Transcription
Yosemite Valley doesn't look real. Somehow a river valley became a giant stone cathedral. Vertical cliffs tower thousands of feet above the valley floor. Entire mountains appear to be sliced in half. I mean, how? And when?
One of the coolest things about Yosemite is that almost everything you're looking at formed underground. The granite cliffs surrounding Yosemite Valley began as magma. And I know you're probably thinking, "Yeah, Rachel, that's how rocks work. They all start as magma." But granite is unique because it starts as magma beneath Earth's surface, but then it also cools and crystallizes directly from that magma still beneath Earth's surface. When magma reaches Earth's surface to form a lava flow, it produces rocks like rhyolite and basalt. But granite forms from magma that never reached the surface.
But what exactly causes bodies of magma to rise within Earth's crust shallow enough to slowly cool and solidify? Well, in the case of the granite that makes up Yosemite, it all started around 150 to 80 million years ago during the age of dinosaurs, when an oceanic plate was diving beneath the western edge of North America. The descent of this plate into the mantle generated enormous amounts of melting and thus magma. And as we know, much of that magma never erupted. It stalled deep beneath the surface and slowly cooled underground. And over millions of years repeated pulses of magma intruded into the crust creating a vast complex of plutons called the Sierra Nevada batholith.
However, it wasn't like there wasn't any magma reaching the surface. Both processes were happening. Magma that reached the surface caused volcanic eruptions of lava and ash forming rhyolite, dacite, andesite, and ash deposits. While the batholith of magma beneath this volcanic arc was like the plumbing system, somewhat analogous to the modern Andes Mountains. However, most of those volcanic rocks have eroded away over the last 100 million years, leaving behind the more resistant granites and granodiorites that cooled underground, essentially the deep roots of that arc system. Yosemite is like looking beneath the volcanoes after the volcanoes themselves have disappeared. The granite that makes up El Capitan, Half Dome, and many of Yosemite's famous cliffs crystallized several kilometers below the surface.
And then during the tens of millions of years of erosion that slowly stripped away the rocks above the batholith to expose the granite, the Sierra Nevada began to rise. Why? Well, it's still an active area of research, but it's thought that three mechanisms likely contributed. First, Basin and Range extension, which began roughly 20 to 15 million years ago in this region, led to crustal stretching and the formation of faults, which caused the eastern side of the Sierra to be uplifted relative to adjacent valleys, and the range to tilt westward. Second, the removal of dense material beneath the Sierra is thought to have potentially contributed to this uplift as well. When the Sierra batholith formed, dense rocks accumulated beneath it. And some recent seismic imaging and mantle studies suggest that between around 10 and 3 million years ago, portions of this dense root detached and sank into the mantle, a process called delamination or foundering. Imagine cutting the anchor off a floating buoy. Once the dense root detached, the crust became more buoyant and the Sierra rose higher. And the third factor that potentially contributed to this uplift was isostatic rebound from erosion. As millions of years of erosion removed rock from the range, weight was reduced and the crust responded by rising slightly. This effect is real, but probably not the dominant cause of Sierra uplift.
But this removal of weight and pressure off the top of the granite domes here did cause a lot of what we call exfoliation and you can see that really clearly if you come here. Exfoliation is a process I talked a lot more about in my video about the Llano uplift in Texas, which is a very similar granite, you know, uplift. And you can see exfoliation weathering happening where because so much weight was removed, the granite is expanding slightly and that causes it to break into sheets sometimes and that causes this dome-like weathering. A lot of these granite plutons that are later uplifted and exposed undergo this exfoliation weathering and end up looking dome-like because of this exfoliation process.
But anyway, back to the uplift, exactly when and how quickly this uplift occurred remains an active area of research, but it's clear that over time the range tilted westward and this created a steep eastern escarpment and encouraged rivers to flow toward California's Central Valley. As mountains rose, rivers began carving downward into the granite and that created the first version of Yosemite Valley, but it didn't look anything like today's Yosemite. At this stage, it would have resembled a typical river valley, narrow, V-shaped and much less dramatic. The iconic Yosemite landscape had not yet taken shape. For that, we need ice.
Around 2.5 million years ago, Earth entered the modern ice age, which has been characterized by glacial cycles of cooling and warming that have caused ice sheets to expand southward and then retreat northward over and over again. And during these glacial cycles, massive glaciers repeatedly formed in the Sierra Nevada, many of which flowed downhill through existing river valleys. But unlike rivers, glaciers are incredibly effective at widening valleys. Rivers mainly erode downward, but glaciers erode downward and outward. So, as giant rivers of ice moved through Yosemite, they scraped, polished, and excavated the granite. And the broad U-shaped valley that we see today is one of the defining characteristics of Yosemite.
