So Earth is geologically active. Okay, how does that relate to rocks? All rock material originally comes from within the Earth. The heat energy left over from formation creates volcanoes and other formations. This renews Earth materials allowing atoms to be rearranged, forming and reforming the minerals that make up rocks. Rocks that form directly from liquid rock called lava or magma, are called igneous rocks.

Igneous rocks can form either on or very near the surface, rapidly cooling and solidifying (extrusive igneous rock), or cooling very slowly within the Earth’s crust (intrusive igneous rock). All igneous rocks form from liquid rock.

Minerals are crystals that make up rocks and in the case of igneous rocks, the rate of cooling will determine what crystals will form and how large they will be. In an extrusive rock, the mineral crystals will be very small. Most of the time you need a microscope to really be able to see them very well. This is because the lava will be erupted onto the surface and cool very rapidly. This doesn’t allow for large crystal formation. An example of an extrusive rock is the rock below called basalt.

In an intrusive rock, mineral crystals are easily seen. Take the example of an intrusive rock called granite in the picture below. There are large chunks or patches of colors. These are mineral crystals easily seen by the naked eye. This occurs because magma that is cooling deep in the crust will cool for very long periods of time. A granite rock like this one may have taken more than a million years to Cool.

So how do these crystals form in the rock like this? Different materials are liquid at different temperatures. Some materials must be very hot to be liquid, while others need less heat. While the magma cools, some atoms will begin to be attracted to other specific atoms and form molecules in a definite shape. They will begin to form solid crystals in the liquid magma. The first minerals to form in a piece of rock like the granite pictured on the right will be two black minerals called augite and biotite (seen in the picture as dark specks). Next the pinkish chunks, the mineral orthoclase, will form. Last to form is the whitish/clear mineral named quartz. This process is called crystallization.

Sedimentary Rocks

Most of the rocks on the planet are igneous rocks. They make up the majority of crustal rock (the outermost layer of the Earth). After the rock becomes exposed at the surface, the process of weathering will begin. Weathering is the altering or breaking down of materials. Weathering can be mechanical or chemical.

In mechanical weathering (also known as physical weathering), rock bits are broken apart without a change in what they are made of. This can happen as plants burrow their roots into the rock, through freeze and thaw cycles (see image below), by salt crystal growth in crevices, by simple impacts of another rock, the repeated motion of animals, or through heat.

In chemical weathering, the rock is chemically altered. Molecules that make up the rock can be dissolved and altered. How does this happen? As rainwater falls from the sky it is naturally slightly acidic, a pH of about 5.6. Now, this is not acid rain. Acid rain must have a pH of 4.0 or lower. Even though it is not acid rain, it is acidic enough to chemically alter the rock (According to the chart on the right, any pH rating lower than 7 is considered acidic).

Life can also chemically break down rocks. There aren’t very many, but some species of fungi, plants, and protists can excrete acids onto rocks in order to extract minerals and nutrients needed by the organism. An example of this is a lichen pictured below.

Mechanical and chemical weathering over time wear down rocks and produce soil by breaking down rock material to produce sediments. Sediments are small particles categorized by size. The largest sediment size is gravel, followed by sand, silt, and the smallest particles are clay.

Fun Fact!

It can take up to 400 years to form soil from newly formed rock. However, it can take up to 3,000 years before that soil is Fertile.

These particles can be transported either by water or wind in a process called erosion. Eventually these materials will settle, called deposition, in a location. During sedimentation, sediments are deposited on beaches and deserts, at the bottom of oceans, and in lakes, ponds, rivers, marshes, and swamps. Landslides drop large piles of sediment at the base of mountains most often in a horizontal position. Glaciers leave large piles of sediments, too. Wind can erode sand and smaller particles.

If these sediments become compacted and cemented together they will form sedimentary rocks. The type of sediment that is deposited will determine the type of sedimentary rock that is formed.

The Delicate Arch inside Arches National Park in Southeastern Utah (pictured below) brings people from all over the world to explore the amazing sedimentary rock formation. The sedimentary rock that makes up the arch is more than a hundred million years old, but the arch itself isn’t as old as that. Slowly the land above the arch was eroded away leaving rock fins (rock formation seen on the right). More rock was eroded and wind and rain created a cavity, eventually creating the arch.

Metamorphic Rocks

The prefix meta, in this case, means change while morph means form. The term Metamorphic then means to change form. Metamorphic rocks are rocks that are changed by pressure and heat inside the crust. The heat is not hot enough to melt the rock material. Instead, the heat and pressure sometimes rearrange the molecules that are in the rock into different minerals. The heat and pressure can also cause folding in the rock, and change its texture. These processes are known as deformation.

The image below is a sedimentary rock, shale, formed from muds. After shale is put under pressure and heat it can change to the sparkly metamorphic rock, schist.

The Rock Cycle

Energy flows and matter cycles in and out of Earth’s systems through many different processes. In the rock cycle, gravity, weathering and erosion, and heat and pressure all play a huge role in recycling atoms, creating soils, and releasing minerals to be used by organisms. These processes don’t destroy or create new matter, but instead rearrange atoms as they move through Earth’s systems.

In the rock cycle, rocks can change to any other rock type. Melting can turn any rock back into magma that will eventually crystallize into an igneous rock. Weathering and erosion can turn any rock type into sediment which through deposition, pressure, and cementation, can turn into a sedimentary rock. Burial, heat and pressure can turn any rock type into a metamorphic rock.