1. A trench forms when one tectonic plate slides beneath another.
2. A ridge forms by rising magma in a zone where two plates are moving apart.
3. When the areas are warmed up, their density drops and they arise, then cool, sink and replicate the manner all over anew.
4. Earth's mantle is caused by convection by currents carrying heat from the interior to the planet's surface. this is added with hot material that cools down by conduction and convection of warmness or heat.
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