#laccolith

[artichokefartichoke]

[geologicaltravels]

2016: Schematic of the spectacular Torres Del Paine laccolith; with additional  detail of the surrounding heavily-folded Cretaceous country rock, which is also riddled with dykes associated with the laccolith formation, no doubt. A beautiful, beautiful, wild place.

[earthstory]

New model for formation of Devil’s Tower (Bear Lodge)

However, there are a number of issues with this model; there are shallow rocks nearby that suggest limited erosion in the 50 million years since Devil’s Tower formed, and there is little density contrast between the shallow sedimentary layers around Devil’s tower. Having one layer be less dense is a big help in getting molten rock to get stuck at a level in the crust. Finally, some details of the tower itself, such as how the top of the tower came out so flat, aren’t well explained by this model. Erosion wouldn’t necessarily carve out a flat top to the tower unless that flat top was the original top of the laccolith, but that model requires some of the tower to be eroded away to fit features of the joints.

Columnar joints form as igneous rocks cool. As they shrink, tension forces develop, eventually cracking the rocks. The cracks always form perpendicular to temperature contours, so for the cracks to be so long and vertical, the tower must have cooled top to bottom. But, as you can see in this photo, there are several complexities: the columns turn at the base, indicating the cooling at the base was not simple, and there is a boundary 2/3 of the way up the tower where the fracture pattern changes.

The boundary 2/3 of the way up the tower could be where cooling from the top and bottom met; columns would start forming at the top and at the bottom, and when they met in the middle there would be an offset boundary between them. Why then are the columns shorter at the top than at the bottom? This pattern requires either that the top was eroded away, which wouldn’t leave the flat top, or that somehow Devil’s tower cooled most rapidly from the bottom.

It is these observations, combined with other details of the local geology and comparisons around the world, that drove a team of scientists led by a geologist at the institute of Geophysics of the Czech Academy of Sciences to suggest that Devil’s Tower did not start as a body of molten rock below the surface, but instead was a thick lava flow that grew out of a maar crater.

Maar craters occur when magma rises up and meets groundwater. The heat from the molten rock triggers explosions which throw blocks of debris out of the way, leaving a crater behind. There are a number of maar craters in the area formed as part of the same volcanic complex, and there are blocks of crustal rocks near Devil’s Tower that could have been ejected as part of maar eruptions. So, starting Devil’s Tower as a maar crater is consistent with the geologic evidence in the area.

Once the explosive eruptions stopped, the magma that fed the maar could have still been there. If molten rock rose up through the edges of this crater, it could have erupted out of the crater and piled up into a thick lava flow. That lava would form either a dome if it went straight up or a coulee if it began flowing down the gentle local slope.

The top of the coulee would be flat because it was the top of a lava flow. The top of the coulee would cool straight down, creating the vertical columns at the top. The bottom of the coulee would have a complex cooling as it might cool as it flowed along the ground and circulating water could also cool that portion if it rained. This more complex cooling caused the columns near the base to turn in multiple directions, creating the unbalanced shape of the whole tower. Furthermore, because the bottom was cooled by water, it cooled more rapidly than the top, making the bottom columns longer than the top columns since the tower cooled mostly from the bottom up.

Since it cooled, most of the dome or coulee eroded away, leaving behind the piles of broken columns found in the area today.

This is an elegant explanation for the Tower’s formation and, IMO, fits the surrounding landscape better than the ideas that require hundreds of meters of sediment erosion over this time period. I’m not sure what needs to be done to further test this model, but I’m already partial to it.

-JBB

Image credit: https://commons.wikimedia.org/wiki/File:Devils_Tower_National_Monument.JPG

Original paper: https://doi.org/10.1130/GES01166.1

[mysecretponyshame]

1st up is Hijacker’s (NSFW link) fierce Night Terror as depicted by LisCas .  Found here live and here in archive

2nd up is another LisCas piece, the always calm and collected Laccolith from Lis’s very own Curse Quest, found here in archive.

[relatetorocks]

[artichokefartichoke]

[earthstory]

Glasshouse Mountains

These peaks are found in Australia’s Glasshouse Mountains National Park, near the ocean coastline in Queensland, Australia. From left to right in this photo, the peaks are named Mt. Tibberoowuccum, Mt. Tibrogargan, Mt. Beerwah, Mt. Coonowrin, and Mt. Ngungun. The area received the name Glasshouse Mountains from James Cook in the 18th century, but each peak retains an originalaboriginal name relating to the story told about the area.

In the tale, Tibrogargan, the largest peak, was the father of tribes in the area, and Beerwah, the next largest peak, was the mother. When Tibrogargan perceived that a flood was about to strike the area, he asked his son Coonowrin to escort his mother to high ground, but Coonowrin panicked and deserted her. When Tibrogargan learned of this, he struck Coonowrin, leaving him with a crooked head, and placed him behind his mother so that Tibrogargan would never have to see him again. When the other members of the family learned of this incident, they cried, leading to the streams that drain the area.

The mountains were a meetingplace for many tribes that lived in the area, with early reports suggesting thousands could gather here at one time. Starting in the 19th century however, the land was logged and these meetings were cleared out. The trees growing here today were replanted starting in the 1930s.

The mountains are all igneous in origin. They are volcanic plugs or laccoliths – volcanic rock that got close to the Earth’s surface but that did not erupt. Some even show columnar jointing up close. The molten rock solidified at depth, and as the surrounding landscape eroded, the strong igneous rocks began to stand tall above the surrounding landscape. These rocks have been dated to be 25 million years old, and they form an age-progressive trend with other igneous rocks in Queensland that have been used to argue for the existence of a hotspot traveling beneath Australia between 35 and 25 million years ago.

-JBB

Image credit: David Molloy https://flic.kr/p/26AF5CM

References: https://bit.ly/2tRD4bd https://bit.ly/2KsXa5U https://bit.ly/2KwALob

[earthstory]

Climb

This gorgeous picture of the Grand Staircase was captured last year by astronauts on the International Space Station. In Utah and Arizona north of the Grand Canyon, sedimentary rocks deposited between the Permian and the Cretaceous have been tilted slightly upwards, so that as they erode, resistant units create an enormous stairstep pattern across the entire region. Several amazing landscapes, including Bryce Canyon and Zion National Parks, are found along steps in the Grand Staircase. The Colorado River has then chopped its way through the heart of this landscape; today that river has been dammed to form Lake Powell and much of that lake is now visible in this shot.

The isolated peak towards the left is Navajo Mountain. The same sedimentary layers in the Grand Staircase have in this mountain been folded upwards by a laccolith; an intrusion of magma pushed the layers up into a dome (https://tmblr.co/Zyv2Js2AXb958).

Until late 2017, Grand Staircase-Escalante National Monument, encompassing much of this landscape, was the largest National Monument in the United States. The Secretary of the Interior has proposed cutting this monument in half.

-JBB

Image credit:

Previous post: https://tmblr.co/Zyv2Js2Sk2bgX