Great Smokey Mountains, Gatlinburg TN, Credit: JGP
Earth, one of the rocky, terrestrial planets, originiated as a soft mass of accreted smaller planetesimals. As the solar nebula cooled, material changed from gaseous to liquid and to solid state. You have observed everyday condensation when water droplets form as air cools near the ground, forming dew. You have observed water freezing (crystallizing) to ice. Other materials undergo the same changes of state as the temperature drops. Once together as a "blob" of material, gravitation settling and chemical fractionation lead to an internal layering of Earth, with an Iron-rich core, a silicate mineral mantle forming its bulk, and a thin outer rind, or crust, of light material. Much material in gaseous form drifted off, but Earth is large enough to have a gravitational pull adequate to hold at least some of the gases. This was the beginning of our atmosphere. The traditional explanation for water in the oceans and on land is that it was steadily released through volcanic activity, to accumulate in such quantity that we should really call our planet "the water planet." There have also been ideas that our water came from comets, but recent work suggests a misfitting chemical characteristic for the water. Other recent work has pointed to asteroids which have a composition better fitting a "water source" chemistry. Another study suggests instead that water clung to the primordial dust. This is a most important question, not just for appreciating Earth history, but for the origin of Life -- here as well as on other planets and moons in our solar system and beyond.
We infer from radiometric dating of moon rocks, Earth rocks, and meteorites, that Earth and the rest of the solar system, formed by 4.6 billion years ago.
Earth, like other planets and moons, underwent an early period of heavy bombardment, as there were still pieces of solidified material orbiting the Sun that were yet to be pulled in and captured by a larger planet. Earth remains hot inside and has been volcanically active during its entire lifetime, and Earth's surface temperature allows water to exist in liquid form. These are reasons why the cratering that happened during the heavy bombardment stage has been obliterated by mountain building, continental assembly, and surface process on Earth. Not so for the Moon, which stands in static testimony to the entire record of bombardment, which long ago tapered off, so that the Moon receives the rare large impact, and the occasional smaller impact.
The Moon, with an inset showing a patch of cratering. The large image of the moon was taken with the 0.9-m telescope and Mosaic CCD camera of Kitt Peak National Observatory, mounted on another image of the background stars taken with the 4-m Mayall telescope. The inset image is from Apollo Over The Moon: A View From Orbit (NASA SP-362) . Craters in the inset image are located in central Mare Serenitatis, one of the darker-colored areas of the Moon. You can observe that a small crater in the center of the inset image has rays of ejected material radiating outward, covering other craters, which indicates a younger age for the crater with the rays.
We have a few young craters clearly visible on Earth, but older ones have been obscured by geological activity over millions of years, by the action of running water and moving ice, and by the action of vegetation covering the ground. Meteor Crater in Arizona is one such young crater. It is only 20,000 to 50,000 years old.
Meteor Crater, Arizona. CREDIT: Smithsonian Scientific Series (1929), taken by the U.S. Army Air Service.
Earth is divided into distinct layers, the core, the mantle, and the crust. There are finer divisions, as shown below:
USGS Interior of Earth Diagram. CREDIT: USGS Dynamic Earth .
core -- The deepest layer, very rich in iron and hot. Inner core is solid, because there is so much pressure at the greatest depths, that the iron-rich compounds exist in the solid state, even though it is super hot. The outer core is molten, because the pressure in this zone is not as great, allowing the iron-rich material to flow freely as liquid.
mantle -- The "middle" layer, rich in silicate minerals dominated by iron and magnesium, and containing most of Earth's volume. Mineral types vary slightly, but the mantle is very uniform overall.
crust -- An outermost "rind" composed of lighter alumino-silicate minerals, and more rigid than the underlying mantle. There are many more mineral and rock types in the crust than in the mantle. The crust is very thin, only a few to about 100 km thick.
asthenosphere -- The so-called "weak layer" is a zone in the upper part of the mantle where the composition of the minerals and the pressure/temperature combination result in partial melting. This "slippery" condition of the asthenosphere allows the overlying lithosphere to move around.
lithosphere -- The lithosphere includes the outer part of the mantle, lying above the asthenosphere, and the crust. The lithosphere is broken into several large and smaller plates that move around, over the asthenosphere. These are the plates of plate tectonics fame. Plates of lithosphere are more rigid, and include continents as well as ocean floor.
General reference is the Moon entry at NASA .