In the Beginning
The formation of Half Dome took place in the womb of the earth about 85 million years ago. The family history of Half Dome, though, goes back in time much further to the birth of the Sierra Nevada mountain range.
500 million years ago, a sea existed along the western edge of North America. The process of erosion deposited vast amounts of sediment along this coast for hundreds of millions of years. This sediment would one day become the building blocks of the oldest peaks in the Sierra Nevada. But how did sediment at the bottom of an ancient sea become the mountain range we see today?
Born of Fire
If we fast forward to about 200 million years ago, the Farallon tectonic plate underneath what is now the Pacific Ocean pushed under western North America. The geologic process of one tectonic plate pushing underneath another plate is called subduction. In the case of the Sierra Nevada, subduction had several effects.
First, it generated a lot of heat and pressure, which compressed the sediment from 500 million years ago into the metamorphic rocks that now form some of the Sierra’s peaks. Metamorphic rocks are born when a pre-existing rock is subjected to extreme heat and pressure, which brings about chemical and physical changes.
As the tectonic plate underneath the ocean pushed under the North American plate, the heat from this friction also caused magma deep beneath the surface of the earth. The magma moved upward through the earth’s crust and erupted at the surface, forming a chain of volcanoes along what is now the Sierra Nevada range. This ancient range of volcanoes was similar to the modern Cascade Range in northern California, Oregon, and Washington.
Not all the magma generated by the friction of subduction became volcanoes. Between 200 and 80 million years ago, magma forced its way up and through the more brittle overlying rock, but some of it failed to breach the surface of the earth. These intrusions of magma amassed into giant blobs of molten rock and slowly cooled underground. Cooled and hardened magma is called igneous rock. A large mass of igneous rock is called a pluton. How common are plutons? Within Yosemite alone, there are close to 100 plutons, and our protagonist is one of them.
The Half Dome pluton lies within a region where the dominant granite type is called Half Dome Granodiorite. This granodiorite is about 85 million years old, making it Yosemite Valley’s youngest igneous rock. It is also the most common rock in the Valley. Upon examination of Half Dome’s granite, curious hikers will notice crystals made of various minerals including biotite, hornblende, and feldspars. These crystals formed as the magma cooled. The large and dark hornblende crystals are of notable beauty.
At this point in the timeline, a distinct mountain range known as the Ancestral Sierra Nevada had formed. Its summits reached up to 15,000 feet. The contemporary Sierra Nevada did not yet exist. Millions of years of erosion and uplift were required to shape the mountains that many people enjoy today.
By 65 million years ago, many of the old volcanoes had lost their breath of fire. Erosion started to expose the underlying masses of plutons, collectively known as a batholith. The Ancestral Sierra Nevada continued to erode faster than it grew. The range transformed into a chain of high rolling hills with none of the fascinating geological features of today’s range.
New Life for an Old Range
About 25 million years ago, things got particularly exciting. Another tectonic plate subducted underneath the worn down mountain range. The subduction of this plate injected new life into the area, and the Sierra Nevada began to grow out of the earth’s crust. More precisely, a block of the crust broke free and uplifted. Today, this block gradually climbs out of the west and drops off precipitously in the east where it broke off from the rest of the crust. The Sierra Nevada continues to grow today at a rate of about 1-2 centimeters a year.
Research suggests that during the drought of 2011-2015, the mountains lost so much water that the weight displacement caused the range to rise almost 1 inch! When snow returned to the range in the following years, the mountains shrank half an inch. This data highlights the importance of mountains as water storage and the massive effect of climate change on mountain ranges. In the case of the Sierra Nevada, this range is the source for much of California’s water.
From 20 million to 5 million years ago, a new batch of volcanoes erupted throughout the range as more magma found its way to the surface. This volcanic period is evident in various features throughout the Sierra, including the basalt columns of Devil’s Postpile and the hot springs of the Long Valley Caldera near the ski town of Mammoth Lakes.
Erosion continued to cut away at the mountains. The higher the mountains rose, the faster the water flowed. Swift rivers and streams carved out steep, deep, and narrow canyons.
The pluton that is Half Dome emerged from the ground as erosion removed the soil around it. The Dome’s sheer Northwest Face that towers above the Valley also began to take shape, though by a different process entirely.
