the

Rockies Thrust Up | National Geographic

on our journey south along the Rocky

Mountains we're entering a very

different landscape from Montana

southward the Rockies are made from

ancient granite 1.7 billion years old

granite makes up much of the deepest

part of the continental crust

that's why geologists call this rock the

basement

the Canadian Rockies are built from

sedimentary rocks piled up on top of the

continental foundations so why does

granite suddenly appear here in the

American Rockies

but there's an even greater puzzle

Mountains usually form close to plate

boundaries but the southern Rockies sit

a long way from the plate margin

the Front Range in Colorado is a

thousand miles from where the Pacific

and North American plates actually meet

geologists have come up with an

explanation

they believe that the subducting

pacificocean plate is responsible oh Sh

in crust had been pushed deep into the

mantle beneath North America for a

hundred million years when something

unusual happened

plate started to subduct at a shallower

angle instead of plummeting steeply it

sliced beneath North America

horizontally this change had dramatic

consequences the big oceanic plate in

the Pacific didn't go deep down he went

in shell like a spatula under a pizza so

something happened sixty-eight million

years ago over in California that plate

drives under North America but instead

of diving deeply it comes in shallow and

a thousand miles away from the coast up

from the ground sprout the Rocky

Mountains for millions of years the

ocean plate scraped along the underside

of North America it created friction

breaking up the basement granite of the

North American plate and punching it

upward

structural geologist Karl Karl strim

demonstrates because it was at a

shallower angle beneath North America it

was scraping along the base of North

America when that happens it puts the

plate under compression like this

because it's being both pushed at the

end and scraped along at the bottom so

it squeezes pushes up the mountains and

it transferred this mountain building

from the edge to great distance from the

plate margin the shallow angle

trajectory of the Pacific Ocean plate

explains why these mountains formed so

far inland and it also explains the

presence of granite thrust up through

layers of sedimentary rock the broken

granite became the Rocky Mountains of

the south

Red Rocks park Denver Colorado a

landscape forged by granite uplift the

granite mountains here have been pushed

up for miles on top of the granite lie

spectacular red slabs of rock known as

flat irons they're the patchwork remains

of sedimentary rock that once blanketed

the entire granite basement Kirk Johnson

demonstrates how these granite peaks

punched up through the layers of red

rock around them I'm going to use my

assistants Veronica and Ian to help me

explain how the Rocky Mountains in

Colorado formed imagine if you will that

Veronica naeun are composed of 1.7

billion year old metamorphic rocks the

basement Rock of Colorado before the

mountains grew layers of sedimentary

rock covered the granite and then in

about sixty eight million years ago the

mountain where ian is starts to break

and lift up and move up so you see the

uplift forming what's happening is that

layer of sediments being deformed is

bent and these are the Flatirons at Red

Rocks as the mountain comes up overlying

sediment eroded off and deposited into

the adjacent basin and eventually Ian's

granitic back is exposed as the core of

the Rocky Mountains

Veronica remains deeply buried beneath

Denver still covered with deep sediments

and the set of us I wrote it off the

uplifting Rocky Mountains