Guided Exploration: The Rock Record

 

HoPE Tour Map: 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? 

Father of Geology

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The father of geology

The Scottish naturalist James Hutton (1726-1797) is known as the father of geology because of his attempts to formulate geological principles based on observations of rocks.


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?

Dike in a granite

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Dike in a granite

This rock is a granite that solidified between 350 and 320 million years ago.


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.

Igneous rocks

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Igneous rocks

Igneous rocks solidify from molten rock (called magma within the Earth and lava on the surface).



Metamorphic Rocks

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Metamorphic rocks

Metamorphic rocks form when sedimentary, igneous, or pre-existing metamorphic rocks are changed by heat, pressure, and chemically reactive waters.


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.


Windblown Desert Deposit

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Windblown Desert Deposit

The Coconino Sanstone was formed when desert winds drove fine grains of quartz sands onto the Colorado Plateau, dune by dune.


Calm Marine Deposit Specimen 1

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Calm marine deposit

The Kaibab Limestone was deposited as organic carbonate fossil fragments, sand grains, and mud 260 million years ago in a shallow, quiet ocean.


A Stream Deposit

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A stream deposit

This is a conglomerate – a rock containing fragments of previously formed rocks – from the Hermit-Supai formations in the Grand Canyon region.


Sandstone

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Sandstone

Composed of grains coarser than those in shales, sandstones are created in environments such as beaches, rivers, and deserts, where waves, currents, or wind sift the sands, removing the finer particles.


 

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.

Granite

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Granite

This igneous rock formed when molten material intruded into the shallow crust, where it cooled and crystallized.


Claystone

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Claystone

Weathering and erosion of rocks like granites concentrate elements that are necessary to form clay minerals, which accumulate as sediments.


Gneiss

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Gneiss

With burial and the associated increase in pressure and heating, shales may be transformed by re-crystallization in the deep crust into metamorphic rock, like this gneiss.