Planet Earth/6e. The Rock Cycle and Rock Types: Igneous, Metamorphic and Sedimentary

What are Rocks?
Rocks are any solid inorganic substance that occurs naturally. This includes nearly all the solids in Earth’s interior and surface, and is the material that is most frequently used in the construction of houses, buildings, roads, and items used in everyday life.

What are the major types of rocks?
Rocks can be divided into three major types based on their formation. Rocks can be solids that have been cooled from molten liquid magma or lava (igneous rocks). Rocks can be changed or altered by deep pressures and intense heat in the subsurface, which does not completely melt the rock (metamorphic rocks). Or rocks can be composed of smaller fragments of other rocks or organic matter that have been glued or adhered together by a process called lithification (sedimentary rocks).

The Rock Cycle
These three types of rocks form a cycle. Magma and lava cool to form igneous rocks, which are eroded at the surface to break apart and be transported, where they are buried and become sedimentary rocks, which can be further buried with increasing pressure and temperature, until they turn into metamorphic rocks, and if they continue to be heated, and melt into magma and are returned to igneous rocks when cooled. This rock cycle is a continuous process on Earth’s surface caused by the continued creation of rocks by cooling magma, erosion caused by water and wind, and the burial of rocks in subduction zones, that result in metamorphic rocks. This long history of recycling Earth’s solid rocks has resulted in a surface shaped over long spans of time into a unique configuration, when compared to the more static rocks found on other planets.

One of the key aspects of the Earth’s rock cycle is that the type of rocks found in different regions of the Earth are directly related to this long-term rock cycle of melt, cool, erode, transport, burial, change, and melt. The ocean floor of the Earth’s surface is dominated by igneous rock that have recently been formed by mid-ocean ridges. These young igneous rocks will move over time into subduction zones and be melted back down into the mantle. Above the subduction zones, magma will also be brought to the surface in volcanoes and mountains which will erode and these transported fragments will be carried into basins both on land, and on the sea floor. These sediments of rock fragments will be buried and form sedimentary rocks. These sedimentary rocks can be eroded themselves when exposed to the Earth’s surface, or undergo metamorphism with deep burial, and change. These metamorphic rocks might be brought up to the surface and eroded forming sedimentary rocks. The interchange of the three types of rocks is common on Earth. If we compare the Earth’s rock cycle to that of the dormant Moon, we see a striking difference. The Moon lacks active volcanoes, tectonic plates, and is only subjected to impact structures as a process for changing rocks on its surface. This means that the moon’s rocks have existed in their current state for billions of years, dating back to when the moon cooled down to the point that magma and lava was no longer melting rocks. Since the moon lacks oceans or an atmosphere, the rocks are not eroded or transported beyond where they have cooled and formed. The only way rocks on the moon are transported is during rare impacts of asteroids or meteors on its surface, otherwise these rocks stay in place. This permeance of the surface of the moon is one reason that the Apollo 11 astronauts’ footprints will last billions of years, while footprints on Earth will last only days. It is also why impact craters are visible, as unlike on Earth, impact structures are fairly permanent features on the Moon’s surface.

Mars is also less dynamic when it comes to its rock cycle. Although it has an atmosphere, which results in wind erosion. Sedimentary rocks are formed on Mars by transported grains by the action of these winds, and the recent discovery of sedimentary rocks by the Martian rovers from water transported grains. These rocks are evidence of a very early history of liquid water on Mars, with transported rock fragments by flowing water several billion years ago. However, the vast majority of rocks on Mars are igneous rocks and cooled from their early formation. Mars has several large volcanoes, but lacks large scale tectonic plates, indicating a cooler mantle that lacks the convection of heat as on Earth.

One of the great mysteries in geology is what makes Earth rock cycle so dynamic? This is likely due to the combined motion of tectonic plates, active volcanism, presence of liquid water on its surface and a thick atmosphere. Exploration of the surfaces of larger gas planets (Saturn, Jupiter and Neptune) or the extremely hot surface of Venus, may reveal rock cycles as dynamic as those found on Earth, but likely dynamic in very different ways.

One of the challenges that will face you when you pick up a rock is how to identify the rock as either igneous, metamorphic or sedimentary. To do this, you need to think about the processes that led to the formation of that rock. But here are some tips for identifying a rock’s type.

