Green Sands Beach, Hawaii
Yesterday, I showed you some sand, including some green sand from Green Sands Beach on the big island of Hawai'i. Today, I'll show you some more images from Green Sands. Let's start by orienting ourselves: We're on the south side of the island, just east of Ka Lae (a.k.a. South Point). Here's a Google Map of the cove (Mahana Bay) where Green Sands Beach (a.k.a. Papalakoa Beach) is located:
To get there from the Ka Lae parking area, you can either drive a four-wheel-drive vehicle over some very rough "roads" or you can hike about 2 miles along the coast. When I visited last week, we hiked. It's a pleasant walk, and there's plenty of green sand to be seen en route to the official Green Sands Beach. Here's the coast: basalt and grassy pastureland, with plenty of wind:
A view looking down into the cove where the green sand beach is located:
So, just why is the sand here green? It's full of olivine, which is weathering out from the local rocks. At first, I assumed the source was the local porphyritic basalt. The fine-grained basalt contains many large phenocrysts of olivine, and when the basalt breaks up, these dense grains tend to be concentrated together. Here's some of that olivine-rich basalt:
But apparently the major local source of olivine are some ash/lapilli layers that make up the prominent headland on the cove's eastern edge, as seen as the "backdrop" in this photo:
A close-up of the sand on the beach, with my fingertip for scale:
And a (repeated showing) of a handful of the stuff:
These green grains don't last especially long -- olivine isn't stable over geologic timescales at the earth's surface, and so it chemically degrades and weathers away. Thus, green sand beaches are extraordinarily rare on the planet Earth (according to Wikipedia, there are two: this one, and one in Guam). You've got to have a source of olivine right there, continually adding new green grains to the mix at a rate which matches or exceeds the rate at which they are being chemically broken down.
On the back side of the beach, draped up against the outcrops of ash and lapilli, is a big slope of sand piled up at the angle of repose. I really liked the patterns made between the olivine and the dark grains as small "avalanches" flowed down the "slip face" of this pile:
I even made a pointless little movie showing these mini-avalanches of green sand:
...Or not pointless? Maybe the sandman, new on the geoblogoblock, can tell me more about what's happening here.
A poorly-lithified chunk of green sandstone (cemented with halite from seawater, apparently, as it crumbled readily in my hands):
Some green sand on a basalt cobble (which itself hosts plenty of olivine phenocrysts):
And now a closer look at some of these ash/lapilli layers which are supposedly the main source of all this olivine. These ash layers were erupted by a cinder cone called Pu'u Mahana, and apparently date to 49,000 years ago:
Some of these layers are better lithified (probably due to welding, a phenomenon that occurs when pyroclastics are erupted at higher temperatures and then deform around one another as the particles settle) and thus stand out as little 'shelves' that are more resistant to erosion by the waves and wind:
Close-up of the ash/lapilli layers, with my fingertip again providing a sense of scale:
After an hour of swimming and relaxing on the beach, we climbed back up to the plateau above the beach, where we noticed this contact between lower ash/lapilli layers and overlying basalt flows:
Notice also all the white stuff filling in fractures here. I'm betting it's calcite, especially considering the little stalactites hanging down, but I didn't have any acid with me, and I neglected to collect any to confirm that assumed identity once I got home. Mea culpa.
Hiking back along the coast to the west, we encountered more beautiful olivine basalt. Porphyritic and vesicular, this stuff just about made me cry, it was so beautiful:
I was delighted when we detoured along one of the little unnamed coves between the official Green Sands Beach and the car, and found this:
This green sand was greener than the official Green Sands Beach:
A reprise of yesterday's image of this beautiful stuff:
We noticed some footprints of a mongoose (introduced species) crossing the green sand:
Close-up of the mongoose tracks:
There was also a nice accumulation of basaltic cobbles (some porphyritic, some not, almost all vesicular) mixed in with chunks of coral:
Wow. What a cool place! Unique in my experience, and pretty close to unique in the world. If you're ever on the big island, you've got to check it out. As a geologist, visiting Green Sands Beach imparts big bragging rights: it will make all your friends green with envy!
