Lesson 3 – Review Question Answers

1. Why is the study of rocks important?  

Rocks are important for both economical and scientific reasons. They contain things like fossils and radioactive elements that help with geological dating, and they have economic minerals such as iron ore, copper, gold, and coal. Rocks also hold fluids like drinking water, crude oil, and natural gas that we use every day. (Section 3.1)  

2. Be able to sketch the rock cycle (figure 3.4) without referring to the diagram!  

The rock cycle shows how rocks change from one type to another through processes like weathering, metamorphism, melting, and recrystallization. Any rock can be weathered into sediment, which can become sedimentary rock through lithification. Heat and pressure turn rocks into metamorphic ones, and if they melt, they become magma that cools into igneous rock. The cycle was very active during creation week, Noah’s Flood, and right after the Flood. (Section 3.2)  

3. Be able to sketch and identify the different types of plutons from photographs.  

Plutons are intrusive igneous bodies that form underground. Concordant ones like sills and laccoliths are parallel to surrounding rock layers. Discordant ones like dikes cut across layers, while stocks and batholiths are larger masses. A sketch would show magma injecting into rocks, with sills horizontal, dikes vertical, and batholiths as big blobs. (Section 3.3)  

4. What are the main criteria in the identification of igneous rocks? Can you sketch out a diagram for the identification of igneous rocks?  

The two main criteria are texture and mineral composition or color. Texture includes sizes like pegmatitic (big crystals), phaneritic (visible crystals), aphanitic (tiny crystals), porphyritic (mixed sizes), glassy (no crystals), vesicular (holes), or pyroclastic (fragments). For large crystals, use minerals like quartz or olivine; for small ones, use color like light or dark. A diagram would be a table with textures on one side and compositions across the top, naming rocks like granite or basalt. (Section 3.3.1)  

5. Can you explain Bowens Reaction Series to someone who is unfamiliar with it?  

Bowen’s Reaction Series shows how minerals form as magma cools, not all at once but over a range of temperatures. It has a discontinuous branch where minerals like olivine form first at high heat, then get replaced by pyroxene, amphibole, and biotite as it cools and silica structures get more complex. A continuous branch has plagioclase changing from calcium-rich to sodium-rich. At the bottom, quartz, potassium feldspar, and muscovite form last at lower temperatures. The original magma’s chemistry decides which minerals end up in the rock. (Section 3.3.2)  

6. Why are radiohalos important in a biblical understanding of earth history?  

Radiohalos are damage scars in biotite crystals from radioactive decay of uranium and polonium, forming only below 150°C. They show up close together because hot water from cooling plutons carries polonium away from uranium quickly. This suggests large granite bodies cooled in days, fitting a young Earth and rapid processes during Noah’s Flood, not millions of years. (Box 3.1)  

7. If you heated up a granite rock and melted it, what would melt first? Second? Third?  

In reverse of Bowen’s series, the last minerals to form would melt first. Quartz would melt first, then potassium feldspar, then muscovite mica. (Section 3.3.2)