As the plane begins its descent to Daniel K. Inouye International Airport, you look out the cabin window and wonder why. Why is the Hawaiian archipelago where it is, smack in the middle of the Pacific Ocean, almost 2,000 miles from the nearest continent? How did these islands happen? They invite interrogation.

You probably know the answer has something to do with volcanic activity, but why here and not somewhere else? And why does only one of the eight major islands — the Big Island — still have an active volcano now? And why is that island so big, and the other islands progressively smaller as they line up in a northwesterly direction?

Curiosity is the only credential that’s required to become an amateur geologist. And once so credentialed, you grow to appreciate the wondrous origins of all the places you visit. Rocks take on the quality of ghosts with “deep time” memories to share. Any place on Earth has layers upon layers of such memories, but would-be rockhounds might find Hawaii, as well as the three travel destinations that follow, especially compelling.

The geologic origins of the Hawaiian Islands remained a mystery until 1963, when the concept of a “hot spot” beneath the Earth’s crust was postulated. And the theory was soon accepted that in the Earth’s mantle, below the crust and above the core, are places unusually hot compared with the surrounding mantle. These hot spots remain stationary while the crust above gradually moves.

As the Pacific tectonic plate moves northwest, magma is forced upward through the seafloor to form one volcanic island after another. The plate moves at a rate of approximately two to three inches per year — about how fast fingernails grow — allowing time to form an island of hardened magma. With even more time, that island is eroded, so the oldest Hawaiian islands become the smallest. The Big Island, where volcanic activity is happening now, welcomes visitors at a national park.

Where continents meet in Iceland

But most volcanoes, as well as earthquakes, occur where tectonic plates converge or pull apart. The pressure at these plate boundaries (or faults) builds mountains and forms oceanic trenches. If you want to see plate tectonics up close and personal, Iceland is the place to go. Here you can see where the North American and Eurasian tectonic plates meet — creating what is known as the Mid-Atlantic Ridge or Rift.

Running along the floor of the Atlantic Ocean, the Mid-Atlantic Ridge is part of the longest mountain range in the world. Only in Iceland does it appear above sea level, splitting the island in two. The Westfjords and Reykjavik rest on the North American plate, for example, while the Vatnajokull glacier and the East Fjords are on the Eurasian plate.

In Thingvellir National Park, a UNESCO World Heritage site, you can walk along the edges of the two plates — your legs, in effect, straddling Europe and America. The park is technically getting ever bigger — at the approximate rate of one inch per year — as the Eurasian and North American plates move apart. Testimony to this movement is the magma that has risen to the surface as the continents spread — to create the park’s lava fields seen today. Though no volcano has recently erupted in Thingvellir, they are not considered extinct. And though generally too minor to be felt, earthquakes continue.

Ravines formed by past earthquakes have filled with glacier meltwater. The water, filtered by volcanic basalt rock, is crystal clear, and so pristine you can drink it. One of these ravines, Silfra, is known for its unique opportunities for snorkeling and scuba diving. Underwater, in what’s called the Big Crack, Silfra’s narrowest section provides the setting for many “dive between the continents” photos posted on social media.

Arizona's Grand Canyon

Far from the clash of tectonic plates is the geologic wonder that is the Grand Canyon. But its location on the North American plate helps explain its high elevation — so high that the South Rim elevation of approximately 7,000 feet can cause altitude sickness. As the plate stretched, starting about 65 million years ago, it allowed the upper level of the Earth’s mantle to expand like a balloon, pushing the crust upward to form the American West’s high plateau.

The canyon itself, created by erosion from the Colorado River, is relatively new, dating from about 6.5 million years ago. About a mile deep, the canyon walls are like a multilayered cake with each strata revealing chapters of the Earth’s past. The color and texture of each layer is slightly different, but there’s an overall reddish hue because of the pervasive presence of the mineral iron.

At the very bottom of the canyon are its “Vishnu Basement Rocks,” about 1.7 billion years old. Yes, that’s billion. It’s a metamorphic rock called schist, compressed by mountains thought to have once been as tall as the Himalayas. Because these rock remnants, together with basalt from more recent lava flows, form some of the river’s most daunting rapids, the 19th-century explorer John Wesley Powell often referred to them as “ugly.”

The younger rock layers on top are mostly sedimentary, evidence of long-ago, shallow inland seas. The mud became shale; beachlike sands morphed into sandstone; and calcified sea creatures became limestone.

Still, the Grand Canyon retains intriguing mysteries — always inviting new, nuanced geologic interpretations. The science is never totally settled about how such a relatively small river created such a huge canyon, as much as 18 miles across at its widest point. As in a good detective story, many clues have disappeared with time. Whodunit becomes “whatdunit.”

