I wish I could remember who said that planets are the ashes of stars; it’s not quite as lovely as “We are all starstuff,” but it’s certainly encapsulates the process that has to occur in order to get the raw materials for planets. You start out with a cloud of dust and gas in interstellar space; some of that dust and gas is from earlier generation of stars. Some of the materials start to stick together, and then more of them. Eventually, if enough of them stick together (and I’m greatly simplifying the process), you get a world.
Once a planet is formed (ignore the fact that I’ve skipped over a few millions of years of accretion), the process isn’t done just because the planetesimals are done banging together.
Nope, world-building continues, just in different directions and by different processes. On early Earth, there was a surface to be tended to. It got bashed in by incoming bits of stuff left over from the accretion days. In fact, everything with a hard surface in the inner solar system was cratered by incoming bits of interplanetary debris. There’s very little evidence of that bombardment (which geologists call the “late, heavy bombardment”) that you can see openly on Earth’s surface today (unless you know what you’re looking for), but you can see more obvious evidence for it in the cratered surfaces of Mercury and the Moon (for example).
Of more interest to us here on contemporary Earth are the processes of tectonism and volcanism, particularly volcanism. There are volcanoes scattered around the Earth’s surface (both on the continents and under the ocean waves), and they do a good job of building up Earth’s surface.
You can visit a place in Hawai’i where the Earth is created new each day. Lava pours out of this volcano, and under the ocean surface a few miles off the coast of the Big Island, another island is being built, entirely through the hard work of another volcano. I’ve hiked over the most active of Hawai’i’s volcanoes and it is a sobering reminder of the creative (and destructive) forces at work in the planet-building and maintenance game. And, of course, there are other volcanoes on Earth which do a great job of re-paving the surfaces of planets and moons (think Io, at Jupiter, or the ice volcanoes of Triton and other icy moons in the outer solar system).
I mentioned tectonism up there… it’s a short term for a complex set of actions in our Earth’s crust (and on other “hard body” planets and moons, too). Essentially on Earth, the crust is divided into chunks called “plates” and those plates are in motion. You and I are riding along on a plate right now. There are dozens of plates in the Earth’s crust. In some of the places where they meet we see volcanoes formed as rock is heated by the friction of two plates rubbing together. In other places one plate dives under another. In other places,they spread apart, like giant conveyor belts carrying the continents away from each other.
Tectonics are also “blamed” for things like earthquakes, which also reshape the surface of the land. Tectonics builds mountains by shoving huge slabs of Earth’s surface up into peaks. What takes them down? Another surfaces-shaping force called “erosion” (or, what planetary scientists like to call “weathering”). Flowing water, blowing winds, the action of sand on a surface—all these erode the surface on Earth. So do sulfuric acid droplets (so-called “acid rain.”). And, you can see evidence of these forces on other planets. Take a look at Mars sometime and see what erosion, tectonics, and volcanism have done to its desolate surface. Or, check out some of the moons of Jupiter or Saturn. Tectonism and volcanism at work there, too.
When it comes to building planets, as you can see, the work doesn’t stop when the accretion and bombardment do.
