As a result of studies during the past century, geologists have a pretty clear sense of what the layers inside the Earth are made of. Let’s now look at the properties of individual layers in more detail (figure above a, b).
When you stand on the surface of the Earth, you are standing on top of its outermost layer, the crust. The crust is our home and the source of all our resources. How thick is this all important layer? Or, in other words, what is the depth to the crust-mantle boundary? An answer came from the studies of Andrija Mohorovicˇic´, a researcher working in Zagreb, Croatia. In 1909, he discovered that the velocity of earthquake waves suddenly increased at a depth of tens of kilometres beneath the Earth’s surface, and he suggested that this increase was caused by an abrupt change in the properties of rock. Later studies showed that this change can be found most everywhere around our planet, though it occurs at different depths in different locations. Speciﬁcally, it’s deeper beneath continents than beneath oceans. Geologists now consider the change to deﬁne the base of the crust, and they refer to it as the Moho in Mohorovicˇic´’s honour. The relatively shallow depth of the Moho (7 to 70 km, depending on location) as compared to the radius of the Earth (6,371 km) emphasizes that the crust is very thin indeed. In fact, the crust is only about 0.1% to 1.0% of the Earth’s radius, so if the Earth were the size of a balloon, the crust would be about the thickness of the balloon’s skin.
The crust is not simply cooled mantle, like the skin on chocolate pudding, but rather consists of a variety of rocks that differ in composition (chemical make-up) from mantle rock. Geologists distinguish between two fundamentally different types of crust oceanic crust, which underlies the sea ﬂoor, and continental crust, which underlies continents.
Oceanic crust is only 7 to 10 km thick. At highway speeds (100 km per hour), you could drive a distance equal to the thickness of the oceanic crust in about ﬁve minutes. At the top, we ﬁnd a blanket of sediment, generally less than 1 km thick, composed of clay and tiny shells that settled like snow out of the sea. Beneath this blanket, the oceanic crust consists of a layer of basalt and, below that, a layer of gabbro.
|A table and a graph illustrating the abundance of elements in the Earth’s crust.|
Most continental crust is about 35 to 40 km thick about four to ﬁve times the thickness of oceanic crust but its thickness varies signiﬁcantly. In some places, continental crust has been stretched and thinned so it’s only 25 km from the surface to the Moho, and in some places, the crust has been crumpled and thickened to become up to 70 km thick. In contrast to oceanic crust, continental crust contains a great variety of rock types, ranging from maﬁc to felsic in composition. On average, upper continental crust is less maﬁc than oceanic crust it has a felsic (granite-like) to intermediate composition so continental crust overall is less dense than oceanic crust. Notably, oxygen is the most abundant element in the crust (figure above).
The mantle of the Earth forms a 2,885-km-thick layer surrounding the core. In terms of volume, it is the largest part of the Earth. In contrast to the crust, the mantle consists entirely of an ultramaﬁc (dark and dense) rock called peridotite. This means that peridotite, though rare at the Earth’s surface, is actually the most abundant rock in our planet! Researchers have found that earthquake-wave velocity changes at a depth of 400 km and again at a depth of 660 km in the mantle. Based on this observation, they divide the mantle into two sublayers: the upper mantle, down to a depth of 660 km, and the lower mantle, from 660 km down to 2,900 km. The transition zone is the interval between 400 km and 660 km deep.
Almost all of the mantle is solid rock. But even though it’s solid, mantle rock below a depth of 100 to 150 km is so hot that it’s soft enough to ﬂow. This ﬂow, however, takes place extremely slowly at a rate of less than 15 cm a year. Soft here does not mean liquid; it simply means that over long periods of time mantle rock can change shape without breaking. We stated earlier that almost all of the mantle is solid. We used the word “almost” because up to a few percent of the mantle has melted. This melt occurs in ﬁlms or bubbles between grains in the mantle at a depth of 100 to 200 km beneath the ocean ﬂoor. Although overall, the temperature of the mantle increases with depth, temperature also varies signiﬁcantly with location even at the same depth. The warmer regions are less dense, while the cooler regions are denser. The distribution of warmer and cooler mantle indicates that the mantle convects like water in a simmering pot; warmer mantle is relatively buoyant and gradually ﬂows upward, while cooler, denser mantle sinks.
Early calculations suggested that the core had the same density as gold, so for many years people held the fanciful hope that vast riches lay at the heart of our planet. Alas, geologists eventually concluded that the core consists of a far less glamorous material, iron alloy (iron mixed with tiny amounts of other elements). Studies of seismic waves led geo scientists to divide the core into two parts, the outer core (between 2,900 and 5,155 km deep) and the inner core (from a depth of 5,155 km down to the Earth’s centre at 6,371 km). The outer core consists of liquid iron alloy. It can exist as a liquid because the temperature in the outer core is so high that even the great pressures squeezing the region cannot keep atoms locked into a solid framework. The iron alloy of the outer core can ﬂow, and this ﬂow generates Earth’s magnetic ﬁeld.
The inner core, with a radius of about 1,220 km, is a solid iron alloy that may reach a temperature of over 4,700°C. Even though it is hotter than the outer core, the inner core is a solid because it is deeper and is subjected to even greater pressure. The pressure keeps atoms locked together tightly in very dense materials.
The Lithosphere and the Asthenosphere
So far, we have identiﬁed three major layers (crust, mantle, and core) inside the Earth that differ compositionally from each other. Earthquake waves travel at different velocities through these layers. An alternative way of thinking about Earth layers comes from studying the degree to which the material making up a layer can ﬂow. In this context, we distinguish between rigid materials, which can bend or break but cannot ﬂow, and plastic materials, which are relatively soft and can ﬂow without breaking.
|A block diagram of the lithosphere, emphasizing the difference between continental and oceanic lithosphere.|
Geologists have determined that the outer 100 to 150 km of the Earth is relatively rigid. In other words, the Earth has an outer shell composed of rock that cannot ﬂow easily. This outer layer is called the lithosphere, and it consists of the crust plus the uppermost, cooler part of the mantle. We refer to the portion of the mantle within the lithosphere as the lithospheric mantle. Note that the terms lithosphere and crust are not synonymous the crust is just the upper part of the lithosphere. The lithosphere lies on top of the asthenosphere, which is the portion of the mantle in which rock can ﬂow. The boundary between the lithosphere and asthenosphere occurs where the temperature reaches about 1280°C, for at temperatures higher than this value mantle rock becomes soft enough to ﬂow.
Geologists distinguish between two types of lithosphere (figure above). Oceanic lithosphere, topped by oceanic crust, generally has a thickness of about 100 km. In contrast, continental lithosphere, topped by continental crust, generally has a thickness of about 150 km. Notice that the asthenosphere is entirely in the mantle and generally lies below a depth of 100 to 150 km. We can’t assign a speciﬁc depth to the base of the asthenosphere because all of the mantle below 150 km can ﬂow, but for convenience, some geologists consider the base of the asthenosphere to be the top of the transition zone.
Credits: Stephen Marshak (Essentials of geology)