Describe Earth's interior from the crust to the inner core, explain where thermal energy within Earth comes from and how it makes magma, and use igneous rocks to read magnetic reversals

Learning goal: Describe the layers of Earth's interior (crust, Mohorovičić discontinuity, upper mantle, lower mantle, outer core, inner core) with their depths, states and compositions; explain the sources of thermal energy within Earth and how it moves; explain the three ways solid rock melts to make magma; explain how Earth's magnetic field is generated and how igneous rocks record magnetic reversals; and distinguish true north, magnetic north and grid north.

Where this lesson sits. Ohio's PG.IMS.1 (Igneous) lists seven topics, and this site teaches them in two lessons meant to be studied together. This lesson is part 2 of 2: Earth's interior (crust, Mohorovičić discontinuity, upper and lower mantle, outer and inner core), thermal energy within Earth, and magnetic reversals and Earth's magnetic field. Part 1, the lesson on classifying igneous rocks, teaches mafic and felsic rocks and minerals, Bowen's Reaction Series, intrusive structures and the three kinds of volcano; this lesson only refers to them. Neither part alone covers PG.IMS.1.

How we know what is inside. No drill hole has gone more than about 12 km down, a fraction of the 6,371 km to the center, so the interior is known from indirect evidence. Earth's mass divided by its volume gives an average density of about 5.5 g/cm³, but crustal rocks are only about 2.7 to 3.0 g/cm³, so the interior must be much denser than the surface. Seismic waves bend and change speed at boundaries inside Earth, and S-waves, which cannot travel through liquid, do not pass through the outer core. Iron meteorites, fragments of the cores of broken early planetary bodies, suggest what a core is made of. Laboratory experiments at high pressure show which minerals are stable at each depth. Each line of evidence alone is weak; together they fix the picture below.

The layers. Crust: the thin outer rock layer. Oceanic crust is about 5 to 10 km thick, basalt over gabbro, density about 3.0 g/cm³; continental crust is about 25 to 70 km thick, thickest under mountain ranges, with an overall felsic to intermediate composition and a density about 2.7 g/cm³. Mohorovičić discontinuity (Moho): the base of the crust, where P-wave speed jumps from about 6 to 7 km/s to about 8 km/s because the rock changes to denser mantle peridotite. Andrija Mohorovičić identified it from records of a 1909 Croatian earthquake, published in 1910. Upper mantle: from the Moho to about 660 km, solid rock of peridotite composition, made mostly of olivine and pyroxene in its upper part, with those minerals changing into denser high-pressure forms between about 410 and 660 km. Peridotite is an ultramafic rock made of minerals from the high-temperature top of Bowen's series. Its uppermost part, together with the crust, forms the rigid lithosphere; below that lies a hotter, weaker zone that flows slowly. Lower mantle: about 660 to 2,900 km, solid rock in which the same elements are packed into denser high-pressure minerals. Outer core: about 2,900 to 5,150 km, liquid iron with nickel and some lighter elements, the only fully liquid layer. Inner core: about 5,150 km to the center at 6,371 km, solid iron-nickel. It is solid even though its temperature is estimated at roughly 5,000 to 6,000 °C, because the enormous pressure raises iron's melting point above that temperature.

The misconception: a sea of magma. Many students picture the mantle as molten. It is not. S-waves travel through the whole mantle, which they could not do through a liquid. The mantle is hot solid rock that creeps very slowly, centimeters per year, the way ice in a glacier flows while staying solid. Magma forms only in small, particular places, which is why volcanoes are concentrated along certain lines rather than everywhere.

Thermal energy within Earth. Earth's interior heat has two main sources. The first is primordial heat left from Earth's formation: the energy of colliding planetary fragments and of heavy iron sinking to form the core. The second is the ongoing radioactive decay of long-lived isotopes of uranium, thorium and potassium, concentrated in the crust and mantle. Heat moves outward in three ways. Conduction carries it through the rigid lithosphere. Convection carries it through the mantle, as hotter, less dense rock slowly rises and cooler, denser rock sinks, and through the liquid outer core. Magma carries it bodily when it rises. In the upper continental crust, temperature typically rises by about 25 to 30 °C for every kilometer of depth; this rate, the geothermal gradient, is lower in old, stable continental interiors and higher near volcanoes and spreading ridges, and it does not continue at the same rate all the way down.

How solid rock melts. Hot rock deep underground is usually below its melting point for the pressure it is under, so heat alone does not make magma. Three changes do. Decompression melting: when hot mantle rises, beneath a mid-ocean ridge or in a hot-spot plume, the pressure on it drops faster than it cools, and its melting point falls below its temperature; a few percent of it melts to basaltic magma. Adding water (flux melting): at a subduction zone, the sinking plate releases water into the hot mantle above it, and water lowers the melting point of rock, producing magma that feeds composite volcanoes. Heat transfer: hot basaltic magma that pools under continental crust passes its heat to the crust, which has a lower melting point, producing felsic magma. The type of melting therefore predicts the rock: basalt at ridges and hot spots, andesite and granite where plates converge. Most magma never reaches the surface. It stalls in the crust and crystallizes as intrusive igneous structures: dikes that cut across layers, sills that slide between them, batholiths built of many merged bodies, and pegmatites from the last water-rich melt, each carrying the crust's heat upward even though it never erupts.

