A planet built in layers
Earth is not a uniform ball of rock. It is organized into layers that differ in composition, temperature and how they behave under pressure. Scientists cannot drill anywhere near the center, so most of what is known about Earth structure comes from indirect evidence: the way earthquake waves travel through the planet, the density of the whole Earth, laboratory experiments on materials under extreme pressure, and the study of meteorites that resemble the planet’s raw ingredients.
From the outside in, the main layers are the crust, the mantle, the outer core and the inner core. The planet’s radius is a little over 6,000 kilometers, so the distances involved are large, and the crust is a very thin skin by comparison.
The crust
The crust is the layer we live on and the only one we can sample directly. It comes in two broad types. Oceanic crust lies beneath the ocean basins; it is relatively thin, often somewhere around 5 to 10 kilometers, and is made mostly of dark, dense volcanic rock. Continental crust is thicker, typically in the range of 30 to 50 kilometers and thicker beneath big mountain ranges, and it is made of a wider mix of rocks that are generally lighter than oceanic rock.
That difference in density matters. Lighter continental crust sits higher, while denser oceanic crust sits lower and holds the oceans. Oceanic crust is also geologically young compared with continents, because it keeps being created at some boundaries and recycled at others.
The mantle
Beneath the crust lies the mantle, which makes up most of the planet’s volume. It extends to a depth of roughly 2,900 kilometers. The mantle is mostly solid rock, but over very long timescales it flows slowly, a bit like extremely stiff putty. Temperatures rise with depth, and hot material tends to rise while cooler material sinks, producing a slow churning called convection. Speeds are commonly compared to the rate fingernails grow, which gives a sense of how gradual it is.
The very top of the mantle is cool and rigid enough that it moves together with the crust. Together they form the lithosphere, a brittle outer shell. Below it is a softer, weaker zone often called the asthenosphere, which allows the shell above to shift. Exactly how much of the mantle churns as one system or several is still an active research question.
The core
The core is mostly iron with some nickel and a small share of lighter elements, though the exact mix is uncertain. It has two parts. The outer core is liquid, and the inner core is solid, kept in a solid state by enormous pressure even though it is extremely hot. Estimates put the inner core’s temperature in the neighborhood of the surface of the Sun, though such figures carry uncertainty.
Movement of liquid metal in the outer core is generally thought to generate Earth’s magnetic field. That field extends far into space and helps deflect charged particles from the Sun. Without a liquid, convecting outer core, the field would likely look very different.
Heat: the engine behind everything
Earth’s interior is hot for two main reasons: leftover heat from the planet’s formation, and heat released by the decay of radioactive elements in the rocks. That heat flows outward, and it drives the convection that moves the mantle. Most large-scale geology can be traced back to this slow escape of heat.
Plates and the moving surface
The lithosphere is broken into large pieces called plates. They move a few centimeters per year, and they slide past, pull apart from, or collide with one another. This is the core idea of plate tectonics. At spreading boundaries, new crust forms as magma rises. At subduction zones, one plate dives beneath another and is recycled into the mantle. Where plates collide, mountain ranges rise.
Most earthquakes and volcanoes cluster along plate boundaries, which is why hazards are so unevenly spread across the world. The details differ from place to place: why earthquakes happen depends on how stress builds along faults, volcanic eruptions depend on magma and gas, and tsunami waves depend on sudden movement of the seafloor. Each of those is a surface expression of the same deep heat engine.
Surface systems that link to the interior
The solid Earth does not operate alone. Rocks are weathered, carried away, buried and sometimes reheated or melted, in a loop known as the rock cycle. Water moves between oceans, air, ice and land in the water cycle. Volcanoes release gases that have helped shape the atmosphere over geologic time, and the air, with its atmospheric layers and ever-changing weather systems, in turn helps break down rock and move sediment.
Ocean currents and the climate system redistribute heat at the surface, while plate motion slowly rearranges continents and ocean basins, which in turn changes those currents over millions of years. Over the very long term, the arrangement of continents, volcanic activity and the composition of the air have all contributed to the swings between ice ages and warmer periods.
Life and the record in the rocks
Rocks also carry a history of life. Fossil formation preserves traces of organisms in sedimentary layers, and the order of those layers gives geologists a way to tell a relative sequence of events. Dating methods based on radioactive decay add numbers to that sequence. Such evidence indicates an Earth roughly 4.5 billion years old, with continents, oceans and climates that have changed many times.
What we still do not know
It is worth being honest about the limits. Seismic waves give a good picture of the layers, but details such as the precise composition of the core, how the magnetic field changes over time, and exactly when plate tectonics began are still debated. Models are improved as new measurements come in. What is well established is the overall layered structure, the role of heat and convection, and the moving plates that connect deep processes to the landscapes, oceans and hazards we experience at the surface.