Lithosphere

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 composed of the crust and the upper portion of the mantle.

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.

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 atmosphere
• The hydrosphere
• The biosphere

The inner boundary of the lithosphere is the asthenosphere with the outer boundary being the atmosphere, hydrosphere and biosphere.

The Geothermal Gradient of the Lithosphere

What is the geothermal gradient?

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.

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.

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).

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.

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 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.

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.

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