Describe the structure of Earth from the Moho to the core and use gravity, magnetism, isostasy and heat flow as evidence

Learning goal: Describe the composition and physical state of each layer of Earth; distinguish the lithosphere from the asthenosphere and locate the Mohorovičić boundary; explain how seismic wave behavior fixes where the layers are; use gravity, magnetism and isostasy as independent evidence about the structure of Earth; and describe thermal energy inside Earth in terms of the geothermal gradient and heat flow; research an active area, plan how to harness its energy and evaluate the efficiency of that plan.

Two ways to slice the planet. Grade 8 introduced the crust, mantle, outer core and inner core. Crust, mantle and core are layers defined by composition (two kinds of silicate rock, then iron-nickel metal); the outer core and inner core are both mostly iron and are told apart by physical state, liquid above solid. Physical geology adds a second set of layers defined by mechanical behavior, and the two sets do not line up. The lithosphere is the rigid outer shell, made of the crust plus the cool uppermost mantle, on average about 100 km thick: thinner and younger under the oceans (close to zero at a spreading ridge, thickening as the seafloor ages), thicker under old continents. It is the lithosphere, not the crust, that is broken into tectonic plates. Beneath it is the asthenosphere, a zone of the upper mantle, extending from the base of the lithosphere to a depth of roughly 200 to 300 km, where the rock is close enough to its melting point to be weak and to flow slowly, at rates of centimeters per year, while still being solid. Seismologists see the asthenosphere as a low-velocity zone: both P- and S-waves slow slightly on entering it, and because S-waves do pass through it, it is solid, not a sea of magma. A tiny fraction of melt along grain boundaries is enough to weaken it.

The Moho. The Mohorovičić boundary (the Moho) is the base of the crust, where seismic velocity jumps: P-waves increase from roughly 6 to 7 km/s in the lower crust to about 8 km/s in the mantle rock beneath. The Croatian seismologist Andrija Mohorovičić recognized it from records of an earthquake in the Kupa Valley in 1909 (published in 1910): stations at moderate distances from it received two P arrivals: a direct wave through the crust and a faster wave that had refracted along the top of the mantle and then come back up. The Moho lies at about 5 to 10 km below the ocean floor and about 30 to 50 km below continents, deepest (up to about 70 km) under high mountain ranges. Note the distinction: the Moho is a compositional boundary inside the lithosphere; the base of the lithosphere is a mechanical boundary inside the mantle.

Composition of each layer. The oceanic crust is basalt over gabbro, dense (about 3.0 g/cm³) and thin. The continental crust is on average close to granite or andesite in composition, richer in silica, less dense (about 2.7 g/cm³) and thick. The mantle has the composition of peridotite. In the upper mantle that chemistry forms mostly the iron-magnesium silicate minerals olivine and pyroxene; deeper, the same elements recrystallize into denser high-pressure minerals, with density rising from about 3.3 g/cm³ at the top to about 5.5 g/cm³ at the bottom as pressure squeezes the minerals into denser crystal structures; the sharp velocity jumps at about 410 and 660 km depth mark these mineral transformations, not changes in chemistry. The core is mostly iron with some nickel and a few percent of lighter elements, about 10 to 13 g/cm³; the outer core is liquid and the inner core is solid. A whole-Earth density of about 5.5 g/cm³ against surface rock of 2.7 g/cm³ was the earliest evidence that the interior is not the same as the surface.

Disputing a homogeneous Earth. Suppose someone claims Earth is homogeneous throughout. The claim can mean three different things, the same material, the same physical state or the same physical properties everywhere, and each piece of evidence rules out a particular one. (1) Average density: the mass of Earth, known from its gravitational pull, divided by its volume, gives 5.5 g/cm³, about twice the density of granite (2.7) and nearly twice that of basalt (3.0), the common surface rocks. Pressure does squeeze rock, but high-pressure experiments show that silicate rock compresses too little to bring the average to 5.5, so a large part of the interior must be a different, denser material: the composition is not uniform. (2) Seismic velocities are not constant: they jump at the Moho, at 410 and 660 km, and drop at the core boundary, so the physical properties are not uniform. A jump on its own does not prove a change of material: the Moho and the core boundary are changes of material, but the 410 and 660 km jumps are the same mantle composition changing to denser crystal structures. (3) The S-wave shadow requires a liquid layer, so the physical state is not uniform. (4) Earth has a strong magnetic field, which requires a large body of moving, electrically conducting fluid, consistent with a liquid iron outer core and impossible for a uniform silicate planet. The lines are independent, and together they rule out a homogeneous Earth in all three senses.