Countless joints, or cracks, run parallel to Half Dome’s face. These cracks suffer the strain of ever-changing seasons. The freeze-thaw cycles of the colder months pry the cracks apart. The intense heat of the summer swells the rock, putting more stress on weak joints. Over time, flakes of granite peel away from the face like the layer of an onion and fall off, often gouging a path of destruction in the ground below. This process of peeling away from the main body of stone is called exfoliation.
On the ground below the face, the exfoliation debris accumulated over millions of years into piles called talus. Periodically, the talus slid into the Valley below. Today, fresh examples of talus fields are visible on the Valley floor near Mirror Lake.
Visitors to Yosemite might notice the countless cracks zigzagging across Half Dome’s face. For rock climbers, cracks like these are the Sirens’ song that brings them to Yosemite. When the magma that is Half Dome cooled miles underground, it was under immense pressure from the overlying rock. This pressure released as Half Dome emerged from the ground. The rock expanded without the weight of the earth bearing down upon it any longer. The stresses of expansion in this massive monolith increased until they surpassed the strength of the granite, causing the outer edges of the rock to split into several layers of shells, much like the layers of an onion. The face of Half Dome continued its emergence into the world.
The Ice Age
Beginning about two to three million years ago, many of Yosemite Valley’s famous features were visible, but lacked the bold definition we see today. The Valley had a deep “V” shape, like the Merced River Canyon to the west. The broad, forest floor and meadows that we know today did not exist.
The Valley and the entire Sierra Nevada went through several ice ages that brought huge glaciers to the region. Geologists have found four distinct glacial periods that occurred in the Sierra: the Sherwin, the Tahoe, the Tenaya, and, lastly, the Tioga.
During these distinct ice ages, the glaciers that poured into the Valley accelerated the process of erosion. Extending to maximum depths of almost 4,000 feet, the glaciers scoured the bottom of the Valley and transformed it into the wide “U” shaped valley that we know.
Here we arrive at a common question: Did a glacier carve Half Dome into the surreal feature it has become?
Interestingly, the geological evidence suggests that glaciers were not a significant player in the formation of Half Dome’s cleft face. Instead, the face of Half Dome likely formed via exfoliation that occurred between glaciations. During the earliest and biggest glaciation, the Sherwin, the glacier that dominated Yosemite Valley nearly reached the top of Half Dome, coming within 500 feet of the summit. The last glaciation to enter the Valley, the Tioga, started about 28,000 years ago and never touched the bottom of Half Dome’s Northwest Face.
The role the glaciers played in Half Dome’s formation was to remove the massive piles of exfoliated rock away from Half Dome’s base and out of the Valley. Additionally, the glaciers may have plucked away rock flakes hanging from the face, thus accelerating the process of exfoliation. The glaciers pulverized this rocky debris in their cold depths until they spit it out at their terminus. The roaring glacial-fed rivers then swept the debris downstream. The rivers swept this debris, along with the older metamorphic rocks, towards California’s now-fertile Central Valley.
Over the millennia, exfoliation transformed the face from a series of small steps and broken cliffs into a sheer wall.
Half a Dome
Thus far, a primary question has gone unanswered: Is Half Dome half a dome?
The answer is no. From the Valley floor, Half Dome appears as a sphere chopped in half, but this view is deceptive. For a more accurate perspective, one must visit the nearby Glacier Point. From there, Half Dome appears as a thin fin of rock and it is evident that the South Face is almost as sheer as the more famous Northwest Face. This view allows visitors to understand why geologists estimate that 80% of Half Dome remains intact.
The geological story of Half Dome did not end when the last glacier receded from the Valley. Exfoliation continues today. For climbers, this fact became evident in July of 2015 when a rock flake 200 feet tall and 100 feet wide fell off of one of Yosemite’s most famous rock climbs, the Regular Northwest Face of Half Dome. While Half Dome’s geological history took place over hundreds of millions of years, these processes can still create cataclysmic events.
While this unique geologic story has made Yosemite famous today, there are many creatures who have learned to thrive amidst the rocks. Let’s now explore these living characters of Half Dome.