A Brief Overview of Geology
Throughout the course, we will explore not only what geologists learn, but how we learn it. For the first few Modules, however, we ask you to take our word for some things. You will see statements such as “The Earth is 4.6 billion years old.” Before we’re done, you will also see where that number comes from, how good it is, and much more about it. But we can’t do everything at once.
Anyway, we believe that the universe started in the “Big Bang” about 14 billion years ago. The Earth is “only” about 4.6 billion years old. We live in a second-generation solar system because the planets and the sun contain abundant chemicals such as iron that were first formed during the death of older stars. So, there were some stars, and they exploded and generated gases and dust, and something (another nearby star exploding?) caused that gas and dust cloud to be compressed a little. Once the dust and gas started falling together, gravity took over. Eventually, most of the mass went to form the sun, which was squeezed enough under gravity that the sun’s hydrogen began fusing to form helium, in the process releasing energy—sunlight! Some of the gas and dust collected into planets.
Assembly of the Earth involved the falling together of lots of big and little chunks. The largest chunk was probably about the size of Mars. It hit the Earth after most of the assembly was finished and blasted enough material off the Earth to make the moon. (Note that there is still a little discussion in the scientific literature about the details of this Mars-sized moon-forming collision, so stay tuned...)
The falling-together of pieces makes heat, as the pieces give up their energy as they stop. (If you have ever smelled the burning brakes on a large truck that has had to stop suddenly on a steep hill, you know that stopping gives up heat. Something similar happened to the Earth.) That heat partially or completely melted the planet. The melting allowed the planet to differentiate or become layered. The denser material sank to make a core, mostly of iron with some nickel and a few other elements. The lowest-density material rose to the top to form a silicate scum, or crust, floating on a vast mantle of denser silicate (see the sidebar on chemistry). The Earth is hottest in the middle and coldest on the outside. Heat favors melting, but higher pressure tends to make most liquids turn solid. These two effects compete in the Earth, so you find both solid and liquid down there. Going down in the Earth, the crust and the upper part of the mantle are solid (together forming the lithosphere) except in special places where volcanoes occur; the deeper part of the mantle is solid but soft and has a zone of about 100 km (60 miles) down in which a little melting occurs. The soft zone in the mantle is the asthenosphere--we will not learn a huge number of new words in this class, but we do get a few great ones! The core has two layers, a solid inner core, and a liquid outer core.
The image is a labeled diagram showing the internal structure of the Earth, both to scale and not to scale. The not-to-scale section is a cutaway illustration depicting the layers of the Earth. At the top, there is a rugged, mountainous landscape representing the Earth's crust. Directly beneath the crust is the asthenosphere, which is shown as an orange layer. Below the asthenosphere is the mantle, depicted in shades of yellow and gray. Further down, the outer core is marked as "Liquid" with a gray color, and the inner core is labeled as "Solid" with a darker gray hue. The diagram also shows distances from the surface downward: 0-100 km for the crust, 2900 km to the boundary of the outer core, 5100 km to the inner core, and the total radius of the Earth at 6378 km. On the left, there is a circular cross-section labeled "To scale," which graphically represents the proportionate thickness of the layers. The lithosphere is also noted as comprising the crust and the uppermost solid mantle.
Some of the Earth’s heat is left over from when the planet formed, and a lot comes from the decay of naturally radioactive materials in the Earth. As the early heat has escaped and the radioactive materials have decayed, the Earth has slowly been cooling off, but plenty of heat remains to drive geologic processes. The Earth has developed an atmosphere, oceans, and life, and a rich sedimentary history of how those developed. The atmosphere and oceans spend their time wearing down mountains, but the heat of the Earth keeps driving processes that build mountains up, so there is a near-balance. And all of this should become clear as we tour the national parks.