Hyginus Crater and Rille There’s a lot to learn from the stories told by the amazing lunar landforms of Hyginus crater and Hyginus Rille. Hyginus Rille is the sharp, 220 km long depression stretching left to right (west to east) in this Moon Trek visualization using data from the WAC camera aboard the Lunar Reconnaissance Orbiter. Hyginus Crater is seen at the elbow-like bend in the rille near the center of the image.
Hyginus Rille apparently formed when magma from the Moon’s mantle forced its way up through a crack in the lunar crust. As the intruding magma widened the crack, parallel faults were formed on the surface above. As the faults pulled away from each other due to the extensional forces below, the land between them dropped down forming a graben.
Beneath the widening Hyginus Rille, chambers of pressurized foam formed at the top of the dike of molten rock, as dissolved gasses bubbled out of the rising magma. The faults above provided a path for that pressure to be relieved through what must have been spectacular pyroclastic volcanic eruptions. A dark blanket of volcanic ash from these eruptions can be seen surrounding Hyginus Crater.
As the pressurized chambers were evacuated by the eruptions, they collapsed, resulting in volcanic pit craters on the surface. Hyginus Crater is the largest of those seen here, but a string of smaller pit craters can be seen extending to the northwest along the rille. Such pit craters are notable for their lack of raised rims. You can explore something similar here on Earth by driving along the Chain of Craters Road in Hawaii Volcanoes National Park.
There was an intense debate for many years as to whether the Moon’s craters formed through volcanic eruptions or meteoroid impacts. We now know that the vast majority of lunar craters formed as a result of impacts. But as these examples along Hyginus Rille demonstrate, volcanic craters can also be found on the Moon.
There’s another very curious aspect to this region. The floor of Hyginus Crater is an example of an Irregular Mare Patch or IMP. The Ina Caldera, about 325 km to the north, is the best-known of these IMPs. These mysterious features exhibit erupted volcanic material. Nothing special there. But the technique of crater counting leads to an estimate of very young ages for these areas, indicating eruptions that are far too recent to be consistent with our current thermal modeling of the Moon. But as Jim Head presented at the 2016 meeting of the Lunar Exploration Analysis Group, the foamy nature of the erupted material could lead us to seriously underestimate the ages of these features based on crater counts.
Finally, take a look at the branching of the rille at the far right of this image. The upper branch actually connects Hyginus to another of the Moon's great straight rilles, the Ariadaeus Rille.