The Hawaiian Islands fit the textbook definition of “the middle of nowhere” perfectly. A glance at any globe shows that the islands are remarkably isolated, lying 3000 km (1850 miles) from the nearest landmass. What might be even more interesting is that the islands sit in 17,000 feet (5000 meters) of water! How did this gorgeous chain of islands form in so remote a location?
The answer to that question lies in the bedrock of the islands, for each island of the Hawaiian Archipelago is – or once was – a volcano. The Big Island, Hawai’i, is a complex of five volcanoes and the only site of active volcanism. Millions of tourists have gazed in awe at Pele’s power, unaware that Mauna Loa is the largest volcano of its type on the planet – and actually taller than Mount Everest.
This shield volcano (a type named for a broad, conical shape resembling a medieval warrior’s shield) rises more than 30,000 feet from its base on the sea floor.
Beyond the Big Island, the other islands are strung like pearls on a necklace; a necklace that lies in a straight line trending northwest. A map of the island chain shows that each island along that string is progressively smaller, with the largest island at the southeast. Aha: a clue!
Let’s add another clue: we’ll call it “Lo’ihi.” At just ten thousand feet tall, the undersea volcano which sits 50 km (35 miles) southeast of the Big Island, is the most active volcano in the chain. Scientists predict that Lo’ihi could breach the surface in just 10,000 years, but for now the summit is almost a kilometer below sea level. This observation is most important, because not only is each island smaller to northwest along the chain, each is older, too.
Early geological theories about the archipelago’s formation called for a “crustal crack” that allowed molten lava to reach the surface, although explanations for the southeastward progress of the volcanoes through time were vague.
It was not until widespread acceptance of plate tectonic theory that scientists were able to put together an explanation that accounts for the chain’s remote location, its volcanic origin, and the northwestward decrease in size and increase in age of the islands. That explanation is the Hawaiian hotspot.
Hotspot? Let’s explain: imagine holding a sheet of plastic above the flame of a candle: the plastic would melt, right? If you were to move that plastic slowly across the flame, the heat would form a line of melted plastic. That is what is happening underneath the Hawaiian Islands.
Plate tectonics tells us that Earth’s surface is a jigsaw puzzle of plates, with each continent and ocean on its own plate. These plates are part of the crust. Each crust plate moves slowly around on top of a stationary layer that’s ever so slightly gooey, termed the mantle. The center of the planet, or core, is hot enough to boil lead.
The heat of the core rises though the mantle in “plumes” of heat at places scattered about the globe: one such spot is the center of the Pacific Ocean – under Hawaii. Such tremendous heat in one place melts the underside of the crust and this causes two things to happen.
First, solid rock expands when it’s hot. Since it can’t expand sideways, it bulges up, except that the top of the crust can’t bulge without cracking a little. Second, melted rock is lighter than the surrounding solid rock and therefore buoyant, meaning it will rise.
And as that molten rock, which geologists call magma, rises it seeps through those cracks. Once it reaches the surface the molten rock, called lava when it’s above ground, flows out to build a volcano.
The Hawaiian hotspot neatly explains the formation of the Hawaiian Archipelago. The Pacific Plate is moving slowly (about 10cm or 0.4 inches per year) northwest while the hotspot is stationary, which explains why the active volcanoes are at the southeast end of the chain and the oldest volcanoes are at the northwest.
The oldest volcanoes have been exposed to the rain and wind longest, which explains – in part – why they are smallest. They are also farther from the bulge caused by the hotspot, and so the crust has cooled. When the crust cools, the bulge disappears and the mountains sink with it.
In fact, the Hawaiian chain is only part of a 3500-mile (5800 km) long chain of extinct volcanoes that reaches all the way to the Aleutian Islands off Alaska. All but the last few of these volcanoes lie completely under water, and are called seamounts.
In the warm waters of the central Pacific Ocean, the extinct volcanoes are crowned by coral reefs that form the beautiful atolls in the Hawaiian Chain’s Leeward Islands. While the foundation of each of these islands is an extinct volcano, the portion above water lies on living rock.
Not only are the Hawaii Islands phenomenally beautiful, they are also a fascinating example of the forces that shape our earth.
