico USA

E4170275 Dolerite Dyke Science Photo Library - S. Stammers Namib Desert Namibia

Multiple Igneous Intrusion Phases Kosterhavet Sweden

Igneous (dolerite) compound dyke -geograph.org.uk B. Bowyer - Isle of Arran Scotland

Above Left image:- "Dolerite dyke in the Namib desert. A dyke is an intrusion of igneous rock between an existing layer of rock. Dolerite corresponds to basalt in its chemical composition but in contrast to basalt, which emerges on the surface as lava, the magma in dolerite dykes solidifies underground. Photographed on the Welwitschia Drive, Namib Desert, Namibia." Sinclair Stammers Science Photo Library

Laxford Bridge Steph Walker - Lewesian Gneisses & Scourie Dykes

Image by Perros Guirec, Brittany, France. SciencePhotoLibrary

'The cuttings excavated in the 1980s as part of the A838 road improvements between Laxford Bridge and Rhiconich, in NW Scotland, provide dramatic exposures of gneisses that give a glimpse of processes that operate in the middle continental crust during mountain building. The age of shear deformation at Laxford can be established by radiometric dating of the granitic intrusions. These have yielded ages of c 1855 million years. However, the grey gneisses are much older (2700-2800 million years). Collectively these outcrops chart the youngest part of the history of the Lewisian complex in NW Scotland while these rocks were still in the middle crust. Subsequently these units were uplifted to close to the earth’s surface so that they were overlain by the Torridonian strata – some 1.2 billion years ago. These outcrops are part of the NW Highlands Geopark.' Professor Rob Butler for The Geological Society

Hartashen - Namibia

Hartashen in Armenia - south of the Caucasus Mountains.

Hartashen - Megalithic Avenue - Google Earth

Hartashen Armenia - Google Earth

Hartashen Armenia Twin Avenues - The Road to Carnac - Google Earth

Hartashen Armenia - Google Earth

Hartashen NE Armenia - Google Earth

Hudson Bay - Canada

The Mackenzie Dyke swarm in the Northwest Territories of Canada is the world's largest - over 311 miles (500kms) wide and 1,864 miles (3,000kms) long.

SW Hudson Bay Canada Mechs 3 Types

Kildonan - Isle of Arran - Scotland

DSCF0865 - Wigton Physics

DSCF0858 - Wigton Physics

Kildonan - South Coast Isle of Arran - Google Earth

Kildonan Isle of Arran with Actual Archaeology - Google Earth

Ship Rock, New Mexico. USA

Google Earth has new features including Historical Imagery in the Primary Database. This is Ship Rock, Navajo Volcanic Province, New Mexico. The dykes are seen emanating from the volcanic plumes. Being harder than the surrounding rock they resist the ground weathering and so are left as narrow walls that arch out across the landscape like ancient battlements. If the sea had cut this buried landscape then the weathered top of the dykes would appear as the dark lines that are sometimes mistaken for stone fish traps. It would be a simple process to test such features by coring and X-ray fluorescence (XRF).

That is a Stop Sign - these are Street View images within the new Google Earth Historical Imagery.

Mafic Dykes radiate from the source as parallel lines which erode to avenues of single stones & may be mistaken for the works of Neolithic man.

South Alligator Floodplain - Closer to home

Looking West on the Kakadu Hwy to South Alligator R. Bridge NT - Mantle Plume - lava chamber at right.

Sth Alligator River. Mantle Plume - 5/2014

Sth Alligator R. Mantle Plume - Dr. Karen Dempsey - late Wet 2015

Woolianna Tuff Dyke & Boat Ramp

Tuff derives from the Latin 'Tufa' for porous rock - it is lighter than other volcanic rock because it is composed of ash and other material ejected during explosive volcanic eruptions and is deposited in the surface fissures. The Romans notably used it in the vaults of their bathhouses where steam would rot timber roofing.

Woolianna Boat Ramp Daly River NT - a PastMasters image June 2025

Woolianna Tuff Dyke Daly River NT - a PastMasters image June 2025

Sills

Sills are horizontal Dykes but are more persistent as they are generally thicker and less exposed to weathering.

Sills-Dikes NPS Nat. Parks Service

Geology Is The Way

The swarm of sills on Baffin Island Canada - Mike Beauregard image.

Arthur's Seat Edinburgh

Hadrian's Wall UK at the Walltown Crags of the Great Whim Sill - English Heritage UK image

Cawfields Roman Wall - an English Heritage UK image

Stone Fish Traps & Mafic Dykes

Australian Mega-myths

Bentinck & Sweers Stone Traps

Nightcliff Stone Fish Traps

World Heritage Dykes

Sources & Resources

Papers:- Halls, 1982; Halls and Fahrig, 1987; Ernst and Baragar, 1992; Coffin and Eldholm, 1994, 2005; Wilson and Head, 2002; Bryan and Ernst, 2008; Ernst, 2014.

Datong - Galiwin'ku Large Igneous Province

Biberkine Mafic Dyke Swarm '1.39 Ga mafic dyke swarm in southwestern Yilgarn Craton marks Nuna to Rodinia transition in the West Australian Craton' 2018 by J. Camilla Stark et al in Precambrian Research Vol. 316, October 2018, Pages 291-304 - see link in Sources & Resources below.

Biberkine Dyke Swarm