The Earth's core is composed of iron in two parts, a solid inner core and a liquid outer core, with a radius of about 2,200 miles. Mars' core, on the other hand, is much smaller and made up of iron and magnesium silicate, with a radius between 900 and 1,200 miles and a weaker magnetic field.
This podcast explains how volcanoes and earthquakes are formed due to the drift and movement of Earth's crust and rubbing of tectonic plates. The absence of active volcanoes at present is also discussed.
Terraforming Venus could make it habitable in tens of thousands of years by artificially stimulating the formation of its thin atmosphere to block solar radiation and create a suitable environment for water.