What geology studies
Geology is the study of Earth: what it is made of, how it is built, and how it has changed over about 4.5 billion years. Geologists read rocks the way historians read documents, using layers, minerals and fossils as evidence for events that no one witnessed. Because most of the planet is buried, they combine direct observation of surface rocks with indirect methods that probe the interior.
The layers of the Earth
Earth is organized into concentric layers, like an onion, separated by differences in composition and temperature.
Crust. The thin, rigid outer skin. Oceanic crust, under the seas, is dense rock rich in a dark volcanic type called basalt and is typically around 5 to 10 kilometers thick. Continental crust is lighter, more varied, and typically around 30 to 50 kilometers thick, thicker beneath big mountain ranges.
Mantle. Below the crust lies the mantle, about 2,900 kilometers thick, which makes up most of the planet’s volume. It is solid rock, but over millions of years it can creep and flow very slowly, like extremely stiff putty. Heat from deep inside drives this slow churning.
Outer core. A layer of liquid, mostly iron and nickel, roughly 2,200 kilometers thick. Motion in this layer is what generates Earth’s magnetic field.
Inner core. A solid ball, mostly iron and nickel, with a radius of roughly 1,200 kilometers. It is extremely hot, in the range of several thousand degrees Celsius, but the pressure is so high that it remains solid.
The planet’s average radius is about 6,370 kilometers, so no one has ever drilled more than a tiny fraction of the way down. The deepest boreholes reach only around 12 kilometers.
How we know what is inside
Scientists learn about the interior mostly from seismic waves, the vibrations produced by earthquakes. Waves speed up, slow down, bend and reflect as they pass through materials with different properties. One type of wave cannot travel through liquid, and its absence on the far side of the planet after large quakes revealed that part of the core is liquid. Laboratory experiments on rocks under high pressure, studies of meteorites and measurements of Earth’s gravity and magnetism add to the picture.
The three families of rock
Rocks are collections of minerals, and geologists group them by how they formed.
- Igneous rocks form when molten rock cools and solidifies. If it cools slowly underground, large crystals can grow, as in granite. If it cools quickly at the surface, crystals are small or absent, as in basalt.
- Sedimentary rocks form from layers of loose material, such as sand, mud, shell fragments or dissolved minerals, that are buried and compacted or cemented over time. Sandstone and limestone are examples. They often contain fossils and record past environments.
- Metamorphic rocks form when existing rocks are changed by heat and pressure without fully melting. Limestone can become marble, and shale can become slate.
Over long periods, any rock can be turned into another type, and this recycling is described in the rock cycle. Many sedimentary rocks, in turn, preserve the remains of ancient life, which relates to how fossil formation works.
Plate tectonics
Earth’s rigid outer shell, the crust together with the uppermost mantle, is broken into large pieces called tectonic plates. There are about a dozen major plates and several smaller ones. They ride on a hotter, softer layer beneath and move at speeds of a few centimeters per year, roughly the rate at which fingernails grow.
The theory of plate tectonics gained wide acceptance in the 1960s, building on the earlier idea of continental drift. Evidence included matching coastlines and fossils on separate continents, the pattern of earthquakes and volcanoes, and the symmetric magnetic stripes in rocks on either side of mid-ocean ridges.
The plates interact at their boundaries in three main ways.
- Divergent boundaries: plates move apart, and new crust forms as magma rises, as at mid-ocean ridges.
- Convergent boundaries: plates move toward each other. If one is oceanic, it usually sinks beneath the other in a process called subduction, forming deep trenches and chains of volcanoes. When two continental plates collide, neither sinks easily, and the crust crumples into mountain ranges.
- Transform boundaries: plates slide past each other horizontally, sometimes locking and then slipping suddenly.
Why this matters on the surface
Most earthquakes and volcanoes occur near plate boundaries, though there are exceptions such as volcanic hot spots in plate interiors. The sudden slip of rock along a fault is the usual cause of earthquake causes in tectonic settings, while magma reaching the surface in volcanic eruptions often reflects subduction or rifting. Large displacements of the seafloor can generate tsunamis.
Over hundreds of millions of years, moving plates have repeatedly assembled and broken up supercontinents, opened and closed oceans, and recycled the seafloor. The oldest ocean floor is only on the order of 180 million years old, because older oceanic crust has been consumed in subduction zones. Continental rocks, in contrast, include material billions of years old.
Deep time
Geology works on timescales that are hard to picture. Slow processes such as erosion, sediment buildup and plate movement, acting for millions of years, can build mountains and wear them down. This idea, that the processes we see today also shaped the past, is a foundation of the field. Scientists use radioactive decay in minerals as a clock to assign ages to rocks, with results consistent across many independent methods.
Key points
- Earth has a crust, mantle, liquid outer core and solid inner core.
- Igneous, sedimentary and metamorphic rocks are linked through recycling.
- Plates move a few centimeters a year, driving earthquakes, volcanoes and mountain building.