Earth's magnetic field. A compass needle aligns with a magnetic field that surrounds Earth and shields it from much of the charged particle stream from the Sun. Near the surface the field resembles that of a bar magnet tilted about 10° from the rotation axis. It cannot actually be a permanent magnet: above a temperature called the Curie point, about 580 °C for magnetite and about 770 °C for iron, materials lose permanent magnetism, and almost all of Earth's interior is hotter than that. Instead, convection of the electrically conducting liquid iron of the outer core, organized by Earth's rotation, generates electric currents, and the currents sustain the field. This self-sustaining process is called the geodynamo.

Magnetic reversals and the igneous record. When lava cools below the Curie point, tiny magnetite grains in it become magnetized in the direction of the field at that moment, and the rock keeps that record. Measurements of lava flows of known age show that the field has reversed polarity many times: a compass would have pointed south. Reversals come at irregular intervals, from tens of thousands to tens of millions of years apart; the most recent full reversal was about 780,000 years ago. The strongest test came from the ocean floor. Magnetometers towed across mid-ocean ridges found stripes of rock alternately magnetized in the normal and reversed direction, lying parallel to the ridge and matching in width and order on both sides. The explanation is that new basalt forms at the ridge, records the field, and moves away on both sides, so the stripes are a tape recording of reversals. Here is a model worked through. A stripe boundary known to be 780,000 years old lies 39 km from the ridge axis. The spreading rate on that side is 39 km ÷ 0.78 million years = 50 km per million years, which is 5,000,000 cm ÷ 1,000,000 years = 5 cm per year. The same boundary on the other side at 39 km would mean the ridge opens symmetrically at 10 cm per year in total.

True, magnetic and grid north. True north points to the geographic North Pole, where Earth's rotation axis meets the surface. Magnetic north is the direction a compass points, along the local field lines, broadly toward the north magnetic pole, which lies in the Arctic but wanders; it has moved hundreds of kilometers over the past century. The angle between true and magnetic north at a place is the magnetic declination, which differs from place to place and changes year by year. Grid north is the direction of the vertical grid lines printed on a map; because a curved Earth is drawn on a flat sheet, grid north can differ slightly from true north away from the grid's central line. Topographic maps print a diagram showing all three, dated, because the magnetic one changes.

Worked example, Earth's average density: Earth's mass is about 5.97 × 10²⁴ kg and its radius 6.371 × 10⁶ m. Volume = (4/3)πr³ = (4/3) × 3.1416 × (6.371 × 10⁶)³ ≈ 1.083 × 10²¹ m³. Density = 5.97 × 10²⁴ ÷ 1.083 × 10²¹ ≈ 5,510 kg/m³, about 5.5 g/cm³. Crustal rock is about 2.7 to 3.0 g/cm³, roughly half that, so the deep interior must be denser than 5.5 g/cm³ on average; an iron-rich core fits.

Worked example, a geothermal gradient: In a hypothetical region the surface rock is 12 °C and the gradient is 25 °C per km. At the bottom of a 3 km mine the rock would be about 12 + 3 × 25 = 87 °C, which is why deep mines need refrigerated air. Extending the same rate to 100 km would give 2,512 °C, far hotter than measured estimates; the gradient decreases with depth, because radioactive heat production is concentrated near the surface and convection carries heat efficiently in the mantle. A gradient is a local measurement, not a law for the whole Earth.

Worked example, reading magnetic stripes: On a hypothetical profile the boundary of a stripe dated 2.6 million years lies 65 km east of the ridge axis. Spreading rate on that side = 65 km ÷ 2.6 million years = 25 km per million years = 2.5 cm per year. If the matching boundary to the west is at 52 km, the western rate is 52 ÷ 2.6 = 20 km per million years, 2.0 cm per year: the ridge is spreading slightly asymmetrically, and the total opening rate is 4.5 cm per year.

Worked example, correcting a compass bearing: A map says the declination is 7° west (a hypothetical value). West declination means the compass needle points 7° west of true north. A compass bearing of 90° therefore lies 7° counterclockwise of true east, at a true bearing of 90 − 7 = 83°. With east declination you add instead. Checking the sign against a sketch of the two north arrows avoids the most common error.

Investigation, mapping a magnetic field and a reversal: Materials: a bar magnet, a small compass, a sheet of paper, and iron filings sealed inside a clear zip bag. Place the magnet under the paper, set the compass at 16 points around it and draw an arrow for the needle at each point; then lay the sealed bag over the magnet to see the field lines. Turn the magnet end for end and repeat at the same 16 points. Record the direction at each point before and after. Result that supports the claim: every arrow reverses while the pattern of lines stays the same, which is what a geomagnetic reversal would look like to a compass. What the model cannot show: the geodynamo is not a permanent magnet but moving liquid iron far above the Curie point, and real reversals take thousands of years and pass through a weak, disordered field.

Representation task: Draw Earth's interior to scale along one radius 63.7 cm long (1 mm = 10 km), using a strip of adding-machine paper. Mark the Moho at an average continental depth of 35 km (3.5 mm), and the boundaries at 660, 2,900 and 5,150 km. Label each layer with its state (solid or liquid), its main materials and one line of evidence. Then add arrows showing where heat moves by conduction and where by convection.

Independent performance: Write an explanation of at least 250 words answering: "Why does Earth have volcanoes in some places and not others, if the mantle is hot everywhere?" Success criteria: it states that the mantle is solid and gives the seismic evidence; it names the two sources of Earth's thermal energy; it explains decompression melting, flux melting and heat transfer, each with a place where it happens; it predicts the kind of igneous rock each produces; and it ends by explaining how a basalt formed at a ridge records the magnetic field of its time.