Not All Rocks Are Old

We usually think rocks are millions of year old. Not all  are. These are less than three years old. Ours is a dynamic earth: creating, destroying, changing.

Just got in some basalt from R.C. at Geological Specimen Supply. He collected these examples himself in Hawaii late last year. It was, pun intended, rocky going, walking on rock as sharp as glass.

For background, “Basalt is a fine-grained, dark brown to black rock that lava solidifies into. It is made of minerals, such as, calcium-rich plagioclase, pyroxene, and lesser amounts of olivine. Basalt is the main type of rock in oceanic environments, such as mid-ocean ridges and hot spots.” University of Oregon website (external link)

I got two specimens from R.C., one shows more olivine than the others. Olivine is commonly known as peridot.

Here’s how R.C. describes these examples. R.C., by the way, is a practicing geologist who spends most of his time wandering California and Nevada deserts looking for teaching examples of rocks and minerals.

Tholeiite basalt aa from the flow that destroyed Kapoho, Hawaii in 2018

Tholeiite basalt, rising from partial melts of peridotite in the mantle, is the most common igneous rock on earth and makes up the oceanic crust. It is the type of basalt found on the moon. Peridotite is composed primarily of olivine and pyroxine. Olivine crystallizes at a high temperature and is one of the first minerals to form crystals in the rising magma. It is common in Hawaiian basalts, weathering out of a Mauna Loa cinder cone near South Point, where it is concentrated in a cove to form a famous green sand beach.

Aa forms when lava cools quickly and moves fast, with the surface breaking up into a clinkery mass of loose jagged pieces. Aa flows tend to be thicker than pahoehoe and are almost impossible to walk on. This aa was formed on the flow that consumed the community of Kapoho in 2018 and completely filled in Kapoho Bay, forming a lava delta.

This aa is extremely scratchy and will impress any student that walking on an aa flow would be a challenge. Textbook example.

Closeup of texture.

For comparison, this is vesicular basalt from San Carlos in Arizona. It displays olivine more prominently. Stevie Joey mines this kind of basalt at his claim on the reservation. His website is Peridot Dreams and he is good people.

I also have some self-collected vesicular basalt from the Dixie National Forest in Utah, however, I am in too much pain right now to find it. Matter of fact, I was lucky to put on my socks this morning

R.C. mentioned “aa.” Cornell says this, “Lavas, particularly basaltic ones, come in two primary types: pahoehoe (pronounced ‘paw-hoey-hoey”) and aa (pronounced “ah-ah”). Aa forms when lava flows rapidly. Under these circumstances, there is rapid heat loss and a resulting increase in viscosity. When the solid surface crust is torn by differential flow, the underlying lava is unable to move sufficiently rapidly to heal the tear. Bits of the crust are then tumbled in and coated by still liquid lava, forming the chunks. Sometimes the crust breaks in large plates, forming a platy aa. ”

A little look at olivine sand from a green sand beach in Hawaii. Old stock. Hawaii’s coastline doesn’t favor beach formation. For decades, the white sand beaches of Waikiki were maintained by bringing in sand dredged from Long Beach Harbor in California.

“Around Kapoho.” Credit: Graeme Churchard: Wikipedia Commons.


“Views from HVO’s helicopter overflight at 1PM HST, show the remains of the Kapoho Beach Lots subdivision and the fissure 8 flow front. The northern flow margin in this area was unchanged from HVO’s morning flight and appeared to have stopped advancing at the time of the flight.” Credit: USGS.

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Hawaii In A Bottle

Self collecting is great but most of us can’t go around the world to chase specimens or go back in time to get them. Case in point is this old stock Hawaiian sand from Geological Specimen Supply, mentioned in my last post.

This sand is over twenty years old, predating the ban on removing sand from the Islands.  This specimen comes with great labeling, a summary of the fuller descriptions RC provides on his site. Did you know that Waikiki’s beaches actually came from Long Beach, California? Read on and then take a look at what I am seeing under my scope.

“Sand is derived from whatever material is locally available. Beaches are uncommon on the seacliff-ringed Island of Hawaii. Occasional small coves have sand derived primarily from basalt unless there is a coral reef offshore. This sand was collected from a small cove near South Point, Hawaii, where a local concentration of cobbles of basalt and coral was the source material for this sand. The sand is primarily coral and basalt, giving it a salt and pepper look, with occasional small shell fragments adding variety to the mix.”

“Hawaii’s white sand beaches at Waikiki were initially an import, from California. In the 1920s and 1930s sand was barged from Long Beach, though the import of California sand was abandoned in the 1970s and the beach is now maintained with locally dredges sand composed of coral and shell fragments. A locally famous black sand beach occupies a cove southwest of Hilo, where the sand is entirely basalt. At Mahana Bay, near South Point, the greenish sand is composed of grains of olivine that weather out of the basalt.”

“Set of five tubes, 16 ml each, optically clear polystyrene with screw caps. The plastic tubes are practical in a classroom and are somewhat student resistant, though a cap can be unscrewed. Good for student examination. When this sand is gone, it’s gone. A recent law prohibits removal of sand from Hawaii’s beaches.”

The focus of our attention.

Here are images just taken under my microscope. These are single focus shots, so the foreground and background focus aren’t the same. Really good photographers take a number of shots at different focal points and then have software combine them to produce a perfectly focused imaged. I’m trying to learn to do that.

White material is probably coral, dark material basalt, and the green is olivine or peridot. Shell fragments, too. Olivine is thrown out from deep within the earth during volcanic eruptions, mixed up with basalt at the same time. Basalt pieces can make up a black sand beach, the lighter peridot may weather out of the basalt and collect together, producing a rare green sand beach.

Closeup of a peridot or olivine grain, exhibiting poor conchoidal fracturing.

A shell, sharpened in Photoshop.

Unsharpened and from a different angle. Any difference?

Another shell, this one red.

Basalt with olivine from the San Carlos Apache Reservation in Arizona. This is what tribal member Stevie Joey mines on his peridot claims. His site is Peridot Dreams and you can learn all about gem grade peridot there. He is receptive to inquiries about touring his claim.

Specimen tube. Buy several to trade.