ENVIRONMENTAL SCIENCE
TECTONIC PLATES
Tectonic plates are massive, puzzle-piece-like slabs of solid rock that make up the Earth's outer shell.
Lithosphere
The lithosphere is the rigid outer layer of Earth that includes:
- The crust
- The very top part of the mantle
Tectonic plates are sections of the lithosphere.
Asthenosphere
The asthenosphere is a softer, hotter layer of the mantle beneath the lithosphere.
Tectonic plates float and move very slowly over the asthenosphere.
Types of Crust
Tectonic plates can contain:
- Oceanic crust → dense and relatively thin
- Continental crust → thicker and less dense
- A combination of oceanic and continental crust
Seven Major Tectonic Plates
The seven major plates are:
1. Pacific Plate
2. North American Plate
3. Eurasian Plate
4. African Plate
5. Antarctic Plate
6. Indo-Australian Plate
7. South American Plate
Types of Plate Boundaries
Divergent Boundary
Plates move away from each other.
This can form:
- Mid-ocean ridges
- Rift valleys
Convergent Boundary
Plates move toward or crash into each other.
One plate may slide beneath another through a process called subduction.
This can create:
- Volcanoes
- Deep ocean trenches
- Mountain ranges
Transform Boundary
Plates slide sideways past each other.
Friction between the plates can build up energy, which is released as earthquakes.
Quick Review
Tectonic plates → pieces of the lithosphere
Lithosphere → crust + uppermost mantle
Asthenosphere → softer layer beneath the lithosphere
Divergent → move apart
Convergent → move together
Transform → slide past each other
Subduction → one plate moves beneath another
Layers of Earth
Earth is divided into four main layers based on their chemical composition and physical properties: the crust, mantle, outer core, and inner core.
As depth increases toward Earth's center, temperature, pressure, and density generally increase.
1. Crust
The crust is the outermost and thinnest layer of Earth. It is a solid, brittle layer of rock broken into large sections called tectonic plates.
Continental Crust
- Found beneath landmasses.
- About 30–70 km thick.
- Thicker and less dense than oceanic crust.
- Mainly composed of granitic rocks.
Oceanic Crust
- Found beneath the ocean floor.
- About 5–10 km thick.
- Thinner and denser than continental crust.
- Mainly composed of basaltic rocks.
2. Mantle
The mantle lies directly beneath the crust and is Earth's largest layer, making up about 84% of Earth's volume.
- About 2,900 km thick.
- Made mainly of dense silicate rocks rich in iron and magnesium.
- Mostly solid, but the intense heat allows the rock to flow very slowly over long periods of time.
- Movement within the mantle is associated with convection, which helps drive tectonic plate movement.
Lithosphere
- Rigid, brittle outer part of Earth.
- Made up of the crust and the uppermost mantle.
- Divided into tectonic plates.
Asthenosphere
- Softer, weaker layer beneath the lithosphere.
- Hot and ductile, allowing tectonic plates to move slowly over it.
3. Outer Core
The outer core lies beneath the mantle.
- Begins at about 2,900 km depth.
- About 2,200 km thick.
- Made mainly of liquid iron and nickel.
- Temperatures are roughly 4,000–5,000°C or higher.
- Movement of the liquid metal generates electric currents that produce Earth's magnetic field.
4. Inner Core
The inner core is the innermost layer at the center of Earth.
- Has a radius of about 1,220 km.
- Made mainly of iron and nickel.
- Temperatures reach about 5,400°C.
- Despite its extremely high temperature, it is solid because of the enormous pressure at Earth's center.
Quick Review
Crust → outermost, thinnest, solid rock
Mantle → largest layer, hot solid rock that flows slowly
Outer Core → liquid iron and nickel, produces magnetic field
Inner Core → solid iron and nickel, highest pressure and temperature
Lithosphere → crust + uppermost mantle
Asthenosphere → softer layer beneath the lithosphere