Earth's Internal Structure and Plate Tectonics
On the MTLE Earth and Space Science (Grades 9–12) exam, this domain tests how Earth's interior is organized, how seismic evidence constrains that structure, and how plate tectonics explains earthquakes, volcanoes, and topography. Expect questions that ask you to interpret boundary types, use rate calculations, and distinguish physical from compositional layers.
From Composition to Physical Layers
Beneath the crust, the mantle extends to about 2,900 km and is dominantly solid silicate rock that deforms ductilely over long times. The outer core is liquid iron-nickel; the inner core is solid despite extreme temperature because pressure is immense. The lithosphere is the rigid crust plus uppermost mantle; it is broken into plates and rides on the weaker asthenosphere. Isostasy explains why thick continental crust sits high and oceanic crust sits low.
Plate Boundaries and Drivers
Divergent boundaries create new oceanic lithosphere at mid-ocean ridges; convergent boundaries recycle it at subduction zones (ocean-ocean, ocean-continent, continent-continent) and build mountain belts; transform boundaries slide laterally. Evidence includes seafloor magnetic stripes, earthquake distribution, hotspot chains, and GPS velocities. Slab pull and ridge push, along with mantle convection, drive motion.
Worked Example: Seafloor Spreading Rate
A magnetic anomaly is mapped 1,500 km from a mid-ocean ridge and dated at 30 Ma. What is the average spreading rate for one plate? Half-spreading rate = 1,500 km / 30,000,000 yr = 5 × 10^-5 km/yr = 5 cm/yr. Full spreading rate across the ridge is 10 cm/yr. The reasoning: distance from ridge equals seafloor age times half-spreading rate; full rate is twice that.
Traps That Cost Marks
- The mantle is not a global magma ocean; it is mostly solid.
- S-waves do not pass through the outer core; P-wave shadow zones reveal the core.
- Continental crust is felsic and less dense than mafic oceanic crust; lithosphere is not the same as crust.
- Not all convergent boundaries produce the same volcanoes; continental collision lacks arc volcanism.
- Hotspots can occur within plates, not only at boundaries.