What Are Earth Layers Made Of?

What Are Earth Layers Made Of? 

A modern view of Earth‘s interior layers.

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).

The Crust 

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.

Introducing the Earth’s Interior

Introducing the Earth’s Interior 

What Is the Earth Made Of? 

The proportions of major elements making up the mass of the whole Earth.

At this point, we leave our fantasy space voyage and turn our attention inward to the materials that make up the solid Earth, because we need to be aware of these before we can discuss the architecture of the Earth’s interior. Let’s begin by reiterating that the Earth consists mostly of elements produced by fusion reactions in stars and by supernova explosions. Only four elements (iron, oxygen, silicon, and magnesium) make up 91.2% of the Earth’s mass; the remaining 8.8% consists of the other 88 elements (figure above). The elements of the Earth comprise a great variety of materials. 

• Organic chemicals. Carbon-containing compounds that either occur in living organisms or have characteristics that resemble compounds in living organisms are called organic chemicals. 
• Minerals. A solid, natural substance in which atoms are arranged in an orderly pattern is a mineral. A single coherent sample of a mineral that grew to its present shape is a crystal, whereas an irregularly shaped sample, or a fragment derived from a once-larger crystal or cluster of crystals, is a grain. 
• Glasses. A solid in which atoms are not arranged in an orderly pattern is called glass. 
• Rocks. Aggregates of mineral crystals or grains, or masses of natural glass, are called rocks. Geologists recognize three main groups of rocks. (1) Igneous rocks develop when hot molten (liquid) rock cools and freezes solid. (2) Sedimentary rocks form from grains that break off pre-existing rock and become cemented together, or from minerals that precipitate out of a water solution. (3) Metamorphic rocks form when pre-existing rocks change in response to heat and pressure. 
• Sediment. An accumulation of loose mineral grains (grains that have not stuck together) is called sediment. 
• Metals. A solid composed of metal atoms (such as iron, aluminium, copper, and tin) is called a metal. An alloy is a mixture containing more than one type of metal atom. 
• Melts. A melt forms when solid materials become hot and transform into liquid. Molten rock is a type of melt geologists distinguish between magma, which is molten rock beneath the Earth’s surface, and lava, molten rock that has flowed out onto the Earth’s surface. 
• Volatiles. Materials that easily transform into gas at the relatively low temperatures found at the Earth’s surface are called volatiles. 

Desert Landscapes and Life

Desert Landscapes and Life 

The popular media commonly portray deserts as endless vistas of sand, punctuated by the occasional palm-studded oasis. In reality, not all desert landscapes are buried by sand. Some deserts  are vast, rocky plains; others sport a stubble of cacti and other hardy desert plants; and still others display intricate rock formations that look like medieval castles. Explorers of the Sahara, for example, traditionally distinguished among hamada (barren, rocky highlands), reg (vast, stony plains), and erg (sand seas in which large dunes form). 

In this post, we’ll see how the erosional and  depositional processes described above lead to the formation of such contrasting landscapes.

Deposition in Deserts

Deposition in Deserts

We've seen that erosion relentlessly eats away at bedrock and sediment in deserts. Where does the debris go? Below, we examine the various desert settings in which sediment accumulates.

Talus Aprons 

Production and transportation of debris and sediment in deserts.

Over time, joint-bounded blocks of rock break off ledges and cliffs on the sides of hills. Under the influence of gravity, the resulting debris tumbles downslope and accumulates as talus, a pile of debris at the base of a hill. Talus can survive for a long time in desert climates, so we typically see aprons of talus  fringing the bases of cliffs in deserts (figure above a).

Weathering and Erosional Processes in Deserts

Weathering and Erosional Processes in Deserts 

Without the protection of foliage to catch rainfall and slow the wind, and without roots to hold regolith in place, rain and wind can attack and erode the land surface of deserts and soil tends to be sparse. The result, as we have noted, is that hill slopes are typically bare, and plains can be covered with stony debris or drifting sand. 

