How Was the Grand Canyon Formed?
Have you ever been to the Grand Canyon before? It’s a remarkable site for geology and Earth’s history.
Like a stack of pancakes, young rock layers pile on top of older layers.
From top to bottom, you are passing through about 2 billion years of time.
We use the superposition and relative dating to find when rocks were deposited.
Once we know this, we can better understand and better estimate Earth’s age.
How to use relative dating to estimate age
Rocks always tell a story. For instance, they reveal stories about colliding continents, meandering streams and volcanic eruptions.
In the case of the Grand Canyon, the multi-layered strata provides insight to the age of the Earth.
We are able to see cross-sections because the Colorado River started cutting through the Grand Canyon about 6 million years ago. Even today, these erosional forces are still at work.
Because younger sedimentary rocks stack on top of older rocks, we use relative dating (superposition) to chronologically order events in the past.
Unconformities are time gaps in the rock record. This is where rocks have eroded away and we are actually missing rocks without evidence of any record for that time.
How old are the sedimentary rocks for the Grand Canyon?
The Grand Canyon is a prime example of rock superposition and relative dating. Continuous horizontal layers of rock are repeated throughout time.
They mostly consist of marine sediments that formed in oceans through compaction. From top to bottom, the Grand Canyon is essentially three main types of sedimentary rocks – sandstone, limestone and shale.
- Sandstone is compacted sand like you can find at a beach.
- Limestone contains large amounts of formerly organic material such as plants, shells and clams.
- Shale begins as mud and gets compacted to form a flakey type of rock.
The base of the Grand Canyon are Precambrian basement rocks. These were formed from flowing magma which cooled and hardened about 1.8 billion years ago.
In the table below, you can find the formation of Grand Canyon rock layers which date back to 2 billion years. As you can see, most of the rocks are sedimentary like shale, limestone and sandstone.
What are the rock layers part of the Grand Canyon?
| Rock Layers | Time (Years) | Thickness (Meters) | Depositional Type |
|---|---|---|---|
| Kaibab Limestone | 250,000,000 | 91-150 | Shallow Marine |
| Toroweap Formation | 255,000,000 | 61 | Shallow Marine |
| Coconino Sandstone | 260,000,000 | 20 | Eolian Sand Dune |
| Hermit Shale | 265,000,000 | 91 | Coastal Swamp |
| Supai Group | 285,000,000 | 300 | Shallow Marine |
| Surprise Canyon | 320,000,000 | 122 | Coastal Estuary |
| Redwall Limestone | 335,000,000 | 137 | Offshore Shallow Marine |
| Temple Butte | 350,000,000 | 30 | Tidal Shallow Marine |
| Muav Limestone | 515,000,000 | 198 | Offshore Shallow Marine |
| Bright Angel Shale | 530,000,000 | 152 | Near Shore Shallow Marine |
| Tapeats Sandstone | 545,000,000 | 70 | Intertidal Marine |
| Chuar Group | 825,000,000 | 1,600 | Shallow Marine |
| Nankoweap Formation | 1,050,000,000 | 110 | Shallow Marine |
| Unkar Group | 1,250,000,000 | 1,950 | Shallow Marine |
| Vishnu Schist | 2,000,000,000 | Unknown | Metamorphosed |
