Mexico and Central America: A Brief Geology & Geography Lesson

Why is the Yucatan Peninsula farther north than Mexico City?  Why are there no rivers there?  Why is Mexico so mountainous?  Did the Isthmus of Panama always exist as a land bridge between North and South America?  How come there is that spindly pencil of land we call Baja, California?  Take a look at this atlas, then read on!

Recently I found a 2007 National Geographic map called The Dividing Link: Mexico & Central America.  It was included as a supplement to the August 2007 issue of the magazine featuring the rise and fall of the Maya.  As I flew from Houston to Mexico City across the Gulf of Mexico and down Mexico’s east coast along the water, I pulled out the map as a reminder of my bearings.

Some general points:

  1. Baja California broke away from the mainland between 12 million and 6 million years ago by the San Andreas Fault system.
  2. 90 million years ago water flowed freely between the Atlantic and Pacific Oceans.  The North American continent ended just south of Oaxaca, more or less.
  3. 50 million years ago volcanic and earthquake forces shifted Mexico and pieces of Central America to the northeast to begin the formation of a land bridge between North and South America.
  4. 20 million years ago most of the Yucatan peninsula was flooded.  Movement from the Cocos Plate narrowed the gap between the Americas and moved islands to attach to land masses.
  5. Panama fused to land about 12 million years ago. The land bridge then completely connected the continents and allowed species to migrate freely between north and south, giving rise to llamas and alpacas.
  6. Barricaded, the Atlantic became saltier and denser; the Gulf Stream sank before reaching the Arctic, and pack ice expanded.  Now, as the Gulf Stream sinks and chills, it draws in warm water from the south, shaping a global system of currents that influences the world’s climate.
Mexican Plateau, Gulf of Mexico and Yucatan Peninsula
  1. The nation’s physical and cultural heartland was uplifted 50 million years ago.  The central plateau where Mexico City is located rises from 4,000 feet in the north to 8,000 feet south of Mexico City.  
  2. The north is “basin and range” terrain, created by crustal stretching and breaking.  
  3. Volcanic outpourings left rich soil in the south where corn was first domesticated 9,000 years ago (think Oaxaca). 
  4. The Sierra Madre Occidental, the western mountain range of Mexico, was created by intense volcanic eruptions 30 to 20 million years ago.  The range’s precipitous canyons are the deepest in the hemisphere and include veins that make Mexico the world’s number one producer of silver
  5. The Sierra Madre Oriental, the older and more weathered eastern mountain range of Mexico, was raised by the same tectonic collisions that built the Rocky Mountains between 40 and 70 years ago.
  6. Mexico’s massive oil deposits from the Gulf of Mexico come from limestone that was formed between 200 million and 160 million years ago. 
  7. The limestone tableland of the Yucatan Peninsula was still submerged 65 million years ago.  No rivers flow across the porous land but rain seeps into underground rivers and sinkholes called cenotes, once used in Maya rituals.
  8. The Trans-Mexican Volcanic Belt passes south of Guadalajara and is crowned by snow-capped Orizaba, at 18,855 feet North America’s third highest peak.  The eruptions are caused by the oceanic Cocos plate that created the subterranean Middle America Trench along the Pacific Coast and forged the Sierra Madre del Sur and coastal mountains the length of Central America.  (This is why we get earthquakes in Oaxcaca!) 
If you love Mexico and want to know more about her geology and geography, see what you can do to get a copy of the National Geographic map.  It’s a great educational resource.



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