Beyond the Forecast: Mauna Loa, Volcanoes and Hot Spots

Fissure Three of Mauna Loa erupting this year ( National Park Service - USGS/M. Patrick, National Park Service - USGS/M. Patrick)

Happy Monday and welcome to another edition of Beyond The Forecast!

As November was wrapping up and some of us were still working on Thanksgiving leftovers (well, maybe some of us were working on still digesting Thanksgiving dinner itself), the big island of Hawaii showed the power of geothermal heat.

Rock turned to liquid by the heat of the planet shot into the air, streaks of red and orange soaring overhead before crashing back down to the surface. Other streams of lava flowed like the lifeblood of the planet itself down the blackened mountainside.

Now I don’t think I’m going to give J.R.R. Tolkien a run for his money anytime soon talking about volcanoes, but instead of Mount Doom (or Orodruin if you want to get fancy about it), this volcano was Mauna Loa in our very own US of A.

Hawaii is famous for its volcanoes. You might already recognize the name Mauna Loa along with Kilauea and Mauna Kea. As a matter of fact, Hawaii is an arcing chain of islands because of the very fact that volcanoes are common there. What you might not expect is that the source of lava in the Earth’s mantle moves very little; the chain of islands is a result of the Pacific Ocean’s tectonic plate moving northwest.

The planet has three main pieces to its makeup: the core (which can be split into inner and outer), the mantle and the crust. The core is made of dense, hot metals like lead that gravity clumped together over millions of years. The surrounding mantle is a collection of extremely hot molecules—mostly rock—that move as a liquid thanks to their heat. The crust is the solid rock surrounding the core with water and life spread out on top of it.

All of the Earth that we experience is only a thin cover to the heated rock within

There are two main types of rocky plates that make up the crust. Dense, heavy plates are mostly made of basalt and slightly less dense plates are largely made of granite.

To us, basalt and granite are both heavier than plenty of things we interact with, but on a planetary scale, granite is noticeably less dense than basalt. That makes it easier for those plates to float a little higher on the mantle. Plates that form continents are granitic ones, while plates that make up the ocean are basaltic.

Continental plates usually slide over oceanic plates causing mountains to form as rock collides

Tectonic plates are responsible for a lot of different features on Earth’s surface, especially mountains and fault lines.

When it comes to volcanoes, there are a number of ways they can pop up, but Hawaii is a great example of a Hot Spot. The heat and rock are not distributed evenly in the mantle. Some spots collect a lot of heat, which then builds pressure against the crust.

When anything gets squeezed too hard, it will eventually burst, and when enough lava builds up in a Hot Spot, it bursts through the crust. Since that lava is just extremely hot rock, it starts to cool when separated from the heat of the mantle.

Hot Spots build up heat again and again over thousands of years, and as the plates move it appears to us as new islands forming in a chain

Lava keeps oozing out for thousands of years off and on and eventually, enough rock builds up to break higher than the ocean’s surface. That’s how we get islands!

The tectonic plates of the crust float somewhat freely over the mantle. A plate moving does not cause a particular Hot Spot below it to move as well. If the plate moves west the next time the Hot Spot bursts, a new island forms to the east of the older island as we see it.

Hawaii does a great job of showing this motion in action. Niihau and Kauai at the northwestern end of the chain are the oldest islands in the state. The Hot Spot bubbled up to form those islands, and the Pacific Ocean’s plate moved north and west before the lava was ready to burst again.

The smallest islands to the northwest of the larger islands in Hawaii are older and therefore more worn by erosion

Oahu was one of the next islands to form; Hawaii (the island itself, not the chain/state) is the newest and therefore largest island.

Elevations tend to be highest on the newest islands in a chain

The ocean erodes away pieces of the islands as the plate moves. Looking outside of the main chain, there are lots of little islands northwest of the state, which were formed even earlier than Kauai and have since eroded. In some cases, the ocean now covers what used to rise well above it.

We certainly don’t have to deal with volcanoes here in the near future, but snow chances are on the rise. Keep up to date on what’s headed our way; you can download our weather app and get Meteorologist Chris Michaels’ latest updates online.

You can always get specific forecast details for your zone, whether it’s the Roanoke Valley, the Lynchburg area, the New River Valley or elsewhere around Southwest and Central Virginia, anytime at Know your zone!

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-- Marshall Downing

About the Author:

Marshall Downing presents the weather Saturday and Sunday evenings at 6:00 PM and 11:00 PM, and you can see him during the week at 12:00 PM and 5:30 PM.