Rootless Cones at Mount Pavagarh, Gujarat(Deccan traps)
Rootless cones, also (erroneously) called pseudocraters, form due to explosions that ensue whena lava flow enters a surface water body, ice, or wet ground. They do not represent primary vents
connected by vertical conduits to a subsurface magma source. Rootless cones in Iceland are well studied. Cones on Mars, morphologically very similar to Icelandic rootless cones, have also been suggested to be rootless cones formed by explosive interaction between surface lava flows and ground ice.
WHAT ARE ROOTLESS CONES AND HOW DO THEY FORM??
Many volcanic terrains all over the globe contain deposits and landforms that indicate interaction of lava, or magma, with surface or near-surface water. Phreatic is the term used for steam explosions resulting from conversion of water to steam
due to magmatic heat, but if juvenile magma is also thrown out of a volcanic vent besides steam and fragments of the country rock, the explosions are described as phreatomagmatic.also thrown out of a volcanic vent besides steam and fragments of the country rock, the explosions are described as phreatomagmatic.or near-surface water and the resulting mild explosions produce scoria and spatter that accumulate to form small cones around the explosion spots
(Thorarinsson 1953). Typical locations where this happens are stream or river beds, lakes, or watersaturated sediments. The typical feature of rootless cones is internal stratification with reverse grading (coarser material toward the top), and the cones
are capped with welded spatter, which sequence indicates decreasing explosivity during the course of their formation, arguably due to volatile depletion. A single cone is usually the result of several explosions attendant upon rapid groundwater recharge
A rootless cone may form when lava flowing through an internal pathway through a lava flow (a lava tube) presses down into the underlying, lower-density, water-saturated sediment (figure
4a). Cracks develop in the lower lava crust and bring about contact of and explosive interaction between the lava and the water-saturated sediment. When an explosion occurs (figure 4b), further
lava flow movement (and further rootless cone formation in the downflow direction) is halted, but more rootless cones can form upflow, and along the new lava tube network that progressively develops.
ROOTLESS CONES ON PAVAGADH HILL: Rootless cones have been described from the Martian surface as well as on earth . The type area is Myvatin, Iceland.Pavagadh cones have been claimed to be rootless cones because :
• they are similar in size to rootless cones known
from Iceland and elsewhere;
• they have a strong morphological resemblance
to the rootless cones from the type locality of M´yvatn, northeastern Iceland;
• the various cones are overlapping, and stand “shoulder-to-shoulder”
They have not produced lava flows themselves (volcanic cones that are eruptive vents, e.g., cinder cones, often have lava flows issuing from them);
• they occur on a basalt flow, are surrounded by the lava flow and are not deformed by motions of the lava, i.e., they formed subsequent to the host lava flow after its surface crust had frozen
— obviously the lava that formed scoria by mixing with water was fed to the lava flow internally, consistent with observations elsewhere in the world;
Though the area left today after erosion through 65–60 million years is relatively small, the Pavagadh cones and craters 837
Pavagadh cones have an apparently random distribution,
similar to that known for Icelandic and Martian rootless cones
Fig. 1: Cones from martian surface
Fig. 2:Cones from Iceland
Reference:Cones and craters on Mount Pavagadh, Deccan Traps:
Rootless cones? by Hetu C Sheth∗, George Mathew, Kanchan Pande, Soumen Mallick and Balaram Jena
Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
Thanks to a good friend of mine Nishil Iyer for this information.