One of the most impressive features within California is a towering volcanic edifice
called Half Dome.
This ancient mass of rock contains an abundance of granodiorite which rises 1,362 feet above
the surrounding landscape.
Yet, scientists weren't exactly sure how Half Dome formed until quite recently.
This video will discuss how Half Dome in Yosemite National Park formed, and mention some nearby
related geologic features.
The towering mass of rock referred to as Half Dome is located in the central east portion
of California.
Specifically, this edifice is located within Yosemite Valley, which is part of Yosemite
National Park.
Around 90 million years ago, much of western California including the area which would
later become Yosemite National Park lied underwater in a shallow ocean.
However, just like the Cascade Range today, the area was filled with numerous active volcanoes.
These volcanoes existed due to an area of plate subduction to the west, which caused
melted crust to migrate upwards and erupt onto the surface.
However, just like volcanoes today, the vast majority of molten rock never reached the
surface.
Most of this rock instead intruded several miles into the crust, before cooling and solidifying
as various types of volcanic rock.
One of these volcanic intrusions which never reached the surface occurred 84.1 million
years ago, which solidified at a depth of approximately 6 miles or 10 kilometers beneath
the surface.
This specific intrusion, which would later become the famous half dome, solidified into
the shape of a lava dome out of a rock type called granodiorite.
So, how do we know that this lava dome didn't erupt onto the surface?
The answer to this is its rock type.
Based on whether magma erupts onto the surface, or solidifies at depth, it forms two different
rock types based on the composition of the magma.
These resulting rock types have an identical composition, but differing crystal sizes.
At depth, magma takes a longer time to cool, which as a result forms larger crystals.
In this case, if the half dome granodiorite had erupted onto the surface, it would have
become a different rock type called dacite.
As millions of years passed, the regional tectonic forces within California changed.
The shallow ocean retreated, which eventually led to a large degree of uplift.
On the surface, the extensive overlying rock was eroded away for tens of millions of years.
So, as the crust was being pushed upwards, the top was being eroded away, exposing various
intrusive volcanic rocks.
By 40 million years ago, the uplift had largely stopped, causing the mass of aforementioned
granodiorite to become exposed on the surface.
Since then, half dome has been rounded by erosion by a type of weathering called exfoliation.
Just like concrete in the hot sun, this mass of rock expanded and contracted due to temperature
variations.
These variations eventually led to the creation of numerous cracks throughout the top of its
edifice.
Eventually, a large circular region of rock would become unstable, causing it to cleave
off and fall to the ground.
This process repeated many times for millions of years, leading to the creation of much
of its rounded appearance.
However, if you were to visit what is now Half Dome 5 million years ago, it would not
have had a rounded appearance.
Rather, the various cracks due to weathering would have caused its edge to have a jagged
and cracked appearance.
This all changed during the last major ice age, which began 2.6 million years ago.
During the next 2.5 million years, glaciers repeatedly advanced and retreated.
Many of these glaciers reached the modern height of Half Dome, after which they began
rounding its edifice.
After the last ice age ended, what remained was the modern and beautiful Half Dome.
Today, weathering and erosion continues much like it has for millions of years.
In 2015, a 2400 ton mass of rock broke off of Half Dome, and fell more than 1000 feet
to the valley floor.
The type of erosion which caused this collapse to occur was exfoliation.
As a final note, I recommend visiting half dome, as Yosemite National Park has many beautiful
features to offer.
I last went to Half Dome about 7 years ago.
I hope that you enjoyed this video!
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