EARTH'S INTERIOR

4 Types of Plate Boundaries

Divergent boundaries (plates pull away from each other)

- New crust is generated

Convergent boundaries (one plate dives under another)

- Crust is destroyed

Transform boundaries (plates slide horizontally past each other)

- Neither produced nor destroyed

Plate Boundary Zones (effects of plate interaction: Unclear)

Divergent Boundaries

Occur when plates moves apart and forms a new crust by magma pushing up from the mantle.
Forms mountain ranges like the submerged mountain range that extends from the Arctic Ocean to beyond the southern tip of Africa called the Mid-Atlantic Ridge. It is one segment of the global mid-ocean ridge system that encircles earth. This known ridge also splits Iceland from the North American and Eurasian plates.
It also resulted to episodes of rifting or, in other words, surface cracking, which took place in the Krafla Fissure Zone.
In addition, volcanic activity are also an outcome of divergent boundaries with an increase of 1-2 meters on the grounds initial height before abruptly dropping which signals an eruption.
This kind of boundary also gave birth to the well known Red Sea which separated Saudi Arabia from the rest of the African Continent.
NOTE: When the crust stretches beyond its limits, the tension crack begins to appear on the Earth’s surface. This, causes the magma to rise and squeeze through the cracks, even erupting and forming volcanoes.

Convergent Boundaries

This type of plate boundary occurs when plates collides or move towards each other.

Oceanic-continental convergence

> Creates a number of long narrow, curving trenches thousands of kilometers long and 8 to 10 km deep cutting into the ocean floor. Pacific Ocean; trenches - the deepest parts of the ocean floor.

> Forms fault lines that produces powerful and destructive earthquakes and eruptive activities.

Oceanic-oceanic convergence

> 2 oceanic plates convergent, one being usually subducted under the other, creating a trench.

> Marianas Trench
> Also causes formation of submarine volcanoes that then rises above sea level forming an island volcano.

Continental-continental convergence

> 2 continents collides together without any getting subducted, usually forming ranges like the Himalayan Mountain Range.

> Due to continental rocks being light, they resist downward motion, causing the crust to buckle and be pushed upward or sideways.

Occurs when 2 continents slide horizontally past one another.
Connects two spreading centers or trenches; convergent boundaries and divergent boundaries (geophysicist J. Tuzo Wilson)
Most are found on the ocean floor.
Offsets the active spreading ridges which produces zig-zag plate margins defined by shallow earthquakes.
Few occur on land (San Andreas Fault Zone in California)
The Blanco, Mendocino, Murray, and Molokai fracture zones are some of the many fracture zones (transform faults) that scar the ocean floor and offset ridges.

Plate-Boundary Zones

not well defined boundaries due to the plate-movement deformation occurring there extends over a broad belt (called a plate-boundary zone).
Mediterranean-Alpine region between the Eurasian and African Plates.
Involves at least 2 large plates and one or more microplates caught up in between them.
complicated geological structures and earthquake patterns.

Why is the Earth's interior hot?

Heat from when the planet formed and accreted, which has not yet been lost.

The earth was formed through the process of accretion.
Meteorites came by the pulling force of gravity and formed bigger objects that attracted bigger masses. This went on until our planet, the earth, reached its size. And accumulated a lot of heat.
When it collides, heat is generated.

Frictional Heating

As the denser core material sink at the center of the Earth, it temperature increases and produces heat.

Heat from the decay of radioactive elements

As the radioactive elements decay it produces heat in the interior of the Earth.
There are four isotopes that contribute to the heating of the earth. These are Uranium-238 , Uranium-235, Potassium-40, and Thorium-232.
We can describe radioactive elements as unstable elements. After its half-life it will produce daughter isotopes which is more stable than parent isotopes.
> Uranium-238 to Lead-206
> Uranium-235 to Lead-207
> Potassium-40 to Argon-40
> Thorium-232 to Lead-208

What happens after magma is formed?

After magma is released from the volcano it cools down and crystallizes to form igneous rocks. Through weathering and erosion the igneous rocks form into sediments. In order to become a sedimentary rock the sediments undergo compaction and cementation. After forming into a sedimentary rock, it is buried to increase the temperature and pressure (metamorphism) to create metamorphic rocks. Once the metamorphic rock melts, it goes back as a magma.

The Rock Cycle

Types of Stress that Rocks Undergo

What is stress?

> It is a force that changes the rocks' shape and volume.

Compression

Rock pushes or compresses against each other.
Rocks can be squeezed until it breaks or folds.
If the compression is horizontal it can cause the crust to thicken or shorten.
If it is a vertical compression stress, it can thin out or break.
Usually occur in convergent boundaries.
An example of compression stress is the Rocky Mountains in North America.

Tension

Rocks move away from each other.
It can happen in two ways:
1. Two different plates move farther away from each other
2. The ends of one plate move on different directions.
It usually occurs in divergent boundaries.
An example of tension stress is the formation of continents from the supercontinent Pangaea.

Shearing

This type of stress is caused by pushing a mass of rock in two opposite directions that break up or slip apart.
This usually occurs in tranform boundaries.
The friction made by shear stress can cause earthquakes.
A known example of the shear stress is the San Andreas Fault in California and Alpine Fault in New Zealand.

Next: Weathering →

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