Earth's Interior

Start with a real problem. Earth's radius is 6,371 kilometers. The Kola Superdeep Borehole in Russia is the deepest hole ever drilled, and it stopped at 12 kilometers when the rock got too hot and soft to keep going. We have direct samples of only the thinnest outer skin. So how can any textbook describe a liquid layer 3,000 kilometers down?

The answer is seismic waves. Every large earthquake sends P-waves and S-waves through the entire planet, and thousands of seismographs around the world record when and how they arrive. Those waves change speed and bend when they cross into material with different properties, the way light bends entering water. By comparing millions of arrival records, scientists have mapped where the wave speeds jump, where the waves bend, and where certain waves vanish entirely. Each of those changes marks a boundary inside the Earth. The planet gets X-rayed for free, thousands of times a year.

Two other clues round out the picture. Earth's overall density, calculated from its gravity, is about 5.5 g/cm³, yet surface rocks average only around 2.8 g/cm³. Something far denser must be hiding in the middle. And iron meteorites, leftovers from the building of the solar system, hint at what that something is made of.

Put all that evidence together and Earth sorts into four main layers. The crust is the cold, brittle outer layer. Under the continents it is granitic rock averaging about 35 kilometers thick. Under the oceans it is denser basaltic rock, often less than 10 kilometers thick. It is the only layer anyone has ever touched.

Below the crust is the mantle, and it makes up most of the planet. It is nearly 2,900 kilometers thick and holds most of Earth's volume. It is made of dense silicate rock rich in iron and magnesium. The mantle is solid, but it is hot enough that over millions of years it turns over in the slow convection currents that drive plate tectonics.

Then the material changes completely. At about 2,900 kilometers down, rock gives way to metal. This is the outer core, made of molten iron and nickel and more than 2,000 kilometers thick. Its slow, swirling currents of liquid metal generate Earth's magnetic field. That field is why a compass works, and it also shields us from solar radiation. At the very center sits the inner core, a ball of iron and nickel roughly two thirds the width of the Moon. It is hotter than the outer core, yet it is solid. That sounds backwards until you think about what is sitting on top of it. The pressure of the entire planet squeezes its atoms so tightly that they cannot flow, no matter how hot they get.

The four-layer model sorts Earth by what it is made of. Plate tectonics needs a second way of sorting, based on how the material behaves. The lithosphere is the rigid outer shell, roughly the top 100 kilometers. It cuts straight across the crust-mantle boundary, because it is the crust plus the uppermost mantle, joined into stiff slabs. Those slabs are the tectonic plates. Beneath them is the asthenosphere, mantle rock hot enough to lose its stiffness. It is still solid, but it can flow about as fast as your fingernails grow. That is the soft layer the plates slide over.

The single most elegant piece of evidence in this chapter is an absence. After a large earthquake, stations on the same side of the planet record both P-waves and S-waves, right on schedule. But stations on the far side of the planet never receive S-waves at all. A huge region of Earth's surface sits in an S-wave shadow.

You already know the rule that explains it. S-waves cannot pass through a liquid. If S-waves stop somewhere in the middle of the planet, then there must be a liquid there. The size of that liquid layer can be measured from the size of the shadow. That liquid is the outer core. P-waves do make it through, but they bend sharply as they cross into the liquid. That bending casts a partial shadow of its own, which confirms the boundary. One earthquake, plus one rule about waves, reveals a layer of molten metal no one will ever see.

Drag the probe from the surface to the center, 6,371 kilometers down. Watch the boundaries go by, from rock to metal and from liquid back to solid. These values are approximate. The exact model is on page 11 of your reference tables.

Three quantities rise the entire way down, and the exam expects you to know it. As depth increases, temperature, pressure, and density all increase. Temperature climbs from the surface toward several thousand degrees at the center, roughly as hot as the surface of the Sun. Pressure climbs because every kilometer down adds another kilometer of rock, and eventually metal, stacked overhead. Density climbs for two reasons. Deeper material is squeezed harder, and the material itself changes, from rocky silicates near 3 g/cm³ to iron near 13 g/cm³ at the center.

The interplay of temperature and pressure explains the strangest fact in this chapter. Melting is a fight between heat, which loosens atoms, and pressure, which pins them in place. In the outer core, heat wins and the metal is liquid. In the inner core, despite even higher temperatures, pressure wins and the metal is solid. Same material, different champion.

These are the deepest earthquakes recorded worldwide in the past 30 days, straight from the USGS. Every one of them happened inside a subducting slab of cold lithosphere sinking through the mantle. The fact that rock can still snap at 500 or 600 kilometers down is evidence in itself. The sinking slabs stay rigid deep inside a mantle that is otherwise too soft to break.

• Model of Earth's Interior Structure (page 11). The whole chapter on one page. Practice reading it in every direction: name the layer at a given depth, read the thickness and density of each layer, and state its physical state. Everything on this page is inferred from seismic evidence, not observed directly.
• Generalized Cross Section Model of Earth's Surface and Interior (page 11). Sits beside the layer model and connects the interior to what happens at the surface: ridges, trenches, hot spots, and volcanoes.

Download the current tables from NYSED (2026 Revised Edition, used from the 2026-27 school year): Reference Tables for Earth and Space Sciences.

• Knowledge of Earth's interior is inferred, mostly from how seismic waves speed up, bend, and disappear as they pass through the planet.
• The four compositional layers: thin rocky crust, thick solid mantle, liquid iron-nickel outer core, solid iron-nickel inner core.
• The S-wave shadow zone proves the outer core is liquid, because S-waves cannot pass through liquids.
• The lithosphere (crust plus uppermost mantle) is rigid and forms the plates; the asthenosphere below it is solid but flows, letting the plates move.
• As depth increases, temperature, pressure, and density all increase.
• The inner core is hotter than the outer core but solid, because extreme pressure keeps it from melting.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.