Did you know that earthquakes happen all over the world, all the time? Most are tiny, measuring less than 3 on the logarithmic Richter Scale. These earthquakes are called microquakes. They're rarely sensed by people, so are often only detected by local seismographs. However, some earthquakes can be powerful and dangerous hazards. Major quakes can lead to ground shaking, soil liquefaction, and destruction of buildings and roads.
Tectonic activity, such as earthquakes and tsunamis, is driven by the lithosphere. The lithosphere is one of five 'spheres' that shape our planet. How does the lithosphere cause earthquakes? Keep reading to find out…
The Lithosphere: Definition
To understand what the lithosphere is, you first need to know about the structure of the Earth.
The Structure of the Earth
Earth is made up of four layers: the crust, the mantle, the outer core, and the inner core.
The crust is the outermost layer of Earth. It's made of solid rock of varying thickness (between 5 and 70 kilometres). That may sound huge, but from a geological perspective, it's very narrow. The crust is split into tectonic plates.
Beneath the crust is the mantle, which is almost 3000 kilometres thick! It's made of hot, semi-molten rock.
Below the mantle is the outer core – the only liquid layer of Earth. It's made of iron and nickel, and is responsible for the planet's magnetic field.
Deep in the centre of the Earth is the inner core, made of mostly iron. Although it's 5200°C (well above iron's melting point) the enormous pressure prevents the inner core from becoming a liquid.
What is the Lithosphere?
Now you've learned about the layers of the Earth, it's time to find out what the lithosphere is.
The lithosphere is the solid outer layer of Earth.
The lithosphere is composed of the crust and the upper portion of the mantle.
The term "lithosphere" comes from the Greek word litho, which means "stone" and "sphere" - the rough shape of the Earth!
There are five 'spheres' that shape our planet. The biosphere comprises all of Earth's living organisms, from microscopic bacteria to blue whales.
The cryosphere constitutes the frozen regions of Earth – not just ice, but frozen soil as well. Meanwhile, the hydrosphere is home to Earth's liquid water. This sphere includes rivers, lakes, oceans, rain, snow, and even clouds.
The next sphere is the atmosphere – the air surrounding Earth. The final sphere is the lithosphere.
You might come across the term 'geosphere'. Don't worry, it's just another word for the lithosphere.
The lithosphere interacts with other spheres to maintain the Earth as we know it. For example:
- The lithosphere provides habitats for plants and soil microbes
- Rivers and glaciers erode the lithosphere at the banks
- Volcanic eruptions affect the atmospheric composition
The five systems work together to support ocean currents, biodiversity, ecosystems, and our climate.
What is the Thickness of the Lithosphere in Miles?
The thickness of the lithosphere varies depending on the type of crust above it. There are two types of crust – continental and oceanic.
The key differences between the two types of crust are summarised in this table.
| Property | Continental Crust | Oceanic Crust |
| Thickness | 30 to 70 km | 5 to 12 km |
| Density | 2.7 g/cm3 | 3.0 g/cm3 |
| Primary Mineral Composition | Silica and aluminium | Silica and magnesium |
| Age | Older | Younger |
Oceanic crust is recycled, so it will always remain geologically younger than the continental crust.
Silica is another term for quartz – a chemical compound made up of silicon and oxygen.
As shown in the table, continental crust is significantly thicker than its oceanic counterpart. As a result, continental lithosphere is thicker, too. It averages a thickness of 120 miles; oceanic lithosphere is much thinner at just 60 miles across. In metric units, that's 193 kilometres and 96 kilometres, respectively.
Boundaries of the Lithosphere
The outer boundaries of the lithosphere are:
The inner boundary of the lithosphere is the asthenosphere with the outer boundary being the atmosphere, hydrosphere and biosphere.
The asthenosphere is a hot, fluid section of mantle found below the lithosphere.
The Geothermal Gradient of the Lithosphere
What is the geothermal gradient?
The geothermal gradient is how Earth's temperature increases with depth. Earth is coolest at the crust, and warmest inside the inner core.
On average, the temperature of Earth increases by 25°C for every kilometre of depth. The temperature change is more rapid in the lithosphere than anywhere else. The lithosphere's temperature can range from 0°C at the crust to 500°C in the upper mantle.
Thermal Energy in the Mantle
The deeper layers of the lithosphere (the upper layers of the mantle) are subject to high temperatures, making the rocks elastic. The rocks can melt and flow below the Earth's surface, driving the movement of tectonic plates.
The movement of tectonic plates is incredibly slow – only a few centimetres per year.
There's more about tectonic plates later on, so keep reading.
