Dike (geology)

A dike or dyke, in geological usage, is a sheet of rock that is formed in a fracture in a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma flows into a crack then solidifies as a sheet intrusion, either cutting across layers of rock or through a contiguous mass of rock. Clastic dikes are formed when sediment fills a pre-existing crack.[1]

BasaltDykes LordHoweIsland 7June2011
Vertical basalt dikes cutting horizontal lava flows, Lord Howe Island, Australia
Geological Dike Cross-Island Trail Alaska
A small dike on the Baranof Cross-Island Trail, Alaska
WestSpanishPeakCO
Magmatic dikes radiating from West Spanish Peak, Colorado, U.S.
ISR-2016-Makhtesh Ramon-Geological dike
A magmatic dike cross-cutting horizontal layers of sedimentary rock, in Makhtesh Ramon, Israel
Minette Dyke in New Mexico USA 01
A dike of lamprophyre near the Shiprock volcanic plug, New Mexico, that has resisted the erosion that removed some of the softer rock into which the dike was originally intruded

Magmatic dikes

An intrusive dike is an igneous body with a very high aspect ratio, which means that its thickness is usually much smaller than the other two dimensions. Thickness can vary from sub-centimeter scale to many meters, and the lateral dimensions can extend over many kilometres. A dike is an intrusion into an opening cross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock; this implies that a dike is always younger than the rocks that contain it. Dikes are usually high-angle to near-vertical in orientation, but subsequent tectonic deformation may rotate the sequence of strata through which the dike propagates so that the dike becomes horizontal. Near-horizontal, or conformable intrusions, along bedding planes between strata are called intrusive sills. The term “sheet” is the general term for both dikes and sills.

Sometimes dikes appear in swarms, consisting of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event. The world's largest dike swarm is the Mackenzie dike swarm in the Northwest Territories, Canada.[2]

Dikes often form as either radial or concentric swarms around plutonic intrusives, volcanic necks or feeder vents in volcanic cones. The latter are known as ring dikes.

Dikes can vary in texture and their composition can range from diabase or basaltic to granitic or rhyolitic, but on a global perspective the basaltic composition prevails, manifesting ascent of vast volumes of mantle-derived magmas through fractured lithosphere throughout Earth history. Pegmatite dikes comprise extremely coarse crystalline granitic rocks - often associated with late-stage granite intrusions or metamorphic segregations. Aplite dikes are fine-grained or sugary-textured intrusives of granitic composition.

The term "feeder dike" is used for a dike that acted as a conduit for magma. Magma flowed along, then out of the dike, then formed another feature.

In contrast to magmatic dikes, a sill is a magmatic sheet intrusion that forms within and parallel to the bedding of layered rock.

Sedimentary dikes

Clastic dike UT
Clastic dike (left of notebook) in the Chinle Formation in Canyonlands National Park, Utah

Sedimentary dikes or clastic dikes are vertical bodies of sedimentary rock that cut off other rock layers. They can form in two ways:

  • When a shallow unconsolidated sediment is composed of alternating coarse grained and impermeable clay layers the fluid pressure inside the coarser layers may reach a critical value due to lithostatic overburden. Driven by the fluid pressure the sediment breaks through overlying layers and forms a dike.
  • When a soil is under permafrost conditions the pore water is totally frozen. When cracks are formed in such rocks, they may fill up with sediments that fall in from above. The result is a vertical body of sediment that cuts through horizontal layers, a dike.

See also

References

  1. ^ Essentials of Geology, 3rd Ed, Stephen Marshak
  2. ^ Pilkington, Mark and Walter R. Roest; Removing varying directional trends in aeromagnetic data, Geophysics, vol. 63 no. 2 (1998), pp. 446–453. abstract
Linear ridge networks

Linear ridge networks are found in various places on Mars in and around craters. These features have also been called "polygonal ridge networks," "boxwork ridges", and "reticulate ridges." Ridges often appear as mostly straight segments that intersect in a lattice-like manner. They are hundreds of meters long, tens of meters high, and several meters wide. It is thought that impacts created fractures in the surface, these fractures later acted as channels for fluids. Fluids cemented the structures. With the passage of time, surrounding material was eroded away, thereby leaving hard ridges behind.

