Why is most of Earth's crust made of igneous rock?

Answer 1

Strictly speaking it isn't.

It's divided into Continental Crust plates (mainly granite), surrounded by the thinner, denser basalt plates of the ocean floor, and all floating on the viscous Mantle.

These are constantly shuffling around the globe, and individual plates can be stretched, thinned and broken by processes in the Mantle, or warped downwards by ice-pressure during the cold phases of ice-ages.

The Atlantic is widening at about 20mm/yr from its Mid-Atlantic [Spreading] Ridge as the Pacific shrinks by subduction around its coasts. The Mediterranean is shrinking slowly, and Africa starting to split in two. India's collision with Asia is still pushing the Himalaya upwards, etc. The Appalachians, Scottish Highlands and Norwegian mountains were all originally one chain, broken by the Atlantic and North Sea developing.

Until plate tectonics was understood in the later part of 20th Century geologists did call the continents "rigid masses" and could not conceive of them moving; but were unable satisfactorily to explain folding, faulting, mountain-building and volcanoes. Once the light dawned thanks to advances in oceanography and seismology, everything fell into place beautifully!

Answer 2

Igneous rock is what you get when hot, liquid rock cools and becomes solid. Early in the history of the Earth, the whole planet was so hot that all the rock was liquid. Hence, it was all igneous at first, when the planet started to cool. Non-igneous rock is rock that used to be igneous but has since undergone further changes (for various reasons).

Answer 3

Because the word crust means a hard outer layer - and that's what it is.

Answer 4

Because if it was not solid it would not be a crust. It has to solidify before you can call it a crust. The outer part of a molten rock will cool first and become the crust.