And while the shape of the valley itself is sufficient evidence of glaciation, there's also another mystery in Yosemite that glaciers helped to explain. Why are there so many waterfalls? If you pay close attention, you'll notice that many of Yosemite's waterfalls don't begin at the valley floor. They spill from high above it. And this is because the main glacier that occupied Yosemite Valley was much larger than glaciers flowing through neighboring valleys. The larger glacier eroded deeper into the landscape, while smaller tributary glaciers couldn't keep up. And when the ice melted, those tributary valleys were left stranded high above the main valley floor. Geologists call these hanging valleys. Today, streams flowing through these hanging valleys plunge over the edge creating spectacular waterfalls. In other words, these waterfalls exist because some glaciers carved deeper valleys than others.
But let's get back to the rocks, shall we? Perhaps the most striking feature of Yosemite is its giant granite cliffs. El Capitan rises almost 1,000 m or over 3,000 ft above the valley floor. So, how exactly did these cliffs become so steep? The answer lies in fractures. As granite cooled underground, it developed cracks called joints. And later, as overlying rock was eroded away and pressure on the granite decreased, fractures formed along these planes of weakness. And glaciers then exploited these planes of weakness, and blocks of rock bounded by joints were plucked away by moving ice. Over time, repeated removal of fractured rock helped create Yosemite's extraordinarily steep cliffs. See, the glaciers didn't carve randomly. They followed the architecture that was already built into the granite.
And no feature better illustrates this than Half Dome. Standing nearly 1,500 m, or around 4,800 ft above the valley floor, it almost looks like somebody cut a mountain in half. For years, people assumed glaciers simply sliced away one side, but the shape actually reflects the interaction of glaciation and pre-existing fractures in the granite. And it wasn't fast, either. Half Dome was sculpted over millions of years by erosion working along zones of weakness within the granite.
So, that's it, right? Yosemite's rocks formed over 100 million years ago and then were later uplifted and carved by glaciers. The end. Well, not actually the end. Yosemite, just like everywhere else on Earth, is still changing today. If you look closely beneath the granite cliffs, you'll find piles, giant piles of broken rock. These are talus slopes, evidence of ongoing cliff collapse. As granite expands and contracts due to temperature and pressure changes, water enters fractures, roots exploit cracks, and stress is released from exposed rock faces, causing pieces of Yosemite to continue to break away.
>> Okay, so that was originally supposed to be the end of the video, but then I realized I kind of lied to you. I said that Yosemite is all granite, but then I went on the trail to Taft Point. And at the very beginning of that trail, there's a huge vein or dike or formation of bright white crystalline material that does not look at all like the granite elsewhere. Turns out it's a quartz and feldspar rich pegmatitic vein or dike that formed during late-stage cooling of the granitic system. So, long story short, as the magma bodies cooled to form granite, they shrink and cracks formed and the very last bits of melted material filled those cracks. And sometimes this is just more granite, which ends up forming granitic dikes, but other times this is late-stage melt that can be super rich in silica, alkalize, and water, which ends up forming large quartz and feldspar crystals, especially if they cool very slowly with lots of fluid. And if you come here, the trick to telling the difference between the quartz and the feldspar is that feldspar breaks along cleavage planes and forms these really flat shiny surfaces, whereas quartz does not. It's massive or structureless and breaks conchoidally rather than along planes. So, anyway, now let's end out the video.
So, I hope you enjoyed learning about the geological history of Yosemite. And if you'd like to come out here and explore it for yourself, I highly recommend getting the Geology Underfoot in Yosemite book, which helps guide you through the incredible geological features out here. And I'll link it down below. And if you'd like to hear more about the different glacial processes that carved Yosemite, I'll be making a whole separate video diving really deep into how glaciers carved these valleys and cliffs and how they actually deposit rocks and what we can learn from different types of glacial features. See you there. Bye.
Okay. This will probably go in the outtakes, but I am at Glacier Point in Yosemite and I was just sitting here enjoying this spectacular view. And you know, I was taking it all in. It's so beautiful. It's so amazing. I feel so lucky to be here. And then, to top everything off, there was like this 6-year-old young girl that walked up the path and ran up, actually. And she immediately screamed like just yelled like, "Oh my gosh, this is so cool." Looking at Half Dome, obviously. And then her mom walked up behind her, obviously a little slower than the the excited kid. And uh and she looked at her mom and said, "Thank you so much for bringing me here." And I don't even have kids or anything, but that just made me so excited for just her. And I don't know, it's just like geology is so amazing, you know? I I don't want to be like cheesy with like oh, you know, geology is amazing, cuz I always say that. But but I mean, I just love that young kids even can look at this and be like, "That is so cool." You know?