Transition zone (Earth)

Deep under our feet is a rocky layer. It is in the middle of the Earth. This part is called the transition zone.

The rock here is very heavy. It changes as it gets deeper. This happens because of high pressure.

At one depth, tiny grains move. They pack together to stay tight. This makes the rock even denser.

At a deeper level, the minerals change too. They turn into new, heavy forms. These changes happen because of heat and pressure.

Scientists study this using earthquake waves. The waves show where the rock changes. It is a busy, hidden place.

Deep inside Earth is a rocky layer. This is the transition zone. It sits between the upper and lower mantle. The mantle is made of a rock called peridotite.

As you go deeper, the weight of the Earth grows. This creates high pressure. This pressure changes the rocks. First, grains of a mineral called olivine move. They form a denser crystal structure. This happens at a depth of 410 km.

At 660 km, things change again. A mineral called ringwoodite turns into two new parts. These are bridgmanite and periclase. These new parts are even denser. This change also makes the rock thicker and harder to move. This is called a viscosity jump.

Scientists study these layers using earthquake waves. These waves travel through the Earth. They reflect or bend when they hit a new layer. This helps us find the exact depth of each change. The depth can change if the area is hot or cold.

Deep inside our Earth lies a special layer. This is called the mantle transition zone. It sits between the upper mantle and the lower mantle. Most people think of the mantle as one big layer. However, it is actually divided into three parts. The transition zone is the middle part of this rocky layer. It is made mostly of a rock called peridotite.

This layer works through changes in pressure. As you go deeper, the weight above grows. This pressure changes how minerals look and act. At 410 km deep, olivine grains rearrange themselves. They form a denser crystal structure because of the weight. Later, at 660 km, ringwoodite changes into new things. These new parts are called bridgmanite and periclase.

Scientists study these deep layers using earthquake waves. They use waves called body waves to see inside. These waves reflect or bend when they hit a boundary. This happens because the rocks change density at certain depths. Scientists also use something called receiver functions. These help show single or double reflections of the waves. This is how we know the layers exist.

There are specific numbers for these deep boundaries. The first change happens at the 410 km discontinuity. The second major change is at 660 km. The depth of these lines can actually change. In cold regions, these boundaries might be deeper. In hot regions, they might be shallower. This is due to the Clapeyron slope.

These changes help us understand how the Earth moves. The change at 660 km creates a viscosity jump. This means the rock becomes harder to move. This jump is important for geodynamical models. Cold material might even pond on this transition. It is like a speed bump for moving rock. This helps us see how the whole Earth works.

The Earth's mantle is a vast, solid, and rocky layer. It is not one single, uniform mass. Instead, it is divided into three distinct parts. These parts are the upper mantle, the transition zone, and the lower mantle. The mantle transition zone, or MTZ, is the middle section. It sits between the upper and lower mantle layers. This zone is highly significant to scientists. It helps us understand how the interior of our planet behaves.

The MTZ consists primarily of a rock called peridotite. Peridotite is an ultramafic igneous rock. This means it is very rich in certain minerals. A large portion of this rock is made of a mineral called olivine. Within the transition zone, the physical properties of these minerals change. These changes happen because of the immense pressure found deep underground. As you move deeper, the weight of the material above increases. This rising pressure forces the minerals to rearrange their internal structures.

There are two major boundaries within this zone. These boundaries are known as seismic-discontinuity depths. The first major change occurs at the 410 km discontinuity. At this depth, olivine grains rearrange themselves. They transform from a phase called alpha-magnesium silicate into beta-magnesium silicate, or wadsleyite. This change creates a denser crystal structure. This process is a phase transition. This means the substance changes its form due to pressure.

The second major boundary is at the 660 km discontinuity. This is a very important transition for the whole planet. Here, a mineral called ringwoodite changes into two new, denser phases. These new phases are called bridgmanite and periclase. This specific change is an endothermic reaction. This means it absorbs heat. This transition also creates a viscosity jump. Viscosity refers to how much a substance resists flowing. This jump makes the rock harder to move through.

Scientists use special tools to see these deep layers. They cannot travel to the mantle, so they use seismic waves. These are body waves that travel through the Earth during earthquakes. When these waves hit a boundary, they react in specific ways. They might be reflected, which means they bounce back. They might also be refracted, which means they bend. Scientists also use receiver functions to study these waves. These functions show single or double reflections of P-to-S conversions.

The exact depth of these boundaries is not always the same. The depth can shift based on the temperature of the area. This is explained by the Clapeyron slope. In cold regions, the boundaries can exist at shallower depths. This happens where cold, subducting slabs of rock sink into the mantle. In warmer regions, the boundaries may be deeper. This occurs where hot mantle plumes pass through the zone. Because of the viscosity jump at 660 km, cold material might even pond on this layer.

Researchers also look for other, smaller transitions. There is a predicted transition at 800 km. This involves olivine changing from the beta to the gamma phase. It also involves garnet in the pyrolite mantle. However, this transition is only seen sporadically in data. Other non-global phase transitions have been suggested at various depths as well. All of these small details help build geodynamical models. These models help us understand the complex movements of the entire Earth.