So I have become interested in the history of the Mali empire, and know little about it. I was surprised learning that it was one of the richest empires in history, yet had many buildings made with mud, although it also had buildings made with stone and other materials.
Without much knowledge these are the potential explanations I came up with:
Am I right about any of these? I would like to be educated on the subject.
This is for the most part not a question for historians-- we have very few documents from Mali discussing their choices in architecture and engineering-- essentially not until after Islam and Muslim travelers visit, and historians need documents. What you're asking about is thus more anthropology, materials science and archaeology. Historians do work, to some degree, with oral traditions and archaeological evidence, but these are much more the bailiwick of other disciplines. The historical approach is particularly awkward in the case of Mali, where Arab histories often mistakenly credit one al-Sahili of North Africa with being the designer of Malian and Sudanese architecture, a line which was taken up enthusiastically by western historians in the 20th century.
Mud architecture is cheap and easy to build, and more appropriate to hot and dry climates than stone, effectively a highly insulating natural cement. You've got an assumption that mud is somehow "worse" or "poorer" than stone, but that's just the way you're looking at it -- not how it would have seemed to someone there at the time. It's never wise to look at some evolved technology and ask "why didn't these people do some other thing that [I think] is better/obvious"; the answer is nearly always "you may think it's 'better' but that's just you". People settling on a technology never do so with a white paper "we chose mud over stone because . . ."; they worked with the materials that made sense to them, without validating it against other possible technologies. If they'd seen stone buildings working well in their environment, they might have thought about it . . . but of course, they didn't.
Mud and associated architectural technologies -- rammed earth, for example- have big advantages over stone in hot dry climates. Stone baking in the sun will radiate heat, and effectively cook you inside. Not pleasant, even if there are good quarries nearby -- which there may not have been.
People around the world in hot climates settled on mud technologies of various kinds, also called "adobe", and they're remarkably efficient. It's also a mistake to look at all adobe structures and say "well, they're all just mud" -- there are many subtle variations in adobe recipes, additives which change properties in important ways, requires systematic inquiry into the precise methods of manufacture/construction for any given adobe to really get at what it is and how people might have tuned its properties for local conditions.
Adobe is one of the oldest building materials in use and abundantly found everywhere with affordable and reasonable cost. It is basically just dirt that has been moistened with water, sometimes with chopped straw or other fibers added for strength, and then allowed to dry in the desired shape. In addition to being simple and economic, adobe bricks are fireproof, durable, non-toxic, possess low sound transmission levels through walls and provide sufficient thermal mass to buildings. Adobe brick walls are load bearing structures, which can be used to build up to several stories high. Also, adobe bricks have ability to build vaults and domes. In addition to this, adobe bricks afford great flexibility in the design and construction process. They can be simply cut, reshaped and can easily be subjected to opening to be use for building’s services. Besides having, these advantages; they have some disadvantages too which can be summarized as follows; moderately low compressive strength, low tensile strength, low scratch resistance and low water resistance. To overcome these defects, additives should be added to the soil to improve its quality and increase its capability to stand against these defects. This paper provides an overview of the mud’s additives that are used in different regions of the world, and their enhancement to improve the soil stability, including mineral, Synthetic, vegetable and animal additives. The paper also includes an experimental study that examines the thermal conductivity of mud brick mixed with some additives (plant oil, ash, soap, and cow dung).
THE MUD ADDITIVES AND THEIR EFFECT ON THERMAL CONDUCTIVITY OF ADOBE BRICKS
Sub-Saharan Africa does of course have stone architecture, famously at Great Zimbabwe, but that's a much cooler climate than Mali. People in very hot climates sometimes built monuments out of stone-- pyramids in central America and Egypt, for example; but they typically lived in buildings made out of much more comfortable materials.
Some civilizations chose to make their monuments out of stone, others elaborated on the mud/adobe that was so useful and with which they were most familiar. The Malians, for example, built the libraries of Timbuktu. And, of course, there's the Great Mosque of Djenne -- while built in 1907, it's on the site of earlier mosques, and is considered one of the great achievements of world architecture (and it is a subject open to historians, because we do have the kinds of records that one needs).
So on the evidence, the Malians built magnificent structures, using materials that worked well in their environment, and which they understood how to handle with great skill. There are all kinds of other materials which they didn't build with-- didn't build log cabins, for example. That doesn't mean anything much . . . it wasn't a choice _not_ to build a log cabin, similarly that they don't have stone architecture and Great Zimbabwe does--- hard to see that that means anything beyond "what makes sense to build with here". People around the world created enormously impressive and very large monuments out of mud/adobe, for example the massive "Pyramid of the Sun"/Huaca del Sol in Moche, Peru.
Sources
Banco, Adobe Mosques of the Inner Niger Delta
The History of the Great Mosques of Djenné
Spectacular Vernacular: A New Appreciation of Traditional Desert Architecture
MONUMENTAL ADOBE ARCHITECTURE OF THE LATE PREHISPANIC NORTHERN NORTH COAST OF PERU
HIGHLAND INCA ARCHITECTURE IN ADOBE
Al-Sahili : the historian's myth of architectural technology transfer from North Africa
For the same reasons sumerians built ziggurats with mud, and persians built large palaces with mud (like arg-é bam), and chinese did as well: it was easier, cheaper and all around. In the south, through Nigeria (Ife and Benin), they choose both mud and wood, as in the coasts they chose wood as well. Far south in southern Africa was different, a lot of small city-states rised, and they used wood and mud to build their houses, but they developed stone traditions, building walls, or terraces (like in Nyanga and Mapella Hill), or simple aqueducts/irrigation systems, call it as you wish (like Ingaruka). But the stone-walled traditions were quite common, the most famous example being Great Zimbabwe (GZ). But they didn't make any larger single stone work or taller like in Great Zimbabwe (as thick maybe), stone towers neither (only in Kweneg, but they had 1-2 meters and it was for grains, though they were thick). Khami ruins are big, but not as big as GZ, and Kweneg had stone houses and a large area like GZ and with stone walls, but most houses were still made of mud. I won't blame them for not doing best than GZ because most of them were in their respective area for only 1000 years, GZ was built 700 years after their arrival, so their primary building way would remain for a long time. That is the point. Even with stone mansory knowledge, they still user mud for most of their buildings, and the same would be with malians, mainly malians, because they were in a area were mud was all over around. I saw a video where they Said that a tablet made of mud/Clay survived a fire in Middle East. This já what happens when you build with mud in a hot place, it lasts. But Ghana Empire and Gao Kingdom had stone architecture. Ghana us really hard to find any pictures, but the palace in Gao was built in 9th, 10th or 11th century.
https://journals.openedition.org/etudesafricaines/17167
You can search for Thulamela ruins, or Kweneg reconstruction, or Thimlich Ohinga, ruins of Khami, Domboshaba, but this cities had many or some mud houses.