Tectonics of the East Pacific
As you cruise past the majestic Andes, take a moment to thank the tectonic plates for their existence.
WORDS BY CAROLYN BEASLEY
Earthquakes and volcanic eruptions are some of the most spectacular natural events on Earth. Equally awe-inspiring are the dramatic mountain ranges like the Andes, which thrust upwards to challenge the sky – influencing cloud formation, ocean currents, and climate. Yet, if you look at the root cause of all these mighty phenomena, you’ll find the movement of giant tectonic plates that are responsible.
But what exactly are tectonic plates, and why do they move?
Tectonic plates are part of the Earth’s outer layer – called the lithosphere – which is composed of the crust (the solid shell) and the top part of the upper mantle. In this layer are vast plates of rock averaging around 125 kilometres in thickness. These plates fit together like pieces of a jigsaw puzzle, wrapping around the planet.
The plates rest on top of a malleable layer below. The convection cells from the intense heat move the tectonic plates by up to 15 centimetres per year. It’s around the boundaries of these tectonic plates where things get really interesting.
Birthing the Andes
Cruise the eastern Pacific, you’ll see stunning examples of what happens when tectonic plates meet in the geographical features of Central and South America. Gazing landwards, it’s impossible to miss one of the longest mountains chains on land, the Andes, as it bisects the continent for more than 7,000 kilometres from Venezuela to Argentina.
Tectonically speaking, the Andes mountains were created by a tectonic boundary called a subduction zone. This is where two plates collide and bend, forcing one upwards and the other downwards. Geologists believe these mountains started rising at least 50 million years ago, mostly in spurts of ‘rapid’ uplift moving up to one mile every several million years.
If you sail further north, you can see the bird-rich Aleutian Islands, which are like a partially submerged mountain range forming stepping stones between Alaska and Asia. These islands comprise 40 active volcanoes that were created by subduction when the Pacific plate sank below the North America plate.
Mountains of the deep
Tectonic movements also generate mountains we can’t see. If your expedition cruise takes you to the east Pacific, you may sail over the East-Pacific Rise. This is part of the world's largest mountain range – the Mid-Ocean Ridge System – which is mostly underwater and stretches over 64,000 kilometres. This is an example of a divergent tectonic boundary, where the plates pull apart from one another and magma pushes upwards through the gap before cooling to form a rocky ridge.
The third type of tectonic boundary in the east Pacific region is the transform boundary, which is caused when two plates grind past each other horizontally in opposite directions. As they jam together, the plates accumulate tension that is suddenly released as earthquakes. Where these slips occur, we have active fault lines like the San Andreas Fault in California.
1. The Andes
Mountain ranges like the Andes are caused by subduction - when two plates converge. In the Andes, the oceanic Nazca plate slipped below the South American plate. This caused the land of South America to buckle upwards and molten material from the sinking plate to reach the surface in the form of volcanoes.
2. The Aconcagua
The highest peak in the Andes, Mount Aconcagua in Argentina has been thrust 6,962 metres above sea level by the converging plates.
3. The Peru-Chile trench
Oceanic trenches like this form where subduction occurs.
4. Aleutian Islands
Where subduction occurs at sea, underwater volcanoes may eventually break the surface in the form of volcanic islands. The Aleutian Islands and corresponding Aleutian Trench formed when the dense Pacific plate slipped below the North American plate.
5. East-Pacific Rise
Divergence occurs when two plates pull apart. Magma from deep in the Earth’s mantle wells up in the gaps and forms new crust, eventually moving upwards and creating underwater ridges. An example is the East-Pacific Rise, part of the planet’s largest (and mostly underwater) mountain range.
6. San Andreas Fault
The San Andreas Fault, a transform boundary, was responsible for the San Francisco earthquake of 1906. Here, the North American plate, moving south, locks margins with the north-moving Pacific plate. The pressure then builds until the plates suddenly lurch in opposite directions, resulting in an earthquake.
