Pit crater
Encyclopedia
Depression (geology)
A depression in geology is a landform sunken or depressed below the surrounding area. Depressions may be formed by various mechanisms.Structural or tectonic related:...
formed by a sinking of the ground surface lying above a void or empty chamber, rather than by the eruption of a volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
or lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
vent
Hawaiian eruption
A Hawaiian eruption is a type of volcanic eruption where lava flows from the vent in a relative gentle, low level eruption, so called because it is characteristic of Hawaiian volcanoes. Typically they are effusive eruptions, with basaltic magmas of low viscosity, low content of gases, and high...
. It is often found in chains or trough
Trough (geology)
In geology, a trough generally refers to a linear structural depression that extends laterally over a distance, while being less steep than a trench.A trough can be a narrow basin or a geologic rift....
s. Several craters may merge into a linear alignment. Pit craters usually lack an elevated rim as well as the ejecta deposits and lava flows that are associated with impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s.
Pit craters are characterized by vertical walls that are often full of fissures and vents. They usually have nearly circular openings.
As distinct from meteor crater
Meteor Crater
Meteor Crater is a meteorite impact crater located approximately east of Flagstaff, near Winslow in the northern Arizona desert of the United States. Because the US Department of the Interior Division of Names commonly recognizes names of natural features derived from the nearest post office, the...
s, these craters are not formed from the clashing of bodies or projectiles from space. Rather, they can be formed by a lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
explosion from a bottled up volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
, (the explosion leaving a shallow caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
), or the ceiling over a void may not be solid enough to prevent the collapse of the overlying material. A pit crater also could result from the collapse of lava tube
Lava tube
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like...
s, dike swarm
Dike swarm
A dike swarm or dyke swarm is a large geological structure consisting of a major group of parallel, linear, or radially oriented dikes intruded within continental crust. They consist of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event and are...
s, or from collapsed magma chamber
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
s under loose material.
A newly formed pit crater has steep overhanging sides and is shaped like a cone inside, growing wider closer to the bottom. Over time the overhangs fall in and the crater fills with talus from the collapsing sides and roof. A middle aged pit crater is cylindrical but the rim will continue to collapse, expanding outward until the crater resembles a funnel or drain, narrower at the bottom than the top.
While pit craters and calderas form from similar processes, the former term is usually reserved for smaller features of a mile or less in diameter.
The term "pit crater" was coined by C. Wilkes in 1845 to describe craters along Hawaii's East Rift Zones.
Hawaii is known for its volcanoes and pit craters. In 1868 an eyewitness saw more than two-thirds of the basin of Kilauea cave in and fill with a lava lake
Lava lake
Lava lakes are large volumes of molten lava, usually basaltic, contained in a volcanic vent, crater, or broad depression. The term is used to describe both lava lakes that are wholly or partly molten and those that are solidified...
. This process happened repeatedly. The modern Halema'uma'u Shield began growing and then collapsed into a deep funnel-shaped pit. This pit filled with lava and for 19 years burned continuously, becoming famous as the Hawaiian Fire Pit. In 1924 the lava lake emptied when the walls of the crater cracked and collapsed and filled with water that turned to steam. After a week and a half Halema'uma'u had widened and was 1,700 feet deep. Rocks that were blasted away from the crater can still be seen on the caldera floor.
Devil's Throat is another good Hawaiian example of a pit crater, especially since we were able to observe its formation through collapse over time. It was first documented by Thomas Jaggar who estimated its dimensions as 15m x 10.5m x 75m. In 1923 William Sinclair was lowered into Devil's Throat on a rope. He found a cavern shaped like an upside down funnel which widened as he approached the bottom. He measured the floor as about 60m in diameter and the crater's depth around 78m. The crater's mouth widened over time and in 2006 the crater's dimensions were measured as 50m x 42m x 49m. This growth is explained by observing pieces of the overhanging roof breaking off and falling to the bottom. These shards gradually piled up on the crater floor, reducing its depth.
The process also happens on the surface of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
and other terrestrial planets. Features resembling pit craters have been observed on Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
.