Geology of Madrid and the Cerrillos Hills - Part 2

(Make sure you read Part 1)

[flickr-photo:id=5350114590,size=m,class=float-left]You see a lot of these little mountain ranges or sets of hills as you drive on the Turquoise Trail: South Mountain, the San Pedro Mountains, the Ortiz Mountains, and the Cerrillos Hills. They're not all that big - no Teton sized ranges. But they're not all that small, either - you see them from miles away as a distinct lumpy piles of rock.

The Ortiz Mountains and Cerrillos Hills are laccoliths, volcanic intrusions similar to sills. At the same time as the sills around Madrid formed, the magma that formed the Ortiz Mountains injected itself between layers of rock and, like in Madrid, it didn't go straight up to the surface. But for laccoliths, the magma doesn't just spread out -- it spreads up, too. The rock above isn't strong enough to keep the magma spread out in just a relatively thin layer, so its weakness allows the magma to go up, as well as out. It's kinda like a volcanic hernia. But these thick hernias of magma still don't break the surface, although they certainly cause a bulge in the overlying sedimentary rock. We get to see the laccoliths now because the 1.5 - 0.5 miles of overlying rock has eroded away.

Here's another, non-medical way of looking at it: it's like slipping your hand into a big book. For sills, slip your hand between the pages. The pages go up a bit, but the book remains relatively flat. For laccoliths, once your hand is in the pages, curl your hand into a fist. The pages go up a lot. Take away the top pages by opening the book, and your fist is like the mountains on the Turquoise Trail with the overlying rock eroded away. All this volcanic injection starts with a weakness in the existing sedimentary rock. Magma squeezes-in where the existing rock lets it. Sills and normal laccoliths only need one weak layer to intrude and spread. But what if the magma squeezed into many separate weak layers?

That's exactly what happened at the Cerrillos Hills laccolith. Going back to our large book, before you put your hand in between the pages, you turn your hand sideways and placed your fingers between separate pages. The top pages then would would be steeper. And if you spread your fingers apart, steeper still. If you're book is not all that big, the top cover could be turned straight up. This happened at the Cerrillos Hills, but not with fingers -- with magma. When the magma moved up, it found not just one but many weak layers it could squeeze between. And just like your fingers in a book, it ended up turning some of the rock straight up. This is called a "Christmas tree" type of laccolith because, well, it looks kinda like a Christmas tree.

from Preliminary Geologic Map of the Madrid 7.5 - minute quadrangle by Stephen R. Maynard, David Sawyer, and John Rogers

Walking around Cerrillos Hills State Park, however, you don't see anything that looks like a big Christmas tree -- not even any Christmas tree ornaments. Most of this laccolith has eroded, leaving a bunch of lumpy hills behind. But if you know where to look, you can find evidence of the remnants of the Christmas tree in two stops right off the road: the Garden of the Gods and Devil's Throne.

[flickr-photo:id=5350118540,size=m,class=float-left]The Garden of the Gods is on Rt 14, 2.8 miles north of the Cerrillos Main Street intersection. Although there's a pull-out, there's not much else. But what you can see are layers of sandstone standing straight up. This sandstone formed between 65 & 100 Ma, and then it was lying horizontally. 35 Ma the intruding magma squeezed into many layers beneath the sandstone, forcing it into its vertical position.

[flickr-photo:id=5350121176,size=m,class=float-right]West of Cerrillos sits the Devil's Throne. Turn west on county road 57 for a half mile and you can't miss it -- it's a huge rock the road has to bend around. If you think this rock doesn't look like the sandstone in Garden of the Gods, you'd be right. Instead of the rock that was pushed around, this was the magma doing the pushing. This is one of those fingers of magma that shoved the other rock straight up. The Devils Throne is a vertical sill. Normally this would be just plain wrong: sills are horizontal intrusions and dikes (a bit more on them later) are vertical. But the other magma intrusions forced this one, just like the sandstone, into a near-vertical orientation. Perfect, it would seem, for some large devil to rule from.

But we're not quite finished. Read on in Part 3 where I finally get around to the 'turquoise' part of the Turquoise Trail.