EXPLORATION: Types of Rock

1. Major Rock Types

Rocks are conglomerations of minerals, and they form the bulk of the Earth. The main classification of crustal rocks is based on their origin.

Igneous rocks form from molten rock called magma, which is emplaced into and onto the crust during volcanic activity. Uplift and exposure of rocks at the Earth's surface can be affected by weathering causing the rocks to break down into smaller grains which are transported and deposited as sediments. The sediments are lithified (compacted and cemented), and sedimentary rocks are formed. Changes in temperature, pressure, and/or rock or fluid chemistry can allow igneous and sedimentary rocks to change physically or chemically to form metamorphic rocks. At higher temperatures, metamorphic (or any other rock type) rocks may be partially melted, and crystallization of this melt will create igneous rocks. Uplift and erosion can expose all rock types at the surface, re-initiating the cycle. The approximate volume proportions of these three rock types throughout the Earth’s crust are: igneous 65%: metamorphic 27%; and sedimentary 8%.

Look at the graphic of the rock cycle below to get a sense of how rocks can change form.

Text and graphics on this page from University College London - Earth Sciences

https://www.ucl.ac.uk/earth-sciences/impact/geology/london/glossary/rocks

2. Igneous Rocks

Igneous
Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock.

Watch the video below to see how igneous rocks are formed. You will learn the difference between intrusive and extrusive igneous rocks.

Examples of this rock type include basalt and obsidian.

3. Sedimentary Rocks

Sedimentary Rocks - are composed of the products of mechanical and chemical weathering. Sedimentary rocks are deposited usually in near-horizontal layers by the agents of transportation. eg wind, water, and ice. Burial produces lithification or hardening.
Sedimentary sources:

• deserts - aeolian deposits
• glacial deposits
• rivers - deltas
• shallow marine areas (fossils)
• deep marine areas
• reefs

Note: deposits from shallow and deep marine areas (i.e. organic deposits) eventually form coal, oil, and natural gas (i.e. fossil fuels)

Watch the video below to see how sedimentary rocks are formed. This video was created for a particular course, so you can ignore directions about filling in a worksheet or looking at a textbook page.

4. Metamorphic Rocks

Metamorphic Rocks - formed by the influence of heat and pressure on other rocks resulting in changes in mineralogy and texture.

• Gneiss: has a banded appearance with alternating composition; may be formed by metamorphism of an igneous or sedimentary rock
• Slate: fine-grained and platy; fissile; formed by the metamorphism of shale.
• Schist: rock flaky minerals (eg mica) aligned in a parallel fashion; formed by the metamorphism of shale and slate.
• Quartzite: hard, mainly composed of quartz; formed by the metamorphism of sandstone.
• Marble: a coarse, but even-grained rock formed by the metamorphism of limestone.

5. Conglomerate Rocks

Conglomerate: composed of large rounded fragments embedded in a fine-grained material (easily eroded because it is made up of tiny particles and layers).

• Sandstone: composed mainly of sand-sized particles, and these are mainly of quartz.
• Arkose: composed mainly of sand-sized particles of quartz and feldspar, the latter making up more than 25% of the total.
• Shale: composed of fine particles predominantly of clay size.
• Limestone: composed of calcite either (1) chemically precipitated in water, or (2) accumulated through the collection of organic remains such as shells, or (3) the result of building up of organic rock through the action of organisms like corals.

Take a look at the image below. See if you can make out different types, sizes, and forms of rocks that make up this conglomerate.