Coastal Maine

When now-retired University of Virginia geology professor Ernest H. Ern was looking for a place to spend his summers, he picked the rocky coast of Maine — not only for its natural beauty but, more importantly, for its geology. Other, equally beautiful places around the world may be geologically interesting, but they are “masked by the overburden,” he explains. That is, forests, farming and layers of soil hide the deep-time narrative. On the Maine coast, ocean swells and especially glaciers (as recently as 12,000 years ago) have stripped away the overburden.

Pemaquid Point is especially noteworthy for its “excellent exposures,” Ern says. “Rocks are compressed, sheared, faulted — and you see it all. It is like opening up a geology textbook, and there it all is, right before your eyes.” Igneous, metamorphic, marine sedimentary rocks — in all shapes, sizes and textures, plus a multitude of colors. There’s a local park at the Pemaquid Point Lighthouse, and a couple of hours up the coast — “down east” — is Acadia National Park, also rich in exposed shoreline rock.

Most dramatically, the bold shoreline is testimony to the collision of tectonic plates 260 million to 325 million years ago creating the supercontinent called Pangea. Resting on top of the folded and scoured bedrock are wondrously named “erratics” — rounded stones and boulders carried by glaciers from somewhere else. The ice sheet at Pemaquid measured as much as two miles thick.

More-subtle stories of Earth’s geologic past are in evidence anywhere you travel. Even cities reflect the geologic forces that led to their creation. Paris, for example, wouldn’t be Paris without its local limestone formed from a long-ago inland sea. Known as Lutetian (after the Roman name for Paris), the limestone forms the building blocks for much of the city’s architecture. Its warm, effusive cream color creates the “City of Light.”

“Stones can speak,” in the words of legendary German geologist Hans Cloos, “if an ear is there to hear them.”

Nicklin is a writer based in Virginia and Maine. Find him on Twitter: @RoadTripRedux.

Please Note

Potential travelers should take local and national public health directives regarding the pandemic into consideration before planning any trips. Travel health notice information can be found on the Centers for Disease Control and Prevention's interactive map showing travel recommendations by destination and the CDC's travel health notice webpage.

If you go

Hawaii Volcanoes National Park

1 Crater Rim Dr., Hawaii National Park

808-985-6011

Extending from sea level to 13,677 feet and encompassing about 523 square miles, Hawaii Volcanoes National Park includes the summits of two of the world’s most active volcanoes, Kilauea and Mauna Loa. The park is a designated international biosphere reserve and UNESCO World Heritage site. About 150 miles of hiking trails through volcanic craters, scalded deserts and rainforests, with views of petroglyphs. Open seven days a week, 24 hours a day, including holidays. Admission valid for seven days; $30 per private non-commerical vehicle, $15 per walk-in pedestrian, and free for youth 15 and under.

Thingvellir National Park

806 Selfoss, Iceland

011-354-482-2660

A 45-minute drive from Reykjavik, Thingvellir National Park was founded in 1930 and designated a UNESCO World Heritage site in 2004 for its historical, cultural and geologic significance. One of Iceland’s most popular tourist destinations, the park lies in a rift valley that marks the crest of the Mid-Atlantic Ridge and the boundary between the North American and Eurasian tectonic plates. Open year-round, parking fees apply. Free entry.

Grand Canyon National Park

20 South Entrance Rd., Grand Canyon, Ariz.

928-638-7888

Grand Canyon National Park encompasses 277 square miles of the Colorado River and adjacent uplands. The park is home to much of the immense Grand Canyon — about a mile deep and up to 18 miles wide. Layered bands of colorful rock reveal millions of years of geologic history. The Grand Canyon is unmatched in the vistas it offers visitors from the rim. North Rim open May 15 to Oct. 15. South Rim open year-round. Admission valid for seven days; $35 per private non-commerical vehicles, $20 per walk-in pedestrian, and free for youth 15 and under.

Acadia National Park

Hulls Cove Visitor Center, 25 Visitor Center Rd., Bar Harbor, Maine

207-288-3338

Acadia National Park, spreading across Mount Desert Island onto the mainland, protects the natural beauty of the highest rocky headlands along the Atlantic coastline of the United States. At about 3.5 million visits a year, it’s one of the top 10 most-visited national parks in the United States. Visitors enjoy 27 miles of historic motor roads, 158 miles of hiking trails and 45 miles of carriage roads. Carriage roads and trails are open year-round, and the scenic Park Loop Road is open April 15 to Dec. 1. Park entrance passes required May to October. Admission valid for seven days; $30 per private non-commerical vehicle, $15 per walk-in pedestrian, and free for youth 15 and under.

— W.N.