The Pressure of the Lithosphere
The pressure of the lithosphere varies, typically increasing with depth. Why? To put it simply, the more rock above it, the higher the pressure will be.
At approximately 30 miles (50 kilometres) below the Earth's surface, the pressure reaches 13790 bars.
A bar is a metric unit of pressure, equivalent to 100 kilopascals (kPa). In context, it's slightly below the average atmospheric pressure at sea level.
Pressure Buildup in the Lithosphere
Thermal energy in the mantle drives the slow movement of the crust's tectonic plates. The plates often slide against each other at tectonic plate boundaries, and get stuck due to friction. This results in a buildup of pressure over time. Eventually, this pressure is released in the form of seismic waves (i.e. an earthquake).
80% of the world's earthquakes occur around the Pacific Ring of Fire. This horseshoe-shaped belt of seismic and volcano activity is formed by the subduction of the Pacific plate underneath neighbouring continental plates.
Buildup of pressure at tectonic plate boundaries can also cause volcanic eruptions.
Destructive plate margins occur when a continental plate and an oceanic plate are pushed together. The denser oceanic crust is subducted (pulled) beneath the less dense continental crust, leading to an enormous buildup of pressure. The immense pressure pushes magma through the crust to reach Earth's surface, where it becomes lava.
Magma is molten rock found in the mantle.
Alternatively, volcanoes can form at constructive plate margins. The tectonic plates are being pulled apart, so magma flows upwards to plug the gap and form new land.
Fagradalsfjall Volcano, Iceland, was formed at a constructive plate boundary. Unsplash
What is the Elemental Composition of the Lithosphere?
The vast majority of the Earth's lithosphere is made of just eight elements.
Oxygen: 46.60%
Silicon: 27.72%
Aluminium: 8.13%
Iron: 5.00%
Calcium: 3.63%
Sodium: 2.83%
Potassium: 2.59%
Magnesium: 2.09%
Oxygen and silicon alone make up almost three-quarters of Earth's lithosphere.
All other elements make up just 1.41% of the lithosphere.
Mineral Resources
These eight elements are seldom found in their pure form, but as complex minerals.
Minerals are natural solid compounds formed through geological processes.
Minerals are inorganic. This means that they are not living, nor created by living organisms. They have an ordered internal structure. The atoms have a geometric pattern, often forming crystals.
Some common minerals are listed below.
| Mineral | Chemical Name | Elements | Formula |
| Silica / Quartz | Silicon Dioxide | | SiO2 |
| Haematite | Iron Oxide | | Fe2O3 |
| Gypsum | Calcium Sulfate | | CaSO4 |
| Salt | Sodium Chloride | | NaCl |
Many minerals contain desired elements or compounds, so they are extracted from the lithosphere. These mineral resources include metals and their ores, industrial materials, and construction materials. Mineral resources are non-renewable, so they need to be conserved.
I hope that this article has explained the lithosphere for you. It's comprised of the crust and the upper mantle. The thickness of the lithosphere varies, but temperature and pressure increase with depth. The lithosphere is home to mineral resources, which are extracted by humans.
Lithosphere - Key takeaways
- Earth has four layers: the crust, the mantle, the outer core, and the inner core.
- The lithosphere is the solid outer layer of Earth, comprised of the crust and the upper mantle.
- The thickness of the lithosphere varies. Continental lithosphere averages 120 miles, while oceanic lithosphere averages 60 miles.
- The temperature and pressure of the lithosphere increases with depth. High temperatures drive the movement of tectonic plates, whilst pressure builds up at tectonic plate boundaries, resulting in earthquakes and volcanoes.
- Over 98% of the lithosphere consists of just eight elements: oxygen, silicon, aluminium, iron, calcium, sodium, potassium, and magnesium. The elements are usually found in the form of minerals.
1. Anne Marie Helmenstine, Chemical Composition of the Earth's Crust - Elements, ThoughtCo, 2020
2. Caltech, What Happens During an Earthquake?, 2022
3. Geological Survey Ireland, The Earth's Structure, 2022
4. Harish C. Tewari, Structure and Tectonics of the Indian Continental Crust and Its Adjoining Region (Second Edition), 2018
5. Jeannie Evers, Core, National Geographic, 2022
6. R. Wolfson, Energy from Earth and Moon, Energy, Environment and Climate, 2012
7. Taylor Echolls, Density & Temperature of the Lithosphere, Sciencing, 2017
8. USCB Science Line, How do the Earth's continental and oceanic crust compare in density?, University of California, 2018
Explanations 
Exams 
Magazine 
