|MadSci Network: Earth Sciences|
Well the dirt comes from many places and there are several systems in nature that contribute to its build-up over time. Water is a major mover of dirt. Rivers move dirt down river and generally deposit it at its base causing a Delta but along the way it twists and turns all over the place moving dirt from the outside of its bends to the inside of it bends. Also it Erodes Rocks which when they become small enough become part of the dirt. Of course, there is also flooding.
Rain, At the heart of every Rain drop is a particle of dirt. It is the seed on which water condenses to form rain.
Wind, The air moves the dirt around here and there so based on the weather patterns that change ever so slowly through the centuries, you get regions of dirt build up and reigns of erosion.
Volcanoes, Earthquakes, and Land-slides, these move TONS of dirt very quickly. A volcano can spread Dirt to the far corners of the earth with just one blast and produce rock that over lots of time also becomes dirt. An Earthquake is Dirt sliding past each other at the continental plates shift. Land slides take a lot of dirt from one spot and spread it out over a wider area.
Life, A large part of dirt is the remains of life. Plants, trees, worms, birds, man, and all manner of things. Without the remains of life in the dirt you would be very hard pressed to get life out of the dirt.
Space Dust and junk, The earth is gaining dirt from space quite a lot of it. Even if a meteor never reaches the earth and burns up its mass is still added to the planet, matter is rarely lost, and this happens every day.
Man, of late we have been moving large amounts of dirt to make places to live, roads, parks, lots of things. Even digging up archaeological sites. ;)
I hope this was of some help to you.
Try the links in the MadSci Library for more information on Earth Sciences.