September 30, 2013

Approaching the Green Sand beach.

Approaching the Green Sand beach. It is always windy in this place

The trail to the Green Sand beach

The trail to the Green Sand beach crosses a long vista of dry grass.

Green Sand beach is located near South Point. Thankfully it is not reachable by normal vehicles and one need to hike 2.5 miles to  the beach. That keeps this treasure relatively untouched by regular tourists.

It’s hard to get descriptive directions to the beach. Take Highway 11 south to Ka Lae. At some point you will see a sign pointing you to the South Point, which is the southernmost point of the U.S. Take that road. This scenic road will take you across windswept pastures, horse farms, and long vistas.  At some point you will see a windmill farm on your right. After ~ 11 miles you approach the end of the road with a gravel parking lot. You can park your car there. That is the point where the trail to the Green Sand Beach starts.

Green sand beaches are very rare, there are only a few in the world and one is on Big Island. The sand that makes the beach green comes from a cinder cone, which rock is made mostly from high temperature green olivine crystals.

Olivine crystals erode out of lava flows. Olivine is a magnesium iron silicate (Mg+2, Fe+2)2SiO4. Its composition includes two heavy elements Mg and Fe. And that makes the crystals heavy – much heavier than the most other crystals which comprise the sand on the beach. Heavy olivine chrystals stay behind while lighter sand grains are washed away by strong wave and wind activity.

Green Sand Beach Top View

View on the Green Sand beach from the top of the cinder cone

The cinder cone, which is a source of green sand, is mostly eroded, only a fragment remains. Both ocean waves and wind are washing it away. This rapid erosion keeps the beach and nearby region supplied with its green sand. But erosion also assures that in some distant future the cinder cone will be completely destroyed  and green sand washed away. And the beach will look like any other.

A small beach on the way to the Green Sand beach

A small beach on the way to the Green Sand beach has sand even more green than on the main beach

Interesting that there is another smaller beach that we passed on our way to the Green sand beach that has sand even more green than on the “main” beach.

Please don’t take sand with you. Keep it there for your grandchildren.

Green sand

Green sand is really green

Olivine is not a single mineral, but a name for mineral series. In olivine, iron (Fe) and magnesium (Mg) can substitute for one another, so the olivine series has two members.

Favalite, formed at lower temperatures (1200 °C), is the iron-rich version, and forsterite, formed at high temperatures (1900 °C),  is the magnesium-rich version. Any combination of magnesium and iron is possible, though.

Olivines  are important rock-forming minerals in mafic igneous rocks such as basalt, gabbro and peridotite. They are common in basalts, especially those found of the ocean-floor.  The gem-quality olivine is called chrysolite.