Earth Systems
8th Grade Science · 8 lessons
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Lessons
- 1Earth's Layers Were Deduced, Not DrilledThe deepest borehole ever drilled reached about 12 km, barely scratching the crust, so the layered picture of Earth's interior comes entirely from how earthquake waves bend and stop. Wave speed jumps sharply at the Mohorovičić discontinuity, fixing the crust-mantle boundary; the S wave shadow zone fixes the outer core as liquid. Density rises from roughly 2.7 g/cm³ in continental crust to about 13 g/cm³ in the inner core, and that gradient is the fingerprint of a planet that sorted itself while molten.Lesson
- 2Slab Pull, Not Floating, Drives the PlatesPlates do not drift on a liquid mantle — the mantle is solid rock that creeps over millions of years. The dominant driver is slab pull: oceanic plate that has cooled and grown dense sinks at a subduction zone and drags the rest of its plate behind it, with ridge push at the spreading centre contributing a smaller share. The evidence is in the speeds: plates with long subducting margins, like the Pacific, move several times faster than plates with almost none, like the African.Lesson
- 3Why Ocean Floor Is Young and Continents Are OldNo sea floor anywhere on Earth is older than about 200 million years, while the oldest continental rocks approach 4 billion. The reason is density: oceanic crust is basalt, dense enough to subduct and be recycled, whereas continental crust is granite, too buoyant to sink, so it simply accumulates. The age map of the sea floor — youngest at every mid-ocean ridge and symmetrically older either side — is the single clearest record that the floor is spreading.Lesson
- 4The Atmosphere's Temperature Profile Reverses TwiceTemperature does not simply fall with height. It drops through the troposphere, rises through the stratosphere because ozone absorbs ultraviolet there, falls again through the mesosphere, and rises in the thermosphere. Each reversal creates a stable layer that caps the one below it, and that is why weather is confined to the troposphere — rising air meets warmer air at the tropopause and stops. It is also why volcanic ash that reaches the stratosphere stays aloft for years.Lesson
- 5Shallow Water Turns a Tsunami into a WallIn deep ocean a tsunami is barely a metre high and can be hundreds of kilometres long, crossing the Pacific at jet-aircraft speed because wave speed depends on water depth. As the water shallows, the front of the wave slows while the back is still travelling fast, so the wave shortens dramatically and the energy it carries is compressed into height. Nothing is added — the same energy in a fraction of the length is what turns an unnoticed swell into a wall at the shore.Experiment
- 6The Geodynamo and Magnetic ReversalsEarth's magnetic field is not produced by a permanent magnet — iron loses its magnetism far below core temperatures. It is generated by convecting liquid iron in the outer core, a self-sustaining geodynamo. The field has reversed hundreds of times, and because basalt erupting at a mid-ocean ridge locks in the field direction of the day it cooled, the sea floor carries symmetrical magnetic stripes. Those stripes both date the crust and independently prove it is spreading.Lesson
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