Arid Weathering and Desert Soil Formation 

In the desert, as in temperate climates, physical weathering happens primarily when joints (natural fractures) split rock into pieces. Joint-bounded blocks eventually break free of bedrock and tumble down slopes, fragmenting into smaller pieces as they fall. In temperate climates, thick soil develops and covers bedrock. In deserts, however, bedrock commonly remains exposed, forming rugged, rocky escarpments.

Chemical weathering happens more slowly in deserts than in temperate or tropical climates, because less water is available to react with rock. Still, rain or dew provides enough moisture for some weathering to occur. This water seeps into rock and leaches (dissolves and carries away) calcite, quartz, and various salts. Leaching effectively rots the rock by transforming it into a poorly cemented aggregate. Over time, the rock will crumble and form a pile of unconsolidated sediment, susceptible to transport by water or wind. 

Although enough rain falls in deserts to leach chemicals out of sediment and rock, there is not enough rain to carry the chemicals away entirely. So they precipitate to form calcite and other minerals in regolith beneath the surface. The new minerals may bind clasts together to form a rock-like material called calcrete.

Caves and Karst

Caves and Karst 

The Development of Caves 

In 1799, as legend has it, a hunter by the name of Houchins was tracking a bear through the woods of Kentucky when the bear suddenly disappeared on a hillslope. Baffled, Houchins plunged through the brambles trying to sight his prey. Suddenly he felt a draft of surprisingly cool air flowing down the slope from uphill. Now curious, Houchins climbed up the hill and found a dark portal into the hillslope beneath a ledge of rocks. Bear tracks were all around was the creature inside? He returned later with a lantern and cautiously stepped into the passageway. After walking a short distance, he found himself in a large, underground room. Houchins had discovered Mammoth Cave, an immense network of natural tunnels and subterranean chambers a walk through the entire network would extend for 630 km.

Most large cave networks develop in limestone bedrock because limestone dissolves relatively easily in corrosive groundwater. Generally, the corrosive component in groundwater is dilute carbonic acid (H2CO3), which forms when water absorbs carbon dioxide (CO2) from materials, such as soil, that it has passed through. When carbonic acid comes in contact with calcite (CaCO3) in limestone, it reacts to produce HCO31- and Ca2 ions, which then dissolve. 

In recent years, geologists have discovered that about 5% of limestone caves around the world form due to reactions with sulfuric-acid-bearing water Carlsbad Caverns in New Mexico serves as an example. Such caves form where limestone overlies strata containing oil, because microbes can convert the sulfur in the oil to hydrogen sulfide gas, which rises and reacts with oxygen to produce sulfuric acid, which in turn eats into limestone and reacts to produce gypsum and CO2 gas. 

Geologists debate about the depth at which limestone cave networks form. Some limestone dissolves above the water table, but it appears that most cave formation takes place in limestone that lies just below the water table, for in this interval the acidity of the groundwater remains high, the mixture of groundwater and newly added rainwater is not yet saturated with dissolved ions, and groundwater flow is fastest. The association between cave formation and the water table helps explain why openings in a cave network align along the same horizontal plane.

The Character of Cave Networks 

Development of karst, dripstone and flowstone.

As we have noted, caves in limestone usually occur as part of a network. Cave networks include rooms, or chambers, which are large, open spaces sometimes with cathedral-like ceilings, and tunnel-shaped or slot-shaped passages. Some chambers may host underground lakes, and some passageways may serve as conduits for underground streams. The shape of the cave network reflects variations in permeability and in the composition of the rock from which the caves formed. Larger open spaces developed where the limestone was most soluble and where groundwater flow was fastest. Thus, in a sequence of strata, caves develop preferentially in the more soluble limestone beds. Passages in cave networks typically follow pre-existing joints, for the joints provide secondary porosity along which groundwater can flow faster (figure above a). Because joints commonly occur in orthogonal systems (consisting of two sets of joints oriented at right angles to each other), passages may form a grid. 

Precipitation and the Formation  of Speleothems 

When the water table drops below the level of a cave, the cave becomes an open space filled with air. In places where downward percolating groundwater containing dissolved calcite emerges from the rock above the cave and drips from the ceiling, the surface of the cave gradually changes. As soon as this water re-enters the air, it evaporates a little and releases some of its dissolved carbon dioxide. As a result, calcium carbonate (limestone) precipitates out of the water and produces a type of travertine. The various intricately shaped formations that grow in caves by the accumulation of dripstone are called speleothems. 

