Guided Exploration: The Rock Record

1. Cast of Rock Outcrop from Scotland

One way geologists learn about Earth’s history is to interpret the structure of rock formations. In general, sedimentary rocks are deposited in horizontal layers, and younger beds lie atop older ones. Examine this cast. Note the two main types of rock: the top section is dark red and made of horizontal beds of sandstone, and the bottom section is blocky gray slate in vertically-oriented layers. What do you think this tells us about how this part of Earth’s crust formed? 

2. Dike in Granite (#3)

Dikes are planar bodies of once-molten rock that intruded across the layering of older rocks. Examine this sample. Which rock formed first — the lighter-coloredgranite or the gray-black basaltic rock?

3. Three Types of Rocks (#4-12)

Geologists use different characteristics to categorize rocks. Read about igneous, sedimentary, and metamorphic rocks on the panel and connect these characteristics to the rocks on display.

4. Gabbro and Vials (#16)

Geologists can date some rocks radiometrically by chemically analyzing them in the lab. Examine the mineral grains in the vials and the related panel to learn about half-life and radioactive dating techniques.

5. Grand Canyon Section

Geologists construct maps to understand how the Grand Canyon and other Earth features formed. Look at the large geologic map on the left, and examine the cross-section views on the right to explore what they tell us about rock formations and age. Then observe the rocks (#18-22) and correlate them to the cross-sections in the “Building the Canyon’s Layers” diagram. Note that the location of different types of rocks and structures can reveal the story of the Grand Canyon’s formation.

6. Granite (#15), Claystone (#16), Gneiss (#17), and Rock Cycle Diagram

Rocks form and transform, some many times over vast expanses of geologic time. Explore the connection between the characteristics of the three types of rocks and the processes at work in the rock cycle.