GEOSC 10: Geology of the National Parks

Plate Tectonics II:Making Mountains & Volcanism

Presented by Dr. Sridhar AnandakrishnanThe Pennsylvania State University

GEOSC 10 - Geology of the National Parks

GEOSC 10 - Geology of the National Parks

GEOSC 10 - Geology of the National Parks

The World’s Volcanoes

GEOSC 10 - Geology of the National Parks

The World’s Ocean Trenches

GEOSC 10 - Geology of the National Parks

The World’s Big Earthquakes

GEOSC 10 - Geology of the National Parks

Overview

•Ocean floor material made at spreading ridges

•Moved off to the side by mantle convection

•Collides with continental crust...

‣ Subduction & accretion

‣ Volcanoes/mountains

‣ Trenches

•Hot spots

GEOSC 10 - Geology of the National Parks

Review

•Mantle made of hot, soft rocks (asthenosphere)

•Upper mantle + crust are rigid (lithosphere)

•Lithosphere broken into plates

•Plates move on mantle convection cells

•Convection cells bring up mantle material and it freezes at pull-apart ridges

GEOSC 10 - Geology of the National Parks

Oceanic Crust

•Let mantle material rise and freeze: Basalt

‣ Basalt is Silica (SiO4) + Iron + Magnesium.

‣ Dark in color, relatively dense.

•Ocean floor is mostly basalt (formed at ridges)

•Earth isn’t getting bigger - oceanic crust has to be destroyed/recycled.

GEOSC 10 - Geology of the National Parks

Subduction

•As oceanic crust moves away from the ridge, it cools, grows more dense, starts to sink back down. (think of the lava lamp...)

•As it is moving sideways, it will run into a continent.

‣ If it is cold enough, already sinking, we get a subduction zone.

‣ If it is warm and buoyant, we get anaccretion zone (Olympics)

GEOSC 10 - Geology of the National Parks

The Fate of the Seafloor

•As it (ocean crust) sinks down, it carries seawater and sediments (ocean bottom mud).

•As it sinks, it gets hot again (remember, it’s hot inside the earth).

•BUT, the added water and sediments help it to melt.

‣ Most things melt better in the presence of water and impurities.

GEOSC 10 - Geology of the National Parks

Volcanic Arcs...

•That melted mix of ocean floor basalt, water, and sediments is low-density, which rises, creating volcanoes at the surface of the Earth.

•This type of volcanic rock is called Andesite (from the Andes Mountains).

GEOSC 10 - Geology of the National Parks

Earthquakes...

•Occasionally that downgoing slab gets stuck and then breaks free with a huge earthquake.

‣ Sumatra, 2004; Peru 1960; Alaska, 1964

‣ The deepest earthquakes aren’t well understood: not like the “stick-slip” we talked about. More like a sudden rearrangement of the solid (phase change).

GEOSC 10 - Geology of the National Parks

Trenches

•Where the slab is subducting, it deforms the over-riding plate - a linear trench is formed. If these trenches are near land, they fill up with sediments from land.

•Those in midocean are the deepest places on the planet.

GEOSC 10 - Geology of the National Parks

Trenches Near Land...

•Trenches near continents aren’t as deep as midocean trenches. Sediment washed out by rivers fills them up quickly.

•Midocean trenches are deeper than Mt. Everest is high!

‣ Marianas Trench off the Philippines is 35,000’ deep.

GEOSC 10 - Geology of the National Parks

First (best!) Science-fiction

•20,000 Leagues Under the Sea by Jules Verne

‣ The 20,000 leagues (about 60,000 miles) is the length of the trip, not how deep it dives - Verne knew better!

GEOSC 10 - Geology of the National Parks

GEOSC 10 - Geology of the National Parks

Density...

•Continents are lowest density (“lightest”)

‣ Mainly silica, v. little iron. Light colored rocks.

•Seafloor is heavier. Basalt.

‣ Silica (SiO4) + iron + magnesium.

•Mantle is heavier.

‣ But cold seafloor is heavier than hot mantle. That’s why seafloor sinks at subduction zones.

•Core is heaviest. Mostly iron.

GEOSC 10 - Geology of the National Parks

•As seafloor runs into continents, the sediment is scraped off and smeared on the continent.