Cave explorers (spelunkers) and geologists have developed a detailed nomenclature for different kinds of speleothems (figure above b). Where water drips from the ceiling of the cave, the precipitated limestone builds dripstone. Initially, calcite precipitates around the outside of the drip, forming a delicate, hollow tube called a soda straw. But eventually, the soda straw fills up, and water migrates down the margin of the cone to form a more massive, solid icicle-like cone called a stalactite. Where the drips hit the floor, the resulting precipitate builds an upward-pointing cone called a stalagmite. 

If the process of dripstone formation in a cave continues long enough, stalagmites merge with overlying stalactites to create travertine columns. In some cases, groundwater flows along the surface of a wall and precipitates to produce drape-like sheets of travertine called flowstone (figure above c). The travertine of caves tends to be translucent and, when lit from behind, glows with an eerie amber light.

The Formation of Karst Landscapes 

Features of Karst landscapes.

Limestone bedrock underlies most of the Kras Plateau in  Slovenia, along the east coast of the Adriatic Sea. The name kras, which means rocky ground, is apt because this region includes abundant rock exposures (figure above a). Geologists refer to regions such as the Kras Plateau, where surface landforms develop when limestone bedrock dissolves both at the surface and in underlying cave networks, as karst landscapes or karst terrains from the Germanized version of kras. 

Karst landscapes typically display a number of distinct landforms. Perhaps the most widespread are sinkholes, circular depressions that form either when the ground collapses into an underground cave below (as we discussed early in this chapter) or when surface bedrock dissolves in acidic water on the floor of a bog or pond. Not all of the caves or passageways beneath a karst landscape have collapsed, and this situation leads to unusual drainage patterns. Specifically, where surface streams intersect cracks (joints) or holes that link to caverns or passageways below, the water cascades downward into the subsurface and disappears (figure above b). Such disappearing streams may flow through passageways underground and re-emerge from a cave entrance downstream. In cases where the ground collapses over a long, joint-controlled passage, sinkholes may be elongate and canyon-like. Remnants of cave roofs remain as natural bridges. Ridges or walls between adjacent sinkholes tend to be steep-sided. Over time, the walls erode, leaving only jagged, isolated spires a karst landscape dominated by such spires is called tower karst. The surreal collection of pinnacles constituting the tower karst landscape in the Guilin region of China inspired generations of artists who portray them on scroll paintings (figure below).

Tower karst forms a spectacular landscape in southern China.

Karst landscapes form in a series of stages (figure below a–c).

The progressive formation of caves and a karst landscape.

• The establishment of a water table in limestone: The story of a karst landscape begins after the formation of a thick interval of limestone in which the water table lies underground. 
• The formation of a cave network: Once the water table has been established, dissolution begins and a cave network develops. 
• A drop in the water table: If the water table later becomes lower, either because of a decrease in rainfall or because nearby rivers downcut and drain the region, newly formed caves dry out. Downward-percolating water emerges from the roofs of the caves; dripstone and flowstone precipitate. 
• Roof collapse: If rocks fall off the roof of a cave for a long time, the roof eventually collapses. Such collapse creates sinkholes and troughs, leaving behind hills, ridges, and natural bridges.

Life in Caves 

Despite their lack of light, caves are not sterile, lifeless environments. Caves that are open to the air provide a refuge for bats as well as for various insects and spiders. Similarly, fish and crustaceans enter caves where streams flow in or out. Species living in caves have evolved some unusual characteristics. For example, cave fish lose their pigment and in some cases their eyes. Recently, explorers discovered caves in Mexico in which warm, mineral-rich groundwater currently flows. Colonies  of bacteria metabolize sulphur-containing minerals in this water and create thick mats of living ooze in the complete darkness of the cave. Long gobs of this bacteria slowly drip from the ceiling. Because of the mucus-like texture of these drips, they have come to be known as “snotites”.
Figures credited to Stephen Marshak.