Question

During a hike you pick up an interesting looking igneous rock. It is dark in color and has a fine, granular appearance with small, fairly round crystals of a glassy, deep green mineral scattered throughout. Answer the following questions about this rock: a. What mineral is the small, rounded, glassy green crystals likely to be? b. Did the magma from which this rock formed likely originate in the mantle or in the crust? Explain. c. Was the magma likely a very high temperature magma or a fairly low temperature magma? Why? d. What other minerals might make up the rest of this rock?

Short answer

a. Olivine b. Mantle c. High temperature d. Plagioclase feldspar, pyroxene

Step-by-step solution

Identify the Green Mineral

The description of small, rounded, glassy green crystals is characteristic of the mineral olivine. Olivine is commonly found as green crystals in igneous rocks like basalts.

Determine Magma Origin Location

Since olivine is a typical mineral found in rocks that originate from the mantle, we can infer that this rock likely formed from magma that originated in the mantle, rather than the crust.

Assess Magma Temperature

Olivine tends to form in high-temperature magmas. Therefore, the magma was likely of a very high temperature as it allowed olivine to crystallize.

Other Possible Minerals

Since the rock is dark in color and likely basaltic, other minerals that could be present include plagioclase feldspar and pyroxene, which are commonly associated with basalt.

Key concepts

Olivine Mineral

Olivine is a fascinating mineral known for its distinctive deep green color and glassy texture. You often find it in igneous rocks, especially those that exhibit a basaltic composition. Basalt, a common volcanic rock, frequently contains olivine due to its formation process. As olivine crystallizes, it usually forms small and rounded crystals, adding a unique look to any rock sample.
These crystals are intriguing both for their appearance and what they signify about the rock's origin. Geologists use the presence of olivine as a clue to deduce the environmental conditions during the rock's formation. If you ever come across a rock with this mineral, it's not just an interesting piece of nature—it's a window into the processes that occur deep within the Earth.

Mantle-Origin Magma

Magma that originates from the mantle is rich in components that form specific minerals. One such mineral is olivine, which is prominent in mantle-derived magmas. When magma forms in the mantle, it can carry these minerals up to the Earth’s surface.
Mantle-origin magma is typically more compositionally uniform than crust-derived magma. This type of magma tends to bring minerals like olivine, which can crystallize at high temperatures, to the surface in volcanic eruptions.
Therefore, when you discover a rock with olivine crystals, it often implies mantle-origin magma. These findings provide insight into geothermal activities occurring beneath the Earth's crust, offering a glimpse into mantle dynamics.

High Temperature Magma

High temperature magma is critical for the formation of certain minerals, including olivine. Magma temperatures vary, but olivine tends to form at hotter temperatures. When magma is extremely hot, it can facilitate the crystallization of olivine, giving the magma a particular composition once it cools and solidifies.
For a rock to contain olivine, the parent magma likely had a very high temperature. Such temperatures allow olivine to crystallize early during the cooling process, making it a primary mineral in rocks like basalt.
Therefore, if you find a rock with visible olivine crystals, it's an indication of the high-temperature conditions under which it was formed, reflecting the intense geothermal processes involved.

Basalt Composition

Basalts are fascinating igneous rocks known for their fine-grained texture and dark color. They often form from lava flows or volcanic eruptions and are predominantly composed of minerals like plagioclase feldspar and pyroxene.
Basalt's composition sometimes includes olivine, especially when the rock is derived from mantle-origin magma. This combination of minerals gives basalt its characteristic appearance and provides information about its formation conditions.
The presence of minerals such as plagioclase feldspar, pyroxene, and olivine indicates basalt's formation environment, often linked to volcanic activity. Basalt covers vast patches of the ocean floor and is a testament to the dynamic geological activity of our planet.