Statement I: Minerals are formed by slow cooling of the magma. Statement II: Very small crystals are formed when lava cools quickly on the surface.

Statement I is true as minerals in igneous rocks form through the crystallization of magma; when this cooling occurs slowly at great depths, it allows for the development of large mineral grains [3]. Statement II is also true because rapid cooling of lava on the Earth's surface inhibits crystal growth, resulting in very small, fine-grained (aphanitic) crystals [4]. While both statements accurately describe the relationship between cooling rates and crystal size, Statement II is not the explanation for Statement I. Statement I describes the general process of mineral formation via slow cooling (typically intrusive), whereas Statement II describes the specific outcome of rapid cooling (extrusive). The two statements describe contrasting geological environments and cooling regimes rather than a cause-and-effect relationship where the second explains the first [3].

Sources

1. [3] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 2: The Earth's Crust > Chapter 2 The Earth's Crust > p. 18
2. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 13: Types of Rocks & Rock Cycle > Extrusive Igneous Rocks (Lava Or Volcanic Rocks) > p. 170
3. [4] https://www.dnr.sc.gov/geology/pdfs/education/Igneous%20Rocks.pdf