Gravity as evidence. The strength of gravity at the surface varies by tiny amounts from place to place, measured in milligals, one thousandth of a centimeter per second squared. Geologists correct each reading for latitude, for elevation and for the pull of the rock standing between the station and sea level. What remains is the Bouguer anomaly, and it reflects the density of the rock beneath. Dense ore bodies or basalt make it positive; low-density salt, sediment or a thick root of continental crust make it negative. Bouguer anomalies over mountain ranges are strongly negative: once the pull of the visible mountain is removed, there is less mass beneath than a uniform crust would hold, which leads directly to isostasy. (The raw gravity reading over a balanced mountain range is not unusually low; the deficit appears only after that correction.)

Isostasy. The lithosphere floats on the weak asthenosphere the way an iceberg floats on water: each block sinks until the mass it displaces equals its own mass. A thick, light block stands high and also has a deep root; a thin or dense block sits low. This is isostasy, and it explains why continents stand about 4 to 5 km above the ocean floors: continental crust is both thicker and less dense than oceanic crust. A fully worked model: take continental crust of density 2.7 g/cm³ floating in mantle of density 3.3 g/cm³, and suppose a mountain range stands 3 km above its surroundings. The extra mass of the 3 km of mountain must be balanced by a root of crust replacing denser mantle. Each kilometer of root removes a density difference of 3.3 minus 2.7 = 0.6 g/cm³ of mantle, so root thickness = 3 × 2.7 ÷ 0.6 = 13.5 km. The range needs a root about four and a half times its height, which is why the Moho is deepest under mountains and why the Bouguer gravity anomaly over them is negative. Erode the mountain and the root rises: this isostatic rebound is also seen where the weight of an ice sheet has been removed, and land once under the ice in Canada and Scandinavia is still rising by millimeters to a centimeter a year.

Magnetism as evidence. Earth's magnetic field resembles that of a bar magnet tilted about 10 degrees from the rotation axis, but no permanent magnet can exist in the core: magnetite loses its magnetism above about 580 °C and iron above about 770 °C, and the core is thousands of degrees. The field is generated by the geodynamo, the convective motion of electrically conducting liquid iron in the outer core, organized by Earth's rotation. This is why magnetism is evidence for a liquid, conducting, moving outer core. The field also reverses polarity at irregular intervals, which becomes crucial evidence in the historical review of plate tectonics.

Thermal energy: geothermal gradient and heat flow. Earth's interior is hot for two reasons: leftover heat from the planet's formation, when impacts and the sinking of iron to the core released gravitational energy, and continuing decay of radioactive isotopes (mainly of uranium, thorium and potassium) concentrated in the crust and mantle. The geothermal gradient is the rate at which temperature rises with depth; in ordinary continental crust it is roughly 25 to 30 °C per kilometer near the surface, higher in volcanic regions and lower in old, stable continental interiors, and it must flatten at depth or the mantle would melt. Heat flow is the rate at which thermal energy leaves the surface per unit area, typically a few hundredths of a watt per square meter, thousands of times smaller than the sunlight the surface receives, yet enough over the whole planet to drive mantle convection, volcanism and plate motion. In the rigid lithosphere heat moves mainly by conduction; in the asthenosphere and deeper mantle it moves mainly by convection, the slow rise of hot rock and sinking of cold rock. Where the gradient is steep, at hot springs, volcanic areas and young rift zones, geothermal energy can be harnessed; where it is ordinary, as in Ohio, the common practical use is the shallow, steady ground temperature for heat pumps; deeper heat for direct heating or power would need deep drilling and suitable rock and water, which depends on the site, the technology and the cost.

