TECTONICS TOPIC 1

TECTONICS TOPIC 1 1.1 PLATE TECTONIC THEORY 3 layers of Earth: Crust → Mantle → Core Layer Thickness Depth (approx) Crust Thinnest, 6–70km Outermost Mantle 2900km In between Core Thickest, 3300km Innermost ● Lithosphere = crust + uppermost mantle (rigid, makes up tectonic plates) ● Asthenosphere = semi-solid layer below lithosphere; heat from core melts rocks here 3 types of plate boundaries: 1. Divergent — plates move apart 2. Convergent — plates move towards each other 3. Transform — plates slide past each other How convection currents cause plate movement: ● Heat from core → mantle material rises, becomes less dense ● Rising material spreads towards surface, drags plates apart ● Material cools, sinks back down → causes divergent plate movement Slab-pull force: ● When 2 plates converge, denser oceanic crust subducts due to gravity ● Pulls rest of the plate with it, contributing to further convergence Keywords: Lithosphere, Asthenosphere, Convection currents, Slab-pull force

1.2 SEAFLOOR SPREADING What is seafloor spreading? = magma rises from deep within Earth through mid-ocean ridges → cools/solidifies → forms new oceanic crust ● Rocks near crest of mid-ocean ridge = youngest ● Rocks progressively older further away from ridge Evidence of seafloor spreading: 1. Rocks at ridge = youngest, get progressively older further away 2. Little sediment accumulation at oceanic ridges; more sediment further away (older crust = more time to accumulate) 3. Oceanic crusts are younger than continental crusts Why evidence supports plate tectonic theory: ● Shows how new oceanic crust is created at mid-ocean ridges + destruction of older oceanic crusts at trenches ● Proves new crust moves laterally away from ridge on both sides Keywords: Seafloor spreading, Mid-ocean ridge, Oceanic trench 1.3 MAGNETIC STRIPING What is magnetic striping? = zebra-like pattern where there are strips of rock w/ alternating magnetic properties (normal & reversed polarity) on the seafloor, symmetrical on both sides of mid-ocean ridge Geographic vs Magnetic North/South: ● Geographic North/South = fixed point (Pole); does NOT change ● Magnetic North/South = direction compass needle points to; CAN shift, reversed multiple times over geological time Normal polarity: magnetic North/South point towards geographic North/South (roughly) Reverse polarity: magnetic North/South point towards geographic South/North (flipped) How striping forms (explain in order!): 1. Magma rises at mid-ocean ridge → cools → forms new oceanic crust 2. Iron partic