I went to Mexico and the pyramids are covered by vegetation. It's only after archeologists clean them that you can actually see the pyramids. Before that they looked like a natural hill, with soil, trees, ...
How is that possible? I can imagine grass or small weeds growing here and there in between the stones with seeds being disseminated by wind or birds. But how did soil get up there? Whatever 'lucky parts' of soil that got up there should be washed off with gravity, wind, rain, no?
I am an archaeologist and I specialize in the ancient Maya. Here's how it happens:
1: The vast majority of ancient Maya buildings are built using a "core and veneer" technique. The bulk of the building's volume is earth and stone rubble, faced with a veneer of nicely-shaped limestone blocks that are themselves covered by a layer of lime plaster (stucco). The geology of the Yucatan peninsula is almost entirely limestone, so they built with what they had available.
2: Most pyramids have multiple construction phases, with each one adding a new layer of rubble/earthen core and cut stone and stucco veneer. The building gets bigger with each renovation.
3: Following abandonment, strong seasonal rains quickly erode the lime stucco on much of the outermost layer. This eroded stucco is chemically and structurally identical to the eroded bedrock that underlies natural soils in the Maya region, meaning that plants can readily grow on it.
4: The limestone used for cut stone veneers is itself really porous and prone to crack, pit, and erode in most parts of the Maya lowlands. I am constantly seeing small trees growing out of ostensibly solid blocks of limestone--their roots can still get plenty of purchase. As such, trees can easily take hold in the space between blocks and in small imperfections in the stones themselves.
5: As the trees grow (and especially as their roots push or pry out loosened veneer stones), the cut stones from the veneer give way, exposing the earthen/rubble core underneath.
6: Other posters are correct that some topsoil builds up as a result of wind-transported dust and decaying plant matter, but this is a very, very small amount, especially on the steeper upper portion of the building (remember, the buildings become less steep overall as they erode, because what had been on the upper part tumbles down and piles up around the base, reducing the gradient). Instead, most of the "soil" was actually put there by the Maya as part of the final construction phase, covered by a layer of modern humus.
7: It follows that in many parts of the Maya world, what you see when you look at a cleared/restored pyramid is not really what it looked like in its final stage. Instead, it's usually a mix of the last and second-to-last construction phases, both having been subject to varying degrees of erosion, with the last phase usually only preserved toward the bottom and at the very top.
The accumulation of soil in such situations takes a long time. The main way soil would have been naturally transported there would be by the wind.
The wind blown soil would get caught and accumulate. Plants, particularly grasses, would grow on the soil and the plant roots would create mesh that would protect the soil from erosion.
You are correct in that some soil would be lost due to erosion (predominantly via rain and wind), but the soil protected by the plant roots would remain. Over time the cycle repeats and soil slowly accumulates. The accumulation of the soil initially occurs at the base of the pyramid and gets deeper, eventually covering the pyramid.
Gravity would not cause soil loss. There may be some localized soil slumping if a soil face was too steep, but it wouldn't more far. The effect of gravity would be to keep the overall long term slope of the soil at the angle of repose of the soil.
Most of the Mexican pyramids are less steep than their Egyptian counterparts, and have niches and terraces which their Egyptian counterparts don't. An important difference is the climate, which is much damper in Mexico, and shows how the rainforest can reclaim areas where it has been eradicated if the site is then abandoned. Yet another factor is the type of stone used, some types weather more easily than others. I think some Mexican pyramids have been built from limestone, which is very susceptible to erosion. The dust and debris from the weathered stone mixes with plant debris and forms soil, making it easier for more trees and bushes to take root. The plant roots help keep the soil in place. In cleared rainforest, jungle indistinguishable from primary rainforest can take over in only 50 years, so no wonder it has re-established on pyramids well over 1,000 years old. They have had the same problem at Ankor Wat in Cambodia, where much clearance of the temples has had to be done.
Disclaimer: this answer is based on what I remember from school and TV.
When a volcano erupts with lava flows, the magma cools and forms an inhospitable, sterile surface. The first life to colonise the rocky surface is lichens. When the lichens start dying, they decay into very thin soil layers. Next comes moss which needs a tiny bit of soil. The roots of the moss keep the soil where it is. Some old moss is decaying while new moss keeps the new soil where it is. Lichens are still present. Eventually wind or bird borne seeds are deposited on the soil and can sprout into small plants, and finally trees.
The American pyramids are similar except that they are in a lush rain forest and were not as sterile as a volcano when they were built. The surface of both the pyramids and cooled magma have enough small cracks that plant roots can get into. Roots can exert an amazing amount of pressure over a long period of time, so they gradually widen the cracks which allows more soil to collect, and more plant growth, accelerating the process.
The main force that prevents erosion is plant roots.
The Accumulation of organic detritus begins when a structure is abandoned or poorly maintained. Plants that thrive without soil (epiphytes, lithophytes) are often the first generation of plants. As they die, new ones take their place and the first layer of accumulated biomass builds to accommodate more sophisticated plants grow in this medium
