Question

Explain how igneous rock forms from lava.

Short answer

Igneous rock forms when lava cools and solidifies at the Earth's surface.

Step-by-step solution

Understand Lava

Lava is molten rock that reaches the Earth's surface. It originates in the mantle, a layer beneath the Earth's crust, and is considered magma while still beneath the surface. Upon eruption, magma turns into lava.

Cooling Process

When lava exits the earth through volcanic activity, it's exposed to much cooler temperatures at the surface compared to its underground conditions. This exposure to cooler temperatures causes the lava to begin cooling slowly.

Formation of Crystals

As the cooling process continues, minerals within the lava start to crystallize. The rate at which the lava cools can affect the size of the crystals – faster cooling results in smaller crystals, while slower cooling can form larger crystals.

Solidification into Igneous Rock

Once fully cooled and solidified, the lava becomes igneous rock. This type of rock is characterized by the mineral crystals that formed during the solidification process and depends on the environment of cooling, such as whether it took place under the ocean or on land (extrusive igneous rock).

Key concepts

Lava

Lava is the flowing, molten rock that emerges from volcanoes when they erupt. It is incredibly hot, reaching temperatures between 700 and 1200 °C. While still beneath the Earth's surface, this molten rock is known as magma. Once the magma breaks through the Earth's crust during volcanic activity, it becomes lava. The dramatic change from underground to the surface means it is suddenly exposed to much cooler conditions.

Cooling Process

The cooling process begins once lava reaches the Earth's surface. It transitions from a hot, liquid state to a solid form as it loses heat. The environment's temperature at the surface is typically much lower than beneath the Earth's crust. This temperature differential is a driving force behind the cooling process.
The rate at which lava cools can vary depending on several factors:

• Environmental temperature: Colder surroundings lead to faster cooling.
• Type of lava: Different lava types can cool at different rates.
• Surface contact: Lava that spreads thin cools more rapidly than lava in thick flows.

These factors, among others, dictate how quickly the transformation from liquid to solid occurs.

Magma

Magma is the term for molten rock located beneath the Earth's surface. It forms from the partial melting of the Earth's mantle and sometimes crust due to high temperatures and pressures. This aggregate contains dissolved gases and sometimes crystals, although it is primarily a viscous liquid. When magma finds its way up through the crust, due to processes like volcanic eruptions, it vents as lava.
The composition of magma can greatly influence the characteristics of the lava and the resulting igneous rock:

• Silica content: High silica magma is thicker and causes explosive eruptions, while low silica magma results in fluid lava flows.
• Mineral composition: Different minerals present in magma crystallize at different stages during cooling, affecting the type of igneous rock formed.
• Gas content: Gases dissolved in magma escape as it reaches the surface, sometimes explosively if pressure is suddenly released.

Crystallization

Crystallization is an essential part of turning lava into igneous rock. As lava cools, the elements within the molten mixture begin to arrange themselves into structured patterns, forming crystals. The size and formation of these crystals largely depend on the cooling rate of the lava.
Here's how crystallization works:

• Slow cooling: This allows time for larger crystals to develop, which is often seen in intrusive igneous rocks forming deep within the Earth.
• Fast cooling: Leads to the formation of smaller crystals, characteristic of extrusive igneous rocks found at the Earth's surface.
• Sudden cooling: Very rapid cooling can sometimes result in volcanic glass, where minerals do not have enough time to form crystals at all.

Overall, crystallization defines the texture and mineral composition of the resulting igneous rock, giving geologists valuable clues about the history of its formation.