Major groups of rocks found in Earth’s lithosphere:-

All the three types of rock namely Igneous, sedimentary and metamorphic make up the Earth’s lithosphere, the outermost layer. The lithosphere is solid rock. Sedimentary rocks are the most abundant rock only on the surface of the Earth, but igneous and metamorphic are abundant deeper into the mantle.

Igneous rocks:-

1. Igneous rocks come in many varieties. However, all igneous rocks began as molten rock (magma) which cooled and crystallized into minerals. Igneous rocks form when molten rock (magma or lava) cools and solidifies.
2. Igneous rocks may look different because of two factors :- They may have cooled at different rates and the mother magma (original melted rock) was of a different composition. Variations in these two factors have created many different types of igneous rocks.
3. When the magma cools at different rates, it creates different sized minerals. Quick cooling magmas have small minerals (with the exception of obsidian, which is actually composed of silica, but has no crystalline structure).
4. Basalt, for example, has small minerals, most of which can only be seen under a microscope.
5. Magma that cools slowly creates rocks like granite which have large minerals that can be seen with the naked eye.
6. 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
7. Examples of this rock type include basalt and obsidian.

Sedimentary rocks :-

1. Sedimentary rocks originate when particles settle out of water or air, or by precipitation of minerals from water. They accumulate in layers. Generally, sedimentary rock is fairly soft and may break apart or crumble easily.
2. However, sedimentary rocks may also form in dry, desert environments or in association with glaciers.
3. Four main processes lithify sedimentary rock:

• Cementation – Large solid sediments are then bound together by substances like silica, iron oxide and calcium carbonate.
• Compression and Compaction – fine sediments are compacted together as overlying sediments and water increase the pressure above them.
• Chemical Action – Dissolved minerals in water may precipitate out of water, or may be left behind as a result of evaporation.
• Biological Processes – Many forms of aquatic life like clams, coral, and diatoms die and settle to the bottom of the water. Over time they can accumulate and become compressed into a solid mass.

Examples of this rock type include conglomerate and limestone.

Metamorphic rocks are igneous, sedimentary, or preexisting metamorphic rocks that have been changed by great pressures and temperatures within the crust and upper mantle of the Earth.

Metamorphic rocks result when existing rocks are changed by heat, pressure, or reactive fluids, such as hot, mineral-laden water.

• Recrystallization – as a result of intense heat and pressure, many rocks will form new crystals, without actually melting.
• Foliation – this is a layered arrangement of crystals in metamorphic rock. The more intense the heat and pressure, the thicker the foliation.
• Distorted Structure – many metamorphic rocks display a curving or folding of foliations as a result of exposure to intense pressures.
• Increased Density – metamorphisized rocks often have a higher density as a result of being exposed to extreme pressures.
• Contact Metamorphism – when magma (intrusive rock) comes into contact with rock beneath the earth’s surface, the heat metamorphisizes the surrounding rocks.

The temperatures were not enough to melt the rock, otherwise, an igneous rock would have formed.
The rocks that result from these processes often have ribbon like layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface.
Examples of this rock type include gneiss and marble.