Harnessing the energy of an active area. The course asks for research on one specific place where geologic processes are active and a plan to use the energy they release. The method is the same for any site. (1) Identify the process and its energy source: heat from shallow magma, hot water circulating through fractured rock, or water falling over a fault scarp. (2) Collect the site's own data: temperature at depth, the flow rate of hot fluid or of the river, the height of fall, and the hazards (earthquakes, eruptions, toxic gas). (3) Choose a conversion: a geothermal power plant, direct use of hot water to heat buildings, or a hydroelectric turbine. (4) Evaluate its efficiency and its limits. Iceland is a well-documented example: it sits on the Mid-Atlantic Ridge above a hot spot, geothermal plants supply roughly a quarter to a third of its electricity, and about 85 percent of its houses are heated with geothermal energy. Efficiency depends on temperature. A heat engine can turn at most the fraction 1 − T(cold) ÷ T(hot) of the heat it takes in into work, with temperatures in kelvin; for geothermal fluid at 250 °C (523 K) and cooling at 25 °C (298 K) that limit is 1 − 298 ÷ 523 = 0.43. Real geothermal power plants convert only about 10 to 20 percent of the heat they extract into electricity, less than a coal plant, because their steam is much cooler; using the same hot water directly to heat buildings delivers most of its heat. A low conversion efficiency is acceptable when the heat is resupplied from the interior, but only if the reservoir is not drained or cooled faster than it recharges. That condition is real: steam output at The Geysers in California fell in the late 1980s as reservoir pressure dropped, and treated wastewater is now piped in and injected to recharge it.

The misconception. The lithosphere and the crust are not the same thing, and the asthenosphere is not liquid. The crust is a chemical layer; the lithosphere is a mechanical one that includes the crust and part of the mantle; the asthenosphere is solid rock that flows very slowly. Confusing them leads to the wrong idea that plates are crust floating on magma.

Worked geothermal gradient: A hypothetical mine in a stable continental region measures (model values) 12 °C at the surface and 42 °C at a depth of 1,200 m. Gradient = (42 − 12) ÷ 1.2 km = 25 °C/km. If the gradient continued unchanged, the temperature at 100 km would be 12 + 25 × 100 = 2,512 °C, above the melting point of mantle rock, yet S-waves show the mantle there is solid. Conclusion: the gradient must decrease with depth, which is expected because most of the heat-producing isotopes are concentrated in the crust and because convection in the deeper mantle carries heat efficiently without a steep gradient.

Worked isostasy with an iceberg: Ice has density 0.92 g/cm³ and seawater about 1.03 g/cm³. A floating block sinks until the displaced water weighs as much as the block, so the submerged fraction is 0.92 ÷ 1.03 = 0.89, or about 89 percent. For an iceberg 40 m thick, about 35.7 m is below water and 4.3 m above. Applying the same principle to crust (2.7) in mantle (3.3): the submerged fraction is 2.7 ÷ 3.3 = 0.82, so about 82 percent of any floating block lies below the level of the surrounding mantle. This is the reasoning behind the deep roots of mountain ranges.

Worked reasoning, how a refracted arrival reveals the Moho (no depth is calculated here): In a model with round-number speeds, two P arrivals reach a station 200 km from a shallow earthquake: a direct wave through the crust at 6.0 km/s and a refracted wave that traveled along the top of the mantle at 8.0 km/s. The direct wave takes 200 ÷ 6.0 = 33.3 s. The refracted wave spends part of its path going down to and up from the Moho and the rest traveling faster along it; beyond some distance the time saved on the fast leg exceeds the time spent on the detour, and the refracted wave arrives first. Mohorovičić saw exactly this crossover in the records of the 1909 earthquake, and the distance at which the refracted arrival overtakes the direct one depends on the depth of the boundary, which is how the Moho depth is measured without drilling.

Worked evidence against homogeneity: A student is told Earth's mass is about 5.97 × 10²⁴ kg and its volume about 1.08 × 10²¹ m³. Density = 5.97 × 10²⁴ ÷ 1.08 × 10²¹ ≈ 5,530 kg/m³, or about 5.5 g/cm³. Granite is 2.7 and basalt 3.0 g/cm³; even mantle peridotite at the top is only 3.3. Pressure squeezes the lower mantle to about 5.5 g/cm³ at its base, but most of the mantle is lighter than that. Since the average must be the volume-weighted mean, some large part of the interior must be much denser than 5.5, consistent with an iron core occupying about one sixth of Earth's volume at densities above 10 g/cm³.

