Porphyritic rocks occur when a rising column of magma cools in two phases. The magma is slowly cooled deep inside the crust in the first stage, resulting in massive crystal grains with diameters of 2 mm or greater. As the rock becomes more porous, heat is lost faster, and the remaining magma rises toward the surface where it melts further crystalline material. This second phase of cooling occurs near the surface, and results in spiky crystals about 0.1-1.0 mm in diameter.
Porphyritic textures are common in volcanic rocks such as pumice and scoria. They are also found in lava flows from volcanoes that do not produce large crystals (such as glassy lavas), and in tuff (volcanic rock) deposits where the original size of the crystals has been ground down by erosion.
The presence of porphyritic texture indicates that a rock has undergone a recent episode of melting and resolidification. Deep within the Earth, temperatures are too low for any significant amount of water absorption or dissolution of minerals, so most rocks contain only small amounts of hydrogen either as gas or hydrated oxide molecules. However, near the surface, temperatures often reach levels at which liquid water can exist for some time.
When magma that has been slowly cooling and crystallising within the Earth's crust is abruptly released at the surface, the remaining uncrystallised magma cools swiftly. This texture is seen in most volcanic rocks. The word "porphyry" comes from the Greek words for "purple" and "stone", because of the color of these rocks when they are first discovered.
Porphyries form when molten rock (magma) rises towards the surface of the planet. As this hot liquid rock reaches the surface, it begins to cool rapidly due to lack of contact with any other substance that could keep it warm. When the rock is still very hot, it can contain fluid inside its pores. But as it cools, this fluid turns to gas, which leaves the rock. Without this fluid, the rock cannot grow crystals.
As the rock continues to cool, any material that was inside the cooled lava tube will be left inside the rock once the lava solidifies. So, porphyries often contain crystal fragments from the original lava flow plus materials such as quartz that were present in the surrounding rock when the lava cooled into stone.
Because magma is an extremely reactive mixture of minerals and fluids, it usually forms steep-sided mountains or volcanoes where it emerges from below ground.
What does a porphyritic texture reveal about an igneous rock's cooling history? It shows that crystals developed at depth (slow cooling), and that the magma was later transferred to a shallow depth or erupted (fast cooling). Porphyry is Greek for "many colored" because the rocks often contain various minerals with different colors.
Porphyries are typically composed of fine-grained materials derived from the alteration of volcanic ash or pumice. They can also include large crystals of quartz, mica, or calcite in a matrix of poorly sorted fine-grained material. The presence of these large crystals indicates that the rock has solidified under conditions of low pressure and high temperature, such as those found deep within a volcano. Volcanic rocks that do not contain porphyric textures are typically rhyolites or tuff.
Porphyries were one of the first rock types identified by Earth scientists. They are common in many parts of the world where volcanoes erupt lava that solidifies before reaching the surface. Because they often contain valuable metals like gold, silver, zinc, copper, and platinum, porphyries are important resources for mining companies.
Porphyries have been used as natural sources of colorants since ancient times. Today, they are used in jewelry and sculpture.
Porphyritic Groundmasses with Fine-Grained Textures This porphyritic pattern shows that the lava rested and cooled somewhat beneath the Earth's surface, allowing time for huge crystals to form before bursting onto the surface and rapidly cooling. Because rapid cooling causes the minerals in the rock to fuse together into a glass like substance that is almost impossible to fracture, most of the rock underneath a porphyritic groundmass will be glass.
If the lava did not restenose (i.e., if it continued to flow after reaching the surface), then only small crystals would form as the molten rock cooled quickly under the Earth's surface. No large crystals would have time to form before the lava was covered by more lava or solidified into rocks below the surface. Thus, this texture indicates that the groundmass was formed by lava that had time to cool and crystallize substantially before being covered by more lava.
People can use this information to estimate how long ago the magma first started to cool after rising from the mantle. If we know the age of the volcano, we can calculate how long it took for the lava to reach the surface after it began to melt. Then we can use this number to estimate how long it has been since the last eruption occurred.
Porphyritic: A rock with well-formed crystals visible to the human eye, known as phenocrysts, set in an extremely fine-grained or glassy matrix, known as the groundmass. When lava erupts at the Earth's surface and cools too rapidly for big crystals to form, an aphanitic texture forms. Phenocrysts can be large enough to see with the naked eye, such as quartz, calcite, pumice, and ash beds.
Aphanitic: Without visible crystals; composed entirely of smooth grains or plates. This is the most common type of rock at Earth's surface. Most lava flows quickly after it gushes out of the volcano mouth and cannot form crystals. Instead, it becomes glassy due to rapid cooling.
Glassy: Having the appearance of clear glass; composed of extremely small particles or molecules that are too small to see with the unaided eye. Lava that has cooled too slowly for crystals to form but not yet hardened into glass can still be quite fluid under certain conditions. When this happens it is called spatter lava.
Spatter lava: Formed when hot liquid flows across relatively flat surfaces and then hardens before reaching the edge of the flow field. The result is a pile of balls of glass with some rocks mixed in. Spatter is very light weight compared to solid rock and so can be lifted by strong winds or water currents and transported great distances from its source.