It is reasonable to think that on Mars impacts broke the ground with cracks since faults are often formed in impact craters on Earth. One could guess that these ridge networks were dikes, but dikes would go more or less in the same direction, as compared to these ridges that have a large variety of orientations. Since the ridges occur in locations with clay, these formations could serve as a marker for clay which requires water for its formation. Water here could have supported past life in these locations. Clay may also preserve fossils or other traces of past life.

These ridges could be formed by large impacts that produced fractures, faults, or dikes made up of melted rock and/or crushed rock (breccia). One formation mechanism proposed by Quinn and Ehlmann in 2017 was that sediment was deposited and eventually the sediment underwent diagenesis which caused a loss of volume and fractures. After erosion exposed the fractures, they were filled with minerals possibly by acid-sulfate fluids. More erosion removed softer materials and left the more resistant ridges behind. If the impact-caused dike is made of purely melted rock from the heat of the impact, it is called a pseudotachylite .

Also, hydrothermalism may have been involved due to the heat generated during impacts. Strong evidence for hydrothermalism was reported by a team of researchers studying Auki Crater. This crater contains ridges that may have been produced after fractures formed with an impact. Using instruments on the Mars Reconnaissance Orbiter they found the minerals smectite, silica, zeolite, serpentine, carbonate, and chorite that are common in impact-induced hydrothermal systems on Earth. Other evidence of post-impact hydrothermal systems on Mars from other scientists who studied other Martian craters.

Because ridges seem to be found in older crust only, it is believed that they occurred early in the history of Mars when there were more and larger asteroids striking the planet.

These early impacts may have caused the early crust to be full of interconnected channels.

These networks have been found many regions of Mars including in Arabia Terra (Arabia quadrangle), northern Meridiani Planum, Solis Planum, Noachis Terra (Noachis quadrangle), Atlantis Chaos, and Nepenthes Mensa (Mare Tyrrhenum quadrangle).A somewhat different ridge formation has been discovered in the Eastern Medusae Fossae Formation; these dark ridges can be 50 meters in height and erode into dark boulders. It has been suggested that there are from lava filling fractures in the Medusae Fossae Formation which is surrounded by lava flows.

Ore resources on Mars

Mars may contain ores that would be very useful to potential colonists. The abundance of volcanic features together with widespread cratering are strong evidence for a variety of ores. While nothing may be found on Mars that would justify the high cost of transport to Earth, the more ores that future colonists can obtain from Mars, the easier it would be to build colonies there.

Sinus Sabaeus quadrangle

The Sinus Sabaeus quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. It is also referred to as MC-20 (Mars Chart-20).

The Sinus Sabaeus quadrangle covers the area from 315° to 360° west longitude and 0° to 30° degrees south latitude on Mars. It contains Schiaparelli, a large, easily visible crater that sits close to the equator. The Sinus Sabaeus quadrangle contains parts of Noachis Terra and Terra Sabaea.

The name comes from an incense-rich location south of the Arabian peninsula (the Gulf of Aden).

Syrtis Major quadrangle

The Syrtis Major quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Syrtis Major quadrangle is also referred to as MC-13 (Mars Chart-13).The quadrangle covers longitudes 270° to 315° west and latitudes 0° to 30° north on Mars. Syrtis Major quadrangle includes Syrtis Major Planum and parts of Terra Sabaea and Isidis Planitia.

Syrtis Major is an old shield volcano with a central depression that is elongated in a north-south direction. It contains the calderas Meroe Patera and Nili Patera. Interesting features in the area include dikes and inverted terrain.

The Beagle 2 lander was about to land near the quadrangle, particularly in the eastern part of Isidis Planitia, in December 2003, when contact with the craft was lost. In January 2015, NASA reported the Beagle 2 had been found on the surface in Isidis Planitia (location is about 11.5265°N 90.4295°E / 11.5265; 90.4295). High-resolution images captured by the Mars Reconnaissance Orbiter identified the lost probe, which appears to be intact. (see discovery images here)

In November 2018, NASA announced that Jezero crater was chosen as the landing site for the planned Mars 2020 rover mission. Jezero crater is in the Syrtis Major quadrangle at (at 18.855°N 77.519°E / 18.855; 77.519)

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