Why do andesite and diorite have large crystals?

Answer 1

The reason why all rocks may have large crystals are because of one of the following reasons:

If the rock is an igneous rock:

Igneous rocks form when magma/lava cools. The slower it cools, the larger the crystals because they have more time to develop.

If the rock is a metamorphic rock:

Metamorphic rocks form from heat and pressure on top of other rocks. The more heat and pressure that builds on top of it, the more likely it will have larger crystals.

If the rock is sedimentary rock:

Sedimentary rock forms when other rocks get eroded, deposited, compacted, and then cemented together. Usually if the rock takes longer in cementation, it'll have larger crystals.

So basically the longer it has to form, the larger the crystals will get.

Answer 2

Diorite is a plutonic igneous rock composed of coarse grains of plagioclase feldspar and less than 40 percent hornblende and biotite , or, more rarely, pyroxene or olivine. Small amounts of potassium feldspar and quartz may also occur, along with traces of magnetite, apatite, sphene, and zircon. Diorite is the plutonic equivalent of the volcanic rock andesite and is intermediate between gabbro and granite. Diorite occurs around margins of granitic batholiths, in separate plutons, and in dikes. It forms by the melting of rocks in the lower crust, by the assimilation of crustal rocks in basaltic magma, or as by metamorphic processes.

Andesite is common in most of the world's volcanic areas. Andesites occur mainly as surface deposits and, to a lesser extent, as dikes and small plugs. Not only the Andes, where the name was first applied to a series of lavas, but most of the cordillera (parallel mountain chains) of Central and North America consist largely of andesites. The same rock type occurs in abundance in volcanoes along practically the entire margin of the Pacific Basin

Answer 3

Andesite are fine-grained extrusive igneous rocks while diorite are coarse-grained intrusive igneous rocks. They have different sized crystals of their chemical and mineralogical composition.

Answer 4

This is controlled by cooling time (longer cooling time = bigger crystals) or the presence of fluids (fluids mobilize atoms and allow them to form larger crystals quicker)