HMXEarthScience - Plate Tectonics

Continental Drift

In the early 1900’s, German Meteorologist Alfred Wegener theorized that the continents were once connected in a super-continent and they have been drifting across the Earth’s surface ever since. Wegener’s super-continent was named “Pangea” and is thought to have existed about 250 million years ago. Wegener had four major pieces of evidence supporting his theory:

The Apparent Fit (South America and Africa appear to fit together like puzzle pieces)
Fossil Correlation (The same exact fossils are found on opposite sides of the Atlantic ocean)
Rock Correlation (The same exact rocks/mountains are found on opposite sides of the Atlantic ocean)
Past Climate Data (There is evidence of glaciers in tropical locations, and deposits of coal in Antarctica)

The theory of Continental Drift was not accepted because it failed to explain what was causing the continents to move (no mechanism).

Earth's Interior

Our model of the Earth’s interior is based on the study of seismic waves. The Earth has a layered structure because when it formed 4.6 billion years ago, it was mostly melted, allowing more dense materials to sink to the center and lighter materials to float to the surface. It is made of the following layers:

Crust, the solid, rocky surface (Continental crust is thick, low density, and composed of granite, oceanic crust is thin, high density, and composed of basalt)
Mantle, composed of the rigid mantle, plastic mantle, stiffer mantle (The Crust and the Rigid Mantle make up the lithosphere, the Moho is the boundary between the crust and the rigid mantle, and the Plastic Mantle is partially melted and known as the asthenosphere
Outer Core (liquid iron)
Inner Core (solid iron and nickel)

As depth within the Earth increases, density, pressure, and temperature increase.

Plate Tectonics

The Earth’s lithosphere is broken into large pieces called tectonic plates. Convection currents in the partially molten asthenosphere move these plates across the surface of the Earth. Locations where one plate interacts with another are called plate boundaries. There are three types of plate boundaries that occur on Earth, convergent, divergent, and transform. Each type of plate boundary has unique characteristics. We also find volcanoes and earthquakes that occur away from plate boundaries. These are called hot spots. Use the tabs below to explore the features found at plate boundaries and hot spots.

Convergent Plate Boundaries: Subduction Zone

This is the boundary between a piece of oceanic crust and a piece of continental crust that are colliding into each other. The dense, thin oceanic crust is forced underground, into the asthenosphere as it collides with the continental plate. A trench is formed where the plates first meet and strong earthquakes are common. On the continental side of the boundary, volcanic mountains are common as the subducting plate melts in the asthenosphere and the melting rock rises and breaks through the surface. A commonly discussed example of a subduction zone is the Peru-Chile trench, on the West coast of South America.

Convergent Plate Boundaries: Island Arc

This is the boundary between two oceanic plates that are colliding. Both dense, thin plates crash into each other and battle to see which one will be forced to subduct. Eventually, one plate will win, forcing the other plate into the hot mantle below. A trench will form at the point where the two plates meet which is also the site of intense earthquakes. Volcanic islands will rise from the sea-floor above the plate that was not subducted. These volcanoes are fed by the melting remnants of the subducting plate. A commonly discussed example is the Aleutian Island chain in the northern Pacific Ocean.

Convergent Plate Boundaries: Collision Zone

This is the boundary between two continental plates that are colliding. The thick, continental crusts crash into each other forcing the land to rise, forming giant, rugged mountains. It is similar to a head-on collision between two cars. As they collide, the cars crumple up. Earthquakes are common in the active plate boundary. Volcanoes are rare as no crust is being forced into the mantle. A commonly discussed example is the Himalaya Mountain chain in Northern India.

Divergent Plate Boundaries: Spreading Centers

This is the boundary between two oceanic plates that are moving away from each other. Magma rises between the plates forming new oceanic crust and forcing the plates apart. The youngest rock is found at the ridge and it gets older as get further away from the ridge. The volcanic mountains at the ridge can grow upwards of one mile above the surrounding sea floor. Molten rock flows over the surface and solidifies instantly. Mild earthquakes are common at spreading centers.

Divergent Plate Boundaries: Rift Valley

This is the boundary between two continental plates that are being torn apart. A crack in the continental crust allows melted rock to escape to the surface, forming new rock and forcing the plates apart. A valley forms between the plates and is eventually filled in with water to create a young sea, similar to the Red sea. Mild earthquakes are common as the land is torn apart.

Transform Plate Boundaries: Transverse Faults

This is a plate boundary in which two plates slide past one another. Friction and pressure are built up as the thick rocky chunks of land are scraped by one another. When this pressure is released, an earthquake occurs. Strong earthquakes are common along transform plate boundaries, though mountains, trenches and volcanoes are rare. A commonly discussed example of a transform plate boundary is the San Andreas Fault in California.

Mantle Hot Spots

Hot spots are active volcanoes that are not located along plate boundaries. The “hot spot” itself is an area of magma (called a plume) that has risen up and broken through the lithosphere, erupting on the surface. It will remain in the same spot while plate moves over it, resulting is a chain of volcanoes, with the only active one directly over the hot spot. As the islands get further from the hot spot, the age increases.

Earthquakes

Where do earthquakes occur?

Earthquakes occur along plate boundaries where two chunks of lithosphere scrape together. The focus is the location where the energy is released and the epicenter is location on the earth’s surface directly above the focus. Earthquakes that occur underwater may trigger a tsunami, a large, fast-moving ocean wave.

Seismic Waves

During an earthquake, several types of waves are generated. The vibrations felt are actually called seismic waves that are traveling through the Earth.

P-Waves

Primary Wave (Compression Wave)
Travels “phastest” so it arrives at a seismic stations “phirst”
Push-pull wave, vibrates forward and backward in the same direction that the wave travels
Pass through solids, liquids (magma), and gasses

S-Waves

Secondary Wave (Shear Wave)
Slow wave, not as fast as the P-wave, arrives at a seismic station second
Shake wave, vibrates side-to-side
Solids wave, only travels through solid layers

Seismographs

A seismograph is an instrument that detects and records seismic waves. By studying a seismogram, we can determine a seismic wave’s distance and size

Seismic Waves Traveling Throughout the Earth

P-Waves travel through solids, liquids, and gasses
S-Waves travel only through solids
Seismic waves travel faster through denser material.
Because of this, the path traveled by a seismic wave is bent towards the surface.
Properties of the material (such as density and pressure) that the waves pass through can be inferred by the speed and angle that the waves travel.
The layers of the earth are determined by the jumps in velocity and “echoes” of seismic waves.
The MOHO is a boundary between the crust and the upper mantle where the velocity of waves jumps up sharply. This sharp increase in velocity is called a discontinuity.
A shadow zone occurs on the opposite side of the earth from an earthquake because of the liquid outer core.
S-Waves are stopped all together as they are absorbed by the liquid outer core. The P-Waves are refracted (bent) as they change velocities in different density materials. This !creates a zone in which no waves are picked up at all.