To get there from the Ka Lae parking area, you can either drive a four-wheel-drive vehicle over some very rough "roads" or you can hike about 2 miles along the coast. When I visited last week, we hiked. It's a pleasant walk, and there's plenty of green sand to be seen en route to the official Green Sands Beach. Here's the coast: basalt and grassy pastureland, with plenty of wind:
A view looking down into the cove where the green sand beach is located:
So, just why is the sand here green? It's full of olivine, which is weathering out from the local rocks. At first, I assumed the source was the local porphyritic basalt. The fine-grained basalt contains many large phenocrysts of olivine, and when the basalt breaks up, these dense grains tend to be concentrated together. Here's some of that olivine-rich basalt:
But apparently the major local source of olivine are some ash/lapilli layers that make up the prominent headland on the cove's eastern edge, as seen as the "backdrop" in this photo:
A close-up of the sand on the beach, with my fingertip for scale:
And a (repeated showing) of a handful of the stuff:
These green grains don't last especially long -- olivine isn't stable over geologic timescales at the earth's surface, and so it chemically degrades and weathers away. Thus, green sand beaches are extraordinarily rare on the planet Earth (according to Wikipedia, there are two: this one, and one in Guam). You've got to have a source of olivine right there, continually adding new green grains to the mix at a rate which matches or exceeds the rate at which they are being chemically broken down.
On the back side of the beach, draped up against the outcrops of ash and lapilli, is a big slope of sand piled up at the angle of repose. I really liked the patterns made between the olivine and the dark grains as small "avalanches" flowed down the "slip face" of this pile:
I even made a pointless little movie showing these mini-avalanches of green sand:
...Or not pointless? Maybe the sandman, new on the geoblogoblock, can tell me more about what's happening here.
A poorly-lithified chunk of green sandstone (cemented with halite from seawater, apparently, as it crumbled readily in my hands):
Some green sand on a basalt cobble (which itself hosts plenty of olivine phenocrysts):
And now a closer look at some of these ash/lapilli layers which are supposedly the main source of all this olivine. These ash layers were erupted by a cinder cone called Pu'u Mahana, and apparently date to 49,000 years ago:
Some of these layers are better lithified (probably due to welding, a phenomenon that occurs when pyroclastics are erupted at higher temperatures and then deform around one another as the particles settle) and thus stand out as little 'shelves' that are more resistant to erosion by the waves and wind:
Close-up of the ash/lapilli layers, with my fingertip again providing a sense of scale:
After an hour of swimming and relaxing on the beach, we climbed back up to the plateau above the beach, where we noticed this contact between lower ash/lapilli layers and overlying basalt flows:
Notice also all the white stuff filling in fractures here. I'm betting it's calcite, especially considering the little stalactites hanging down, but I didn't have any acid with me, and I neglected to collect any to confirm that assumed identity once I got home. Mea culpa.
Hiking back along the coast to the west, we encountered more beautiful olivine basalt. Porphyritic and vesicular, this stuff just about made me cry, it was so beautiful:
I was delighted when we detoured along one of the little unnamed coves between the official Green Sands Beach and the car, and found this:
This green sand was greener than the official Green Sands Beach:
A reprise of yesterday's image of this beautiful stuff:
We noticed some footprints of a mongoose (introduced species) crossing the green sand:
Close-up of the mongoose tracks:
There was also a nice accumulation of basaltic cobbles (some porphyritic, some not, almost all vesicular) mixed in with chunks of coral:
Wow. What a cool place! Unique in my experience, and pretty close to unique in the world. If you're ever on the big island, you've got to check it out. As a geologist, visiting Green Sands Beach imparts big bragging rights: it will make all your friends green with envy!
4 Comments:
(With apologies for the delay)Re your "pointless little movie" and your hoping that I could make a sensible comment. It's certainly not pointless - I've spent a lot of time starting and watching sand avalanches, as have a number of eminent physicists. Quite apart from being a hypnotic sight, sand avalanches reveal some fundamentals of nature - self-organized criticality, no-linear systems, power laws and much more, not to mention a great illustration of our inability to predict anything very well. And then there's the question of whether sand, as a granular material, is a solid, a liquid, or something else entirely when it behaves like this.
So, I don't know that I've added much - but it's a great post and getting me warmed up for a granular post on my blog!
Michael
P.S - having collected some sand from the beaches of Pele's realm, I presume that you're aware of how much the goddess resents such removals and the consequences of her wrath???
is this the only green sand beach on the big island?
WAY cool blog. Just got back from the islands myself where I spent a scant 2 days on the Big Island, viewing all that nature has to offer. As a microbiologist, I was fascinated, but your geological discussion of the green sands was the best I've found. Mahalo.
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