The surface of the Moon is covered in craters, which were formed by meteorite impacts. Meteorites have hit the Earth too, but you don’t see many craters on the surface of the Earth. That’s because the Earth’s surface is young – it is constantly shifting, eroding, crumpling and being recycled and regenerated. On the other hand, the Moon’s surface is old, and we see craters preserved from billions of years ago. Something I didn’t know until last week is that there is (probably) tectonic activity happening on the surface of the Moon to this day.
In a paper published last week (Geology, v. 43, pp. 851-854) Watters et al. describe landforms called ‘lobate scarps’ that are found all over the surface of the Moon.
‘Lobate scarp’ is a funny word, but it’s just a landform that develops when you compress the crust, a fault forms, and one side of the fault is pushed up. This creates a steep bank or escarpment (‘scarp’), and the morphology of these features is often ‘lobate’, or with lobes.
Here are some images from the paper showing a lobate scarp feature on the Moon. Since we know lobate scarps form by thrust faulting, that means there has been thrust faulting on the surface of the Moon.
But how do we know when these faults formed?
The scientists who wrote the paper identified two features that suggest these faults must have formed quite recently.
- In the middle image, the lobate scarp cross-cuts small craters and deforms them slightly. This means the fault must have occurred after the craters formed, and the craters are thought to be no more than 800 million years old.
- In the right-hand image, there are some shallow, elongated troughs which were formed at the same time as the lobate scarp, when the crust got a bit bent. These troughs are no more than 1-2 m deep in some places. Dust and sediment would accumulate very slowly on the Moon, at a rate of about 5 cm per million years… but this means that the troughs can’t be more than about 50 million years old.
Plus, there are earthquakes on the Moon (moonquakes), some of which come from shallow levels and may be related to these faults.
So that’s exciting! Active faulting on the Moon… but… why? What causes this faulting? On Earth, faults are generally associated with plate tectonics, but the Moon doesn’t have mantle convection and separate plates… so what’s going on?
It turns out there are three main sources of stress that act on the crust of the Moon.
- The Moon was once much hotter, so as it cools down it contracts (this creates compressional forces)
- The Moon was once closer to the Earth, feeling a stronger gravitational pull, which made it bulge more. It’s been moving away slowly (a few centimetres per year) and so those stresses are relaxing
- The Moon’s orbit is elliptical, so when it comes closer to the Earth, it feels a stronger gravitational pull which causes some bulging of the rock
If the only force at play was contraction due to cooling, the faults should occur with random orientations; but they don’t. The authors of the study show that all three of these sources of stress combine to generate these faults.
In other words: the tug of the Moon’s gravity on the Earth causes tides… but the tug of the Earth’s gravity on the Moon causes moonquakes!