Plate Tectonics

And Their Effects on the Earth

What are tectonic plates?

• Remember that the lithosphere is made of plates of crust (both oceanic and continental).

• We call these plates of crust tectonic plates.– The tectonic plates float on the liquid mantle– Plates have boundaries (edges) that come into

contact with neighboring plates, causing earthquakes and/or volcanic activity

How did tectonic plates form?

• Over time, smaller plates combined to form larger plates. The thinnest parts of the larger plates split to form the “edges” we see today.

What are Subduction zones?Areas where

oceanic plates collide with continental plates

The oceanic crust is pulled under the continental plate, where it melts in the hot magma.

What is Continental Drift?• Proposed by Alfred Wegener in 1915• States that the continents were once connected as a

larger land mass, before they were pulled apart. They are still moving today.

• This theory was not accepted until we discovered seafloor spreading years later.

Evidence for Alfred Wegener’s Evidence for Alfred Wegener’s TheoryTheory

a. South America and Africa would fit remarkably well, shoreline to shoreline.

b. If the Americas were moved next to Africa and Europe, there would be a match of ancient continental rocks and tectonic (fold and fault) structures.

c. PangaeaPangaea – when Wegener placed all the continents together like a puzzle, it formed a large landmass which he called Pangaea.

Fossil Evidence

Why do the plates move?

Plates move because heat is being released from deep inside the earth.

Convection currents causes hot material to rise and expand (plates diverge) and cooler material to sink and contract (plates converge).

How does new crust form?• Hot magma from the mantle spews out of

specific areas of the earth

• The magma cools, turning to rock

Three Types of Tectonic Plate Boundaries

1. Divergent Boundries

2. Convergent Boundaries

3. Transform Plate Boundaries

Divergent Boundries “pulled apart” AKA “rifting”

• Divergent plate boundaries are locations where plates are moving away from one another.

• The rising current pushes up on the bottom of the lithosphere, lifting it and flowing laterally beneath it. This lateral flow causes the plate material above to be dragged along in the direction of flow.

• At the crest of the uplift, the overlying plate is stretched thin, breaks and pulls apart producing a mid-ocean ridge

ConsequencesEx. Krafla Volcano in northeastern

Iceland. Existing ground cracks have widened

and new ones appear every few months.

From 1975 to 1984, numerous episodes of rifting (surface cracking) and volcanic eruptions took place along the Krafla fissure zone.

Before erupting, the ground would gradually rise 1-2 m before abruptly dropping,

Between 1975 and 1984, the displacements caused by rifting totaled about 7 m.

East AfricaEast Africa may be the site of

the Earth's next major ocean.

Plate interactions in the region provide scientists an opportunity to study first hand how the Atlantic may have begun to form about 200 million years ago.

Geologists believe that, if spreading continues, the three plates that meet at the edge of the present-day African continent will separate completely, allowing the Indian Ocean to flood the area and making the easternmost corner of Africa (the Horn of Africa) a large island.

Convergent Boundaries“plates colliding together”

• Two plates meet and push on one another, until the pressure gives way

• Responsible for forming mountains, and producing volcanic activity

• Biggest contributor towards earthquakes

• Subduction zones occur at convergent boundaries

Convergent plates can occur:1. Between an oceanic and a

continental plate

2. Between two oceanic plates, causes the formation of a trench, subduction zone

3. Between two continental plates, causes the formation of mountains, the crust tends to buckle and be pushed upward or sideways.

Subduction Zones • When two oceanic plates collide, the younger of the

two plates, because it is less dense,* will ride over the edge of the older plate.

•The older, heavier plate bends and plunges steeply through the athenosphere, and descending into the earth, it forms a trench that can be as much as 70 miles wide, more than a thousand miles long, and several miles deep.

Trenches and Subduction• Trenches are the deepest parts of the ocean floor

and are created by subduction. • If we could pull a plug and drain the Pacific Ocean,

we would see a number of long narrow, curving trenches thousands of kilometers long and 8 to 10 km deep cutting into the ocean floor.

Formation of Volcanoes

When subduction occurs over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until the volcano rises above sea level to form an island volcano.

Such volcanoes are typically strung out in chains called island arcs. Volcanic island arcs, which closely parallel the trenches, are generally curved.

Two Continental Plates Colliding

• Example: The Himalayan mountain– The collision of India into Asia 50 million

years ago caused the Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights.

– Most of this growth occurred during the past 10 million years.

– The Himalayas, towering as high as 8,854 m above sea level, form the highest continental mountains in the world.

Transform Plate Boundaries“sliding across”

• The zone between two plates sliding horizontally past one another

• Most transform faults are found on the ocean floor.

• They commonly offset the active, divergent spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes.

• However, a few occur on land– Example - the San Andreas fault zone in

California – This fault connects the East Pacific Rise,

a divergent boundary to the south, with the South Gorda -- Juan de Fuca -- Explorer Ridge, another divergent boundary to the north.

San Andreas Fault• The San Andreas fault zone is

about 1,300 km long and, in places, tens of kilometers wide

• It slices through two thirds of the length of California.

• Along it, the Pacific Plate has been grinding horizontally past the North American Plate for 10 million years, at an average rate of about 5 cm/yr.

• Land on the west side of the fault zone (on the Pacific Plate) is moving in a northwesterly direction relative to the land on the east side of the fault zone (on the North American Plate).

Earthquakes

• 80% of all earthquakes occur along the rim of the ring of fire

• Most earthquakes occur at fault zones, where tectonic plates collide or slide against each other

• Immense stress builds up between plates. When the stress is released quickly, it sends massive vibrations to the surface

Ring of Fire

Tsunamis• A series of ocean waves

that sends surges of water onto land

• Caused by large, undersea earthquakes at tectonic plate boundaries

• When the plate rises or falls suddenly, it displaces the water above it, forming rolling waves that will become the tsunami

Hotspots

HotspotsAn area of volcanoes that form in the middle of a

tectonic plate

As the plate moves, the volcanoes on the surface move with it and are eventually cut off from its magma source

According to the hotspot theory, the volcanoes of the Hawaiian chain should get progressively older and become more eroded the farther they travel beyond the hotspot.

Kauai

5.5 million years old

Hawaii

.7 million years old

• Here are several calderas (large craters formed by the ground collapse accompanying explosive volcanism) that were produced by three gigantic eruptions during the past two million years, the most recent of which occurred about 600,000 years ago.

YELLOWSTONE

Yellowstone• The thermal energy of the presumed Yellowstone hotspot fuels

more than 10,000 hot pools and springs, geysers (like Old Faithful), and bubbling mudpots (pools of boiling mud).

• A large body of magma, capped by steam and hot water, still exists beneath the caldera.

• Recent surveys demonstrate that parts of the Yellowstone region rise and fall by as much as 1 cm each year However, These ground movements are most likely caused by hydrothermal pressure changes and do not necessarily mean it will erupt any time soon.