Plate Tectonics III:

Making Mountains, Obduction, & Tsunamis GEOSC 10: Geology of the National Parks Presented by Dr. Sridhar Anandakrishnan The Pennsylvania State University

Go Dog Go

PJ Eastman, 1961

GEOSC 10 - Geology of the National Parks

Appalachians

GEOSC 10 - Geology of the National Parks

Sideling Hill, West VA

GEOSC 10 - Geology of the National Parks

Review

• • Tectonics is driven by heat.

• • The plates (8 major ones, few small ones) move on the surface of the Earth.

‣ Oceanic plates are basaltic.

Initially hot and buoyant... later cool and sink Continental plates are silica-rich, low-density, and buoyant.

GEOSC 10 - Geology of the National Parks

Review (2)

• When cold ocean plates collide with continental plates, they “dive under” the continents.

• ‣ Subduction leads to Stratovolcano chains (Andes, Cascades, Aleutians) ‣ Trenches (unless filled by sediments) ‣ ‣ Deep earthquakes Tsunamis

GEOSC 10 - Geology of the National Parks

When Continents Collide?

• Appalachian mountain range from Newfoundland to Alabama, and again in Oklahoma.

• • Continents are rarely destroyed - so the story ‣ gets very complicated.

A colleague’s office - he never throws anything away, so the piles of papers and books get jumbled up Oceanic crust is created, then destroyed...

GEOSC 10 - Geology of the National Parks

GEOSC 10 - Geology of the National Parks

Susquehanna Valley from Space

Image courtesy NASA, MISR

GEOSC 10 - Geology of the National Parks

Appalachians Complicated

• About 300 Million years ago, N. America collided with Europe/Africa to create a chain of mountains (perhaps 15,000 ft high?).

• Similar to what is going on today with India and Asia.

GEOSC 10 - Geology of the National Parks

Continent-continent Collision

GEOSC 10 - Geology of the National Parks

Mt. Everest, looking N

Image courtesy USGS, photo by Gimmy P Li

GEOSC 10 - Geology of the National Parks

How to Shorten a Continent...

• ‣ Thrust fault Shorten up a continent by sliding one part up and over another part.

‣ This is what happened in Great Smokies area.

GEOSC 10 - Geology of the National Parks

How to Shorten...(2)

Susquehanna valley from space, image courtesy NASA, MISR

• • • Farther north (around here), the rocks “wrinkled up” like a kicked rug.

The “rug” is layered (hard and soft layers).

As time goes by, the soft layers get eroded and the hard layers form the ridges.

GEOSC 10 - Geology of the National Parks

Cross Section Through SC

GEOSC 10 - Geology of the National Parks

Eventually the Collision Stopped

• This area spread apart (similar to Death Valley pull-apart.

• • Atlantic Ocean formed... still spreading.

‣ Mountains stopped being pushed up.

Erosion scrapes away the tops of mountains and deposits the remains (sediment) in the low areas or in the ocean.

GEOSC 10 - Geology of the National Parks

Appalachians Still High...

• • But, mountains have “deep roots.” By the principle of

isostasy,

mountains that stick up high above the landscape also have a thickened crust below them that sticks down into the mantle.

GEOSC 10 - Geology of the National Parks

Isostasy, Erosion, Icebergs...

• As these mountains are eroded, they remain high because material from below is rising up.

• ‣ Like an iceberg floating in water.

Chop off the part above water ‣ Part below water will rise up.

It will be almost as high.

Depends on the density difference.

GEOSC 10 - Geology of the National Parks

Plate Collisions...

• • • • Pull-apart (saw that first week - Death Valley) Subduction (saw that last time - Crater Lake) Obduction (this time) ‣ Slide-past (San Andreas fault) Saw this briefly when we talked about earthquakes

GEOSC 10 - Geology of the National Parks

Slide-past Tectonics

• • ‣ Sometimes 2 plates slide past each other.

Usually not smoothly (stick slip... and offset of fences, etc.) ‣ If there is a “kink” in the boundary, then the sliding-past behavior can make mountains.

The 2 plates push together at the kink rather than sliding past (as they do on the straight bits) ‣ Or the 2 plates pull apart at the kink...

GEOSC 10 - Geology of the National Parks

Project Habakuk

• • • During WW2, proposal to build an aircraft carrier from an iceberg.

Mixture of wood pulp and ice.

Small one built on Lake Louise in Canada.

GEOSC 10 - Geology of the National Parks

Rocky Mtns - Leading Idea

• • Collision of N. America with oceanic crust. Normally oceanic crust will subduct under continental crust.

• But, if the oceanic crust is hot, it will slide under the continent, but scrape along the ‣ bottom.

Continent gets deformed way inland ‣ Rockies are 1500 miles from the Pacific

GEOSC 10 - Geology of the National Parks

Rocky Mtns - Leading Idea

• • Collision of N. America with oceanic crust. Normally oceanic crust will subduct under continental crust.

• But, if the oceanic crust is hot, it will slide under the continent, but scrape along the ‣ bottom.

Continent gets deformed way inland ‣ Rockies are 1500miles from the Pacific.

GEOSC 10 - Geology of the National Parks

Cross Section of West

Used to be far offshore Warm Push up far inland Cold

GEOSC 10 - Geology of the National Parks

Metamorphic Rocks

• ‣ Some rocks in the Rockies and Great Smokies are

metamorphic rocks.

Means they have been changed from their original form.

‣ Under the heat, pressure, and chemical action, they change from (usually) sedimentary rocks to a harder, more-resistant rock.

They have been “cooked” ‣ The look different - folded, squeezed, with pretty colors (and funny names).

GEOSC 10 - Geology of the National Parks

Metamorphic Rocks

GEOSC 10 - Geology of the National Parks

Igneous Rocks

Andesite, courtesy USGS Pele’s Hair, courtesy USGS

GEOSC 10 - Geology of the National Parks

Why are They at the Surface?

• As the overlying rocks get eroded and removed, the deeper rocks rise up.

• ‣ The deeper rocks rise up because of isostasy.

Like the iceberg rising up as the part above water is lopped off, and the alien emerging...

GEOSC 10 - Geology of the National Parks

Rock Cycle

Courtesy USGS

GEOSC 10 - Geology of the National Parks

Types of Rocks

• ‣ Igneous rocks Formed by melting and then solidifying of magma ‣ Or during a volcano • • ‣ Sedimentary rocks Formed by erosion of other rocks and then deposition ‣ Metamorphic rocks Formed by “cooking” rocks

GEOSC 10 - Geology of the National Parks

Hazards

• • • ‣ Most of us will die of old age.

Wear a About 1% or fewer die of “outside causes” Car crashes and handgun deaths.

‣ The main geologic hazards.

Tsunamis, volcanoes, earthquakes, landslides.

GEOSC 10 - Geology of the National Parks

Tsunami Warning

• • Because the water wave can take minutes to hours to get across the ocean to land, there is time for warning.

The earthquake/landslide/meteor strike can be detected almost immediately, so we can send out warning by radio/internet/smoke signals.

GEOSC 10 - Geology of the National Parks