Geological history of Nevada

Now, I must profess I’m not a geologist nor do I have any formal education in geology or the like. What follows is my telling of the geological history of Nevada from reading several sources on the topic since living here. Nevada has a vast array of geology to see, much of which is visible from the road; any road for that matter in Nevada. It’s one of the richest states for geological variety and shares this with Utah next door.

Like most descriptions which involve geological time, the figures are staggering. In fact, it’s quite difficult to comprehend just how much time is required for the landscape to change. Hundreds of thousands of years is a short time in geology, but an extremely long time compared our very limited life on Earth. To illustrate this I read a good analogy not long ago. If you drew a pencil line 4500 kilometres long, and concentrated on the last 2 millimetres alone, that’s how long humans have been on the Earth. Those sort of figures help to explain some of the concepts in geology that most people struggle with – the amount of time it takes to move, erode and change rocks to get the landscape we see today. Even if you lived to be 100 years old, a mountain range in your area would hardly change at all, however, that mountain range probably wasn’t even there 200 million years ago. It’s the hardest concept I’ve had to understand and I still struggle with it, but seeing some of the geology first hand, rather than just reading about it, helps to grasp the concept.

So, starting about 500 million years ago, what we now know as Nevada didn’t exist. It was a part of the eastern edge of a large landmass called Laurentia, and the central and eastern part of the state was underwater. This is why we can find marine fossils and see underwater geologic formations in Nevada today. As we move through 500 million years ago to 250 million years, the tectonic plates to the east started moving west, subducting (diving under) the continental landmass of Laurentia. As this plate moved westward, it was met with pieces of land moving east on the Farallon Plate the crashed into the eastern edge of Laurentia. This happened many times, and each time an additional piece of land was added to the eastern coast. Eventually what we now know as California was formed. The addition of these successive land masses is called accreted terranes. As an analogy, think of a car pile up on a freeway. As each car hits the car in front of it, it folds up like an accordion, leaving a jumbled mess of twisted steel that were once cars, but each car is still discernible from the others – these are the fault lines found throughout Nevada that generally run northwest to southeast. Overtime this increased the size of Laurentia, which increased the landmass of Nevada, removing the ocean and creating the Sierra Nevada mountain range that separates California and Nevada today. These accreted terranes and the thrust sheets (the folded over rock as a result of the rock “pile-up” just like the cars) they formed can still be seen today when looking at a map of Nevada. Distinct NE-SW running lines of mountains form the topology of Nevada and are the edges of these terranes.

From 110 – 45 million years ago, the Farallon Plate (the part of the Pacific Plate subducting under California and Nevada) stopped subducting deeply and flattened out, then started again. This created a period of intense volcanic activity across most of Nevada. As plates subduct under continental rock they trap water which starts to boil from the intense pressure of the rock mass above it. When mixed with magma, it causes the magma to rise to the surface, creating volcanoes and volcanos eruptions. A super-volcano was created in what is now central Nevada and when this erupted, with the force many times greater than Krakatoa, it left the landscape vastly changed. This resulted in a great depression in the centre of Nevada and helped develop what we now know as basin and range landscape.

As the landmass was stretched, the earlier fault lines became the location for large block movements
As the landmass was stretched, the earlier fault lines became the location for large block movements

Over time, the fault lines created from the earlier accreted terranes and the ‘thrust’ caused by them became active again, this time acting as the separation location for segments for large blocks of rock to move. As the whole of Nevada started to stretch from the westward movement of the continental plate (approximately 6mm\year), these faults were the locations where large blocks ‘shuffled’ into place.  These blocks, often tens of miles across and hundreds of miles long, became the mountains, or ranges, we now see across the whole of Nevada. In between these are the plains or basins, hence the name ‘basin and range’ landscape. Drive either east or west across Nevada and you’ll soon see the landscape follows the regular pattern of range, then basin, then range, then basin. The western slope of the ranges are typically at 60 degrees, with the eastern side much steeper – this is as a result of the westward movement of the continental plate they are part of. This is why the western side of the Ruby Mountains behind Elko have a gentler slope, whilst around the back, or eastern slope, the terrain is much steeper.

This graphic of the Sierra Nevadas which forms the westernmost edge of the basin and range landscape shows the tilt of the fault block
This graphic of the Sierra Nevadas which forms the westernmost edge of the basin and range landscape shows the tilt of the fault block

Apart from the basin and range landscape that dominates the scenery across Nevada, there is fascinating geology at every turn. As most of Nevada was under the ocean at some point in time millions of years ago, sedimentary rocks are visible at many locations, especially where road cuts have been made through the landscape. 20 minutes west of Elko is the Carlin Canyon, a deep ravine made by the Humboldt River as it makes it way west to where it eventually disappears into a sink. Where the river has cut away the bank over thousands of years, it’s made it easier to see two different types of rock – one 340 million years old, the other 300 million years old – interacting with each other.

As the picture below shows, the older rock, which was once horizontal sedimentary layers, has been turned on its edge 90 degrees. This is apparent by the bands of sediment, laid down over millions of years of sea level change, now sitting vertical. Separated by the red line is another layer of sedimentary rock, this one 300 million years old, stacked against it. Imagine putting two dominos next to each other edge on. With some difficulty you can eventually get one on top of the other, but as you start to tilt one up, the other doesn’t tilt to the same degree – it slides off to the side and results in an angle less then 90 degrees. This is how these two rock formations have ended up the way they are. What were the forces that lifted the older rock through 90 degrees? A combination of tectonic movement and resultant earthquakes over millions of years. All this can be seen by simply pulling off the I-80 just west of the Carlin tunnel.

The Carlin Canyon unconformity which is simply a location between two rock masses that don't conform with each other
The Carlin Canyon unconformity, which simply means “a location between two rock masses that don’t conform with each other”

So there you have it – 500 million years of geology in about 500 words. If only it were really that simple. The best way to understand the geology around here is to come and see it. Spend many hours driving from one side of the state to the other and you’ll see all sorts of geology.

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