Igneous rocks
The best way to identify igneous rocks is that they have crystals that intergrow with each other. This is because the rock has cooled down from molten hot magma or lava, which allows crystals to grow as it cooled. The problem you may face is when the crystals are very tiny, these tiny crystals are a result when the rock cooled very quickly from magma or lava, before the crystals could grow to a large size. Often you need to look very closely at the rock with a hand lens and see if there are tiny crystals present. Crystals in igneous rocks tend to be sparkly, bright, sometimes transparent, often with a variety of colors. But often igneous rocks are mostly black or gray, with tiny crystals that are not very shiny or colorful, unless you look at them closely.

Sedimentary rocks
The best way to identify sedimentary rocks is to see if they are composed of grains or pieces of other rocks. Pieces of other rocks are called clasts. A clast, is a fragment of another rock. Grains or clasts are typically dull colored, because they have been eroded and transported by wind or water. They can be well rounded or angular in shape. These grains or clasts are glued together by cement composed of minerals that easily dissolve in water, such as calcite and silica. This glue can be shiny, and form large crystals if there are spaces or cavities in the rocks. Geodes are frequently found in sedimentary rocks resulting from a cavity or space within the rocks that was infilled with cement or glue that grew into crystals, other geodes can form inside igneous rocks resulting from cavities in magma or lava, in which large crystals grow into. Some sedimentary rocks can be sparkly, but the majority are dull colored. Not all sedimentary rocks are composed of grains or clasts (often called clastic sedimentary rocks), some sedimentary rocks can be formed from organic matter. Limestone is a unique sedimentary rock composed of fossilized shells of aquatic organisms. These organisms when they die result in the burial of calcium carbonate (CaCO₃), the material that they form shells with. This accumulation of calcium carbonate is deposited and buried on the ocean floor and lake bottoms. These sedimentary rocks are called carbonate sedimentary rocks. Some organic matter, such as wood, when buried will turn into coal, another sedimentary rock. Any organic matter buried and turned to stone is a sedimentary rock. Sedimentary rocks are the only rock type that preserves fossils, and the only type of rock in which fossil fuels (coal, petroleum, and natural gas) are found.

The last type of sedimentary rock are rocks formed from the evaporation of salt water, that leaves behind salts. These are evaporitic sedimentary rocks. These salt deposits are crumbly and typically white to gray in color. They can form unusual rock layers, due to their propensity to dissolve or flow in the presence of ground waters.

Metamorphic rocks
Metamorphic rocks have undergone intense pressure and heat to result in changes in the structure of the rock and crystals, without completely melting the rock. Metamorphic rocks typically are very shiny and will sparkle in the light, because the grains or crystals have partially melted or recrystallized with this intense pressure and heat. The one thing that characterizes metamorphic rocks is something called foliation. Foliation is sheet-like wavy planar structures in the rock caused by shearing forces or differential pressures. These resemble wavy lines, often where one mineral has formed by partial melting or recrystallization. Unlike sedimentary rocks, which can have bedding or individual layers, foliation is typically wavy. The sparkly and wavy patterns in metamorphic rocks typically make them easy to identify, although there is a gradient between sedimentary and igneous rocks and metamorphic rocks. Low grade metamorphic rocks are rocks that have not been subjected to heat and pressure may closely resemble sedimentary or igneous rocks, while rocks subjected to intense heat and pressure will be radically different. This swishy definition can be somewhat subjective. A classic type of metamorphic rock is marble, which is limestone that has undergone intense pressure through burial and heat, recrystallizing the rock, making it sparkle and adding wavy lines of color (foliation).

All rocks can be classified into one of these three types; igneous, sedimentary or metamorphic. Geologists will often specialize in the study of one of the rock types. Such as the colloquial use of the terms hard rock geologists, to refer to geologists that study only igneous and metamorphic rocks, and soft rock geologists, to refer to those that study only sedimentary rocks; ironically, despite all three rock types being equally hard. Each rock type can be divided into unique rock names, which are classified based on the mineral content of each rock. Before you can learn how to identify rock names, you will need to learn about minerals, and how you can identify those in hand samples.