Questions: Fossils in Rocks

Question 1

Why are fossils almost always found in sedimentary rocks and not in igneous or metamorphic rocks?

• A. Sedimentary rocks are older than other rocks
• B. Sedimentary rocks form in layers from settling sediment, which can gently bury and preserve dead organisms; igneous rocks form from extreme heat that would destroy remains, and metamorphic rocks involve heat and pressure that would deform them
• C. Animals only lived in places where sedimentary rocks form
• D. Fossils are painted onto sedimentary rocks by scientists

Answer: (B)

Fossils need gentle burial to be preserved. Sedimentary rocks form when layers of mud, sand, or silt settle over time -- a process gentle enough to preserve the shapes of dead organisms. Igneous rocks form from melted rock at extreme temperatures that would destroy any remains. Metamorphic rocks involve intense heat and pressure that would crush and deform any fossils. That is why nearly all fossils are found in sedimentary rock.

Question 2

Fossils are mainly the bones of dinosaurs.

• T. True
• F. False

Answer: False

Fossils include far more than dinosaur bones. Fossil plants, fossil fish, fossil seashells, fossil insects, and even fossil footprints and burrows have been found. Fossils preserve evidence of all kinds of ancient life -- not just dinosaurs. In fact, many fossils are of sea creatures like trilobites and ammonites that are far older than the dinosaurs.

Question 3

How does a fossil form? Describe the basic steps.

Model answer: An animal or plant dies and is quickly buried by sediment like mud or sand. Over time, more layers of sediment pile on top. The sediment slowly hardens into rock. Meanwhile, minerals from water seep into the remains and replace the original bone, shell, or plant material, creating a rock copy of the original organism. Millions of years later, erosion or digging may expose the fossil.

The key condition for fossil formation is rapid burial -- if the organism is exposed to air and scavengers, it decomposes before it can be preserved. Understanding this explains why fossils are relatively rare: most dead organisms decompose completely without being preserved.