Worked energy plan and efficiency for a model site: A hypothetical volcanic valley has wells that deliver 40 kg of water per second at 200 °C, and the plant returns it underground at 80 °C. Heat extracted per second = mass flow × specific heat × temperature drop = 40 × 4,186 × 120 ≈ 20,100,000 J/s, about 20 MW of heat (treating the pressurized fluid as liquid water, a simplification). An upper bound using the inlet temperature, with cooling at 25 °C, is 1 − 298 ÷ 473 = 0.37 (the true limit is lower, because the water cools as it gives up heat); a real plant achieving 12 percent would produce 0.12 × 20.1 ≈ 2.4 MW of electricity, enough for about 2,000 homes at a model average use of 1.2 kW each. Piping the same 20 MW to heat buildings through a district network that delivers 85 percent would supply about 17 MW of heat. The plan is not finished until it states its assumptions and evaluates sustainability and hazard: reinject the cooled water so the reservoir keeps its pressure, and monitor for small induced earthquakes.

Representation task: Draw the top 660 km of Earth to scale (1 cm = 50 km) as a column: continental crust to 35 km, the Moho, lithospheric mantle to 100 km, the asthenosphere to about 250 km, and the rest of the upper mantle to 660 km. Beside the column write the composition, the density and the seismic velocity behavior of each layer, and draw a second column for oceanic lithosphere to show that its crust is thinner and its lithosphere thinner. Then add a small isostasy sketch: three blocks of the same wood but different thickness floating in water, labeled with the fraction submerged.

Safe investigation, isostasy and density: (a) Cut three blocks of the same softwood 2 cm, 4 cm and 6 cm thick, float them in a tub of water and measure how much of each stands above the water; calculate the submerged fraction and compare with the wood's density measured by mass and volume. Add a coin to the thickest block and record how much further it sinks, and remove it to see rebound. (b) Measure the mass and volume (by water displacement) of samples of granite, basalt and any dense metal object and calculate the densities; compare them with Earth's average of 5.5 g/cm³. Record all masses, volumes, densities and submerged fractions in a table. What the model cannot show: water is a true liquid with no strength, while the asthenosphere is a solid that flows only over thousands of years; the blocks reach equilibrium instantly, whereas real isostatic rebound takes millennia; and floating wood cannot model the compositional Moho inside the block.

Research and energy plan: Choose one real area with active geologic processes, such as Iceland, The Geysers in California, the island of Hawaii, New Zealand's Taupō Volcanic Zone or the East African Rift in Kenya. From published sources record the process, temperatures at depth or flow rates, and hazards. Build a working model of a direct-use system scaled to the site, a heat exchanger: warm tap water (no hotter than 50 °C, handled by the teacher) is poured slowly through a coil of tubing that sits in a measured mass of cool water in an insulated container, which stands for a building's heating loop. Collect the warm water as it leaves the coil. Measure the mass of warm water used and its temperature before and after the coil, and the mass of the cool water and its temperature at the start and at the end of the run. Heat given up = mass of warm water × 4,186 J per kg per °C × its temperature drop; heat delivered = mass of cool water × 4,186 × its temperature rise; transfer efficiency = heat delivered ÷ heat given up. For example (model values), 2.0 kg of warm water cooling from 48 to 40 °C gives up 2.0 × 4,186 × 8 ≈ 67,000 J; if 1.5 kg of cool water warms from 20 to 28 °C it gains 1.5 × 4,186 × 8 ≈ 50,200 J, a transfer efficiency of 75 percent. Say where the missing heat went (into the tubing, the container and the room) and note that this measures heat transfer only, not conversion to electricity. Then scale the result to the site's numbers, calculate the heat or power available and the efficiency of your chosen conversion, and compare it with one other energy source. State what would keep the resource renewable and one hazard your design must manage.

Independent performance: Write an evidence-based argument of at least 300 words that disputes the hypothesis that Earth is homogeneous throughout and locates its main internal boundaries. Success criteria: it uses average density, seismic velocity jumps (Moho, 410 km, 660 km, core boundary) and the S-wave shadow as separate lines of evidence; it distinguishes lithosphere from crust and asthenosphere from liquid; it explains isostasy with one calculation; it names gravity and magnetism as further evidence and says what each shows; and it describes the geothermal gradient and heat flow with typical values and their conditions.