Mars may contain ores that would be very useful to potential colonists. The abundance of volcanic features together with widespread cratering are strong evidence for a variety of ores. While nothing may be found on Mars that would justify the high cost of transport to Earth, the more ores that future colonists can obtain from Mars, the easier it would be to build colonies there. How deposits are made Ore deposits are produced with the help of large amounts of heat. On Mars, heat can come from molten rock moving under the ground and from crater impacts. Liquid rock under the ground is called magma. When magma sits in underground chambers, slowly cooling over thousands of years, heavier elements sink. These elements, including copper, chromium, iron, and nickel become concentrated at the bottom. When magma is hot, many elements are free to move. As cooling proceeds, the elements bind with each other to form chemical compounds or minerals. Because some elements do not bond easily to form minerals, they exist freely after nearly all the other elements have bonded into compounds or minerals. The remaining elements are called incompatible elements. Some of them are quite useful to humans. Some examples include niobium, a metal used in producing superconductors and specialty steels, lanthanum and neodymium, and europium for television monitors and energy-efficient LED light bulbs. After the mass of magma has cooled and has mostly frozen or crystallized into a solid, a small amount of liquid rock remains. This liquid bears important substances such as lead, silver, tin, bismuth, and antimony. Sometimes minerals in the magma chamber are so hot that they occupy a gaseous state. Others are mixed with water and sulfur in aqueous solutions. The gases and mineral-rich solutions eventually work their way into cracks and become useful mineral veins. Ore minerals, including the incompatible elements, remain dissolved in the hot solution, then crystallize out when the solution cools. Deposits created by means of these hot solutions are called hydrothermal deposits. Some of the world's most significant deposits of gold, silver, lead, mercury, zinc, and tungsten started out this way. Nearly all the mines in the northern Black Hills of South Dakota came to be because of hot water deposits of minerals. Cracks often form when a mass of magma cools because magma contracts and hardens when it cools. Cracks occur both in the frozen magma mass and in