plate tectonics and isostasy

what are plate tectonics?

• can be oceanic, continental, or both depending on location of boundary

• “slabs” of the earths crust and mantle, they move over the asthenosphere

continental plates

plates are much thicker, but less dense, made of granitic rock

oceanic plate

on the bottom of the ocean, thinner and much more dense, made of basalt

types of plate boundaries

• convergent

• divergent

• transform

convergent plate boundary

• two plates coming together →←

• form either subduction zones, or create mountain ranges

• continental plates + continental plates

• continental plates + oceanic plates

• oceanic + oceanic

continental plates + continental plates =

mountain ranges, no subduction

continental + oceanic =

subduction zones, denser oceanic dives beneath continental, volcanic arcs/stratovolcanoes

oceanic + oceanic =

older, more dense oceanic plate subducts underneath the younger, less dense one

divergent plate boundary

• two tectonic plates spreading apart ← →

• can happen in ocean or on land

divergent plate boundaries in the ocean

new crust is created, magma rises and creates new oceanic crust

divergent plate boundary on land

continental rift valley, two tectonic plates stretch the crust and thin it out, example east african rift system

transform plate boundaries

• tectonic plates “slip” past one another (in opposite directions), instead of directly converging or diverging

hot spots

• plumes of magma rise from the mantle of the earth, melt the overlying crust, and create shield volcanoes

seamount

• created from hot spots

• as hotpsot creates volcanoes, the crust that volcano sits on is always moving but the mantle plume is stationary

• the movement of the ground makes a seqeunce of volcanoes (seamount) that are moving away from the hotspot/mantle plume

isostasy

gravity-controlled equilibrium between lithosphere and asthenosphere

isostasy equilibrium: how do tectonic plates sit on top of the mantle?

lithospheric plate floats above the asthenosphere because the floating depends on the thickness and density of the lithosphere. asthenosphere is thick and hard but still maleable. the heat from mantle helps the asthenosphere act as a solid to hold up the lithosphere

• lithosphere is transformed by plate tectonics surface is constantly being modified)

• lithosphere weight and thickness changes through geologic time, triggering isostatic adjustments