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Crustal electrical conductivity varies with mineralogy, temperature, fluid content, and pressure. Mafic rocks are more conductive than felsic rocks; water-saturated rocks are much more conductive than dry rock. Conductivity increases steeply with temperature in the lower crust.
From your understanding of Earth's interior structure, you know that the crust is composed of diverse rock types arranged in layers. What electrical conductivity adds is a way to "see" the state of those rocks — not just their composition, but whether they contain fluids, how hot they are, and whether they are partially melting. Electrical conductivity (the inverse of resistivity) measures how easily electric current flows through a material, and in crustal rocks it varies over many orders of magnitude depending on conditions.
In dry, crystalline rocks at the surface, conductivity is extremely low because most rock-forming silicate minerals are electrical insulators. Current flow in these rocks happens primarily through electronic conduction in metallic or semiconducting minerals like magnetite, graphite, or sulfides. A granite with no metallic minerals might have a resistivity of 10,000 ohm-meters or more. But introduce even a small amount of interconnected saline fluid into the pore spaces and fractures, and resistivity can drop by a factor of 100 or more. This happens because dissolved ions in the fluid carry charge efficiently — a mechanism called ionic conduction. This is why the single most important factor controlling upper-crustal conductivity is usually the presence and connectivity of aqueous fluids.
Temperature is the dominant control in the deeper crust. As you descend and temperatures rise above roughly 300–400°C, the conductivity of silicate minerals themselves begins to increase exponentially through thermally activated charge carriers. By the time you reach lower-crustal temperatures of 600–800°C, even dry rocks become moderately conductive. The presence of partial melt amplifies this effect dramatically, because silicate melts are far more conductive than solid minerals. This is why electromagnetic methods can detect magma chambers and zones of partial melting beneath volcanoes and rift zones.
Composition also matters in predictable ways. Mafic rocks (basalt, gabbro) tend to be more conductive than felsic rocks (granite, rhyolite) because mafic minerals contain more iron and have higher intrinsic conductivity. Graphite films along grain boundaries, even in trace amounts, can create anomalously high conductivity because graphite is an excellent electronic conductor and tends to form interconnected networks along shear zones. Some of the most conductive features ever mapped in the crust — conductivity anomalies in ancient suture zones — are attributed to thin graphite films deposited during past subduction of carbon-rich sediments.
Understanding these controls is essential for interpreting geophysical surveys. When a magnetotelluric or resistivity survey reveals a high-conductivity anomaly at depth, the question becomes: is it fluids, partial melt, graphite, or simply hot rock? Answering that question requires combining the electrical data with knowledge of the local geology, temperature structure, and tectonic setting — turning a measurement of how easily current flows into a window on the physical state of the deep crust.