Magma and lava are both molten rock, but they are different. Magma is molten rock under the Earth’s surface. Lava is molten rock that comes out during a volcanic eruption. These rocks are key in shaping our planet.
The main difference is where they are found. Magma is molten rock under the ground, in the mantle or crust. When magma comes to the surface through vents or cracks, it becomes lava. Lava then cools and turns into igneous rocks, a big part of the Earth’s crust.
Volcanoes have helped humans over time. They create rich soil that grows food and supports life. Knowing about magma and lava helps scientists understand the Earth and its changes.

Chemical Composition: Magma vs. Lava
The chemical makeup of magma and lava is key to their behavior and properties during eruptions. They are classified by their silica levels, which affect their flow, temperature, and the rocks they create through magmatic processes.
Mafic Magmas and Lavas
Mafic magmas and lavas have low silica, between 45% to 55% by weight. They form when the Earth’s mantle melts, often near hot spots or where tectonic plates move apart. These magmas are quite fluid, making them flow easily and produce smooth lava when they reach the surface.
Some key traits of mafic magmas and lavas are:
- High temperatures, between 1,000°C and 1,200°C (1,832°F to 2,192°F)
- Low gas content, leading to less explosive eruptions
- Forming basaltic rocks like basalt and gabbro when they cool
- Common in shield volcanoes and oceanic islands, like Hawaii

Silicic Magmas and Lavas
Silicic magmas and lavas have more silica, between 65% to 75% by weight. They come from melting the continental crust or differentiating from mafic magmas. These magmas are thicker and cooler, causing different eruption styles and rock types.
Some key traits of silicic magmas and lavas are:
- Lower temperatures, between 700°C and 850°C (1,292°F to 1,562°F)
- More gas, making eruptions more explosive
- Creating felsic rocks like rhyolite and granite when they cool
- Common in stratovolcanoes and continental volcanic arcs
Magma/Lava Type | Silica Content | Temperature Range | Viscosity | Associated Rock Types |
---|---|---|---|---|
Mafic | 45-55 wt% | 1,000-1,200°C (1,832-2,192°F) | Low | Basalt, Gabbro |
Silicic | 65-75 wt% | 700-850°C (1,292-1,562°F) | High | Rhyolite, Granite |
Viscosity and Flow Characteristics
The viscosity of magma and lava is key to understanding volcanic eruptions. It affects the shape of volcanoes and the type of lava flows. Knowing how viscosity affects flow is vital for predicting volcanic hazards.
High-viscosity magmas, like those rich in silica, create steep volcanoes. These volcanoes have slopes of 30 to 35 degrees. They can produce dangerous flows and ash.
Low-viscosity magmas form shield volcanoes with gentle slopes. These volcanoes are wide and can be very tall. For example, Mauna Kea and Mauna Loa in Hawaii are shield volcanoes that rise over 9 km above the sea.
Magma Type | Viscosity | Volcano Type | Slope Angle | Example |
---|---|---|---|---|
Basaltic | Low | Shield | < 10° | Mauna Loa, Hawaii |
Andesitic | Moderate | Stratovolcano | 30-35° | Mt. St. Helens, USA |
Rhyolitic | High | Stratovolcano | 30-35° | Crater Lake, USA |
The chemical makeup of magma affects its viscosity. Basaltic magma has more iron and magnesium but less silica. Rhyolitic magma has more silica and less iron and magnesium. Higher silica content means higher viscosity.
The viscosity of magma and lava is a fundamental property that shapes the Earth’s landscape through volcanic eruptions and contributes to the ongoing rock cycle.
