Plate Tectonics Explorer

Earth's lithosphere is divided into tectonic plates that move on the asthenosphere. Interactions at plate boundaries produce earthquakes, volcanoes, and mountain building.

• Divergent — plates move apart, creating new crust at mid-ocean ridges
• Convergent — plates collide, causing subduction or mountain building
• Transform — plates slide past each other along strike-slip faults

Plate velocities range from 1 to 10 cm/year, roughly the rate your fingernails grow.

Radioactive isotopes decay at a constant rate described by their half-life. Measuring the ratio of parent to daughter isotopes gives the age of a rock.

Where $t_{1/2}$ is the half-life, $N$ is the current amount of parent isotope, and $N_0$ is the original amount (parent + daughter).

Carbon-14 (t½ = 5,730 years) dates recent organic material. Uranium-238 (t½ = 4.47 billion years) dates ancient rocks and meteorites.

The rock cycle describes how rocks transform from one type to another through geological processes over millions of years.

• Igneous — formed when magma/lava cools (granite, basalt)
• Sedimentary — formed by compaction of sediments (sandstone, limestone)
• Metamorphic — transformed by heat and pressure (marble, slate)

Any rock type can become any other through the right sequence of processes. Tectonic forces drive the cycle by causing uplift, burial, and melting.

Earth has a layered structure determined by density, composition, and physical state. We know this primarily from seismic wave analysis.

• Crust — 5-70 km thick, rocky silicates
• Mantle — to 2,890 km, slow convective flow drives plate tectonics
• Outer Core — 2,890-5,150 km, liquid iron-nickel (S-waves blocked)
• Inner Core — 5,150-6,371 km, solid iron-nickel under extreme pressure

S-waves (shear waves) cannot travel through liquids. Their absence in the outer core is how we know it is liquid, which generates Earth's magnetic field.