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Unlock This CourseDeep beneath our feet lies a world of extreme heat and pressure that drives many of the geological processes we see on Earth's surface. The mantle makes up about 84% of Earth's volume and is the source of the molten rock called magma that creates volcanoes, new ocean floor and mountain ranges. Understanding the mantle and magma is crucial for marine science because these processes shape the ocean basins and create the underwater features that affect marine life.
Key Definitions:
Think of Earth like a hard-boiled egg. The thin shell represents the crust (where we live), the white is like the mantle (hot, thick rock) and the yolk is the core (mostly iron and nickel). The mantle is by far the largest layer, making up most of our planet's bulk and containing most of its heat.
The mantle isn't just one uniform layer - it has distinct zones with different properties and behaviours. As you go deeper, the temperature and pressure increase dramatically, changing how the rock behaves.
The mantle is divided into several important zones, each with unique characteristics that affect how heat and material move through our planet.
Extends from 30-670km depth. Contains the asthenosphere - a partially molten layer that allows tectonic plates to move. Temperature ranges from 500-900°C.
Found at 410-670km depth. Rock structure changes due to extreme pressure. Acts like a barrier that can trap water and affect convection patterns.
Extends from 670-2,900km depth. Much denser and hotter (1,600-4,000°C). Rock behaves more like thick treacle due to extreme conditions.
The mantle contains about 10^21 tonnes of water - that's several times more water than in all the oceans combined! This water is locked in the crystal structure of minerals and plays a crucial role in how the mantle behaves and how magma forms.
The conditions in the mantle are truly extreme. Temperature increases by about 25°C for every kilometre you go down, whilst pressure increases dramatically due to the weight of all the rock above.
Under normal surface conditions, rock is solid and brittle. But in the mantle, the combination of heat and pressure creates some fascinating behaviours that are crucial for understanding how our planet works.
High temperatures make rock atoms vibrate more, weakening the bonds between them. This allows solid rock to flow very slowly over long periods - like thick honey. Even though it's solid, mantle rock can move and circulate in convection currents.
Enormous pressure keeps most mantle rock solid despite the high temperatures. However, when pressure drops (like when rock moves upward), it can start to melt even without getting hotter - this is called decompression melting.
Magma forms when mantle rock melts, but this isn't as simple as ice melting into water. Rock is made of many different minerals that melt at different temperatures, creating a complex mixture with unique properties.
There are three main ways that solid mantle rock can melt to form magma and each process is important in different geological settings.
When mantle rock gets hot enough (usually above 1,200°C), it starts to melt. This happens near the core-mantle boundary and in mantle plumes - columns of extra-hot rock rising from deep in the Earth.
When mantle rock rises toward the surface, pressure decreases faster than temperature. This pressure drop allows the rock to melt without getting hotter - like opening a fizzy drink bottle.
Adding water or other chemicals lowers the melting point of rock, just like salt melts ice. This happens where ocean water gets dragged down into the mantle at subduction zones.
At mid-ocean ridges like the Mid-Atlantic Ridge, mantle rock rises up to fill the gap as tectonic plates move apart. As it rises, pressure drops and the rock melts through decompression melting. This creates the basaltic magma that forms new ocean floor - about 3.4 cubic kilometres of new oceanic crust every year!
Not all magma is the same. Different types have different compositions, temperatures and behaviours that affect how they move through the Earth and what happens when they reach the surface.
Geologists classify magma based on its silica content, which determines many of its other properties. Understanding these differences helps explain why some volcanoes are explosive whilst others produce gentle lava flows.
Low silica (45-52%), very hot (1,000-1,200°C), runny consistency. Forms most ocean floor and creates gentle volcanic eruptions. Common at mid-ocean ridges and hotspots.
Medium silica (52-63%), moderate temperature (800-1,000°C), medium thickness. Named after the Andes mountains. Creates moderately explosive eruptions at subduction zones.
High silica (63-77%), cooler (650-800°C), very thick and sticky. Creates highly explosive eruptions because gases can't escape easily. Forms continental volcanoes.
The mantle is constantly moving in slow circulation patterns called convection currents. These movements transport heat from Earth's core to the surface and drive plate tectonics - the process that shapes our planet's surface and ocean basins.
Just like water boiling in a pot, hot mantle rock rises whilst cooler rock sinks. This creates circulation patterns that have been operating for billions of years, constantly recycling material between Earth's surface and deep interior.
Hot mantle rock is less dense than cooler rock, so it rises slowly toward the surface. This creates upwelling zones at mid-ocean ridges and hotspots, where new magma is generated and volcanic activity occurs.
Cooler, denser rock sinks back down into the mantle. This happens at subduction zones where oceanic plates dive beneath other plates, carrying water and sediments deep into the Earth.
The Hawaiian island chain was created by a mantle plume - a column of extra-hot rock rising from deep in the mantle. As the Pacific Plate moves northwest over this stationary hotspot, it creates a chain of volcanoes. The Big Island sits over the hotspot now, whilst older islands like Kauai have moved away and become extinct.
Mantle processes and magma formation have profound effects on marine environments. They create the ocean basins, shape underwater landscapes and influence ocean chemistry and marine life.
The movement of mantle material directly controls the formation and destruction of oceanic crust, creating the underwater features that define marine habitats.
At mid-ocean ridges, rising mantle creates new oceanic crust through seafloor spreading. This process adds about 3.4 km³ of new basaltic rock to the ocean floor each year, gradually widening ocean basins.
Magma creates underwater mountains, volcanic islands and hydrothermal vents. These features provide unique habitats for marine life, including chemosynthetic bacteria that form the base of deep-sea food webs.