‣ Most of that sediment came from the continent, so the continents grow slowly if at all.

‣ Sometimes, an oceanic mountain/volcano runs into the continent, and the continent grows.

•Oceans are never really old (oldest is 160 million years old).

•Continents are old! 4 billion years old.

Accretion

GEOSC 10 - Geology of the National Parks

Review...

•Mantle is hot and flows in convection cells, called the asthenosphere.

•Upper mantle and crust is rigid and broken into a few plates. This is called the lithosphere.

•Plates meet at pull-apart, push-together, and slide-past boundaries. Mountains built here (mostly).

•Heat (from radioactive decay) drivesthe whole thing.

GEOSC 10 - Geology of the National Parks

GEOSC 10 - Geology of the National Parks

Subduction Volcanoes

•When basalt + water + sediments heat up, they melt and rise, forming andesitic volcanoes.

•The magma tends to polymerize. It makes stringy, lumpy rocks. As the magma comes out the top of the volcano, it solidifies right away.

‣ The volcano is tall, steep, and symmetrical

‣ A stratovolcano.

GEOSC 10 - Geology of the National Parks

Subduction Volcanoes

•The magma doesn’t polymerize inside the earth because the water and CO2 in the magma keeps the magma fluid.

•When it gets to the surface of the earth, the water and gas escapes and the rock quickly solidifies.

•Sometimes, the rock forms a cap.

•The pressure builds.

GEOSC 10 - Geology of the National Parks

Mt. St. Helens, 20 May, 1980

GEOSC 10 - Geology of the National Parks

Mt. St. Helens, 20 May, 1980

GEOSC 10 - Geology of the National Parks

Stratovolcano Eruption

•Magma, containing water, CO2 rises.

•Earlier flows have capped the volcano.

•Pressure builds in the magma pocket.

•Something (small earthquake?) cracks the top of the volcano.

‣ Pressure drops... releasing gas/water... which cracks open the top some more... which dropsthe pressure... releasing gas/water...

GEOSC 10 - Geology of the National Parks

Hot Spots

•Sometimes a plume of magma will rise up from deep inside the asthenosphere.

‣ As deep down as core/mantle boundary?

•The magma has lots of iron, so it doesn’t polymerize. At the surface, the magma spreads out making a broad gently-sloped mountain.

•Hawaii is a shield volcano. Broad, gently sloped (like a gladiator’s shield).

GEOSC 10 - Geology of the National Parks

Hot Spots

•The location of the hot spot is fixed in the asthenosphere... but because the plate moves over it, the surface location of the hot spot moves in a straight line (plates move straight - usually).

GEOSC 10 - Geology of the National Parks

Hot Spots

GEOSC 10 - Geology of the National Parks

GEOSC 10 - Geology of the National Parks

Hazards

•Don’t be on the volcano.

•Gasses are hot (300+); cloud blast is fast (100+ miles/hr); gasses are heavy, so they flow along the ground

‣ nuee ardente

•Ashes and cinder: pyroclastic flows

•Landslide/avalanche

GEOSC 10 - Geology of the National Parks

Hazards

•Lake Nyos, Cameroon is on a hot spot.

•CO2 seeped into the lake and built up on the bottom of the lake.

•Something (small earthquake?) disturbed the lake and the CO2 escaped.

•Suffocated 100s of people living downhill from the lake.

•Now they pump the CO2 out

GEOSC 10 - Geology of the National Parks

Tsunamis

•Move a big volume of water, and it will create a wave that travels across the ocean, eventually striking land.

•Landslides, volcano eruptions, even meteorite strike

• In subduction zone, during an earthquake, the overlying plate will “snap” upwards, moving water.

GEOSC 10 - Geology of the National Parks

26 Dec 2004

QuickTime™ and aCinepak decompressor

are needed to see this picture.

GEOSC 10 - Geology of the National Parks

26 Dec 2004

QuickTime™ and aMotion JPEG A decompressor

are needed to see this picture.

GEOSC 10 - Geology of the National Parks

Shorelines are Inundated

•As the wave gets closer to shore, the sea bottom is rising (water is getting shallower).

•The wave gets bigger and bigger.

•Strikes with great force, as well as flooding and washing out villages & people.

•Roads, infrastructure gone... disease, hunger...