7A: Meteorites

Meteorites

Iron meteorites consist of nearly pure iron-nickel alloy. Stony meteorites consist of olivine, pyroxene and feldspar, which are common minerals. Some meteorites also contain diamonds and other gemstones (including olivine and peridot).

Most meteors originate from the asteroid belt between Jupiter and Mars. Thousands of meteorites fall to Earth every year, but only a few of them are found. As it enters the Earth’s atmosphere, a meteor burns and creates a light phenomenon that we can see as a shooting star. The surface of a discovered meteorite is in fact often bumpy, burnt black or rusty.

Most of the meteorite discoveries in Finland have been stony meteorites, as well as one iron meteorite.

1. Stony meteorites:

a) Chondrites contain silicate grains, aka chondrules, often olivine and pyroxene as well as usually 15–30% of iron. This is the most common type of meteorite.
b) Achondrites do not contain chondrules (metamorphosis).

2. The foremost stony-iron meteorite is the pallasite, which has olivine crystals (peridot gemstone!)

3. Iron meteorites always contain 5–25% of nickel, as well as rare metals that help to identify them, such as germanium, gallium and iridium. They are strongly magnetic. The surface of iron meteorites is bumpy, burnt black or rusty. A total of around 5,000 meteorites have been identified. Around 500 meteorites weighing over 0.5 kilograms fall to Earth every year, two thirds of which fall into the sea. Only 5–6 of them are discovered annually. The best location for meteorite discoveries is the Antarctic.

The Crater of Lake Lappajärvi

The crater was around 20 kilometres in diameter. Several glacial periods shaped the crater considerably, and glacial meltwater eventually filled it and turned it into a lake. The round shape of Lake Lappajärvi still recounts the story of an ancient meteorite impact. In addition to the crater, other signs in the surrounding nature show traces of the Lappajärvi meteor – namely the unique rock types. Even though the impact destroyed the meteorite itself, the great heat also created new, unusual rock types, such as kärnäite.

The Cretaceous mass extinction

According to present knowledge, the extinction event was caused by the impact of a massive asteroid. One of the determining discoveries has been the elevated concentration of iridium in the sedimentary layers at the end of the Cretaceous–Paleogene boundary. Iridium is rare in the Earth’s crust but much more abundant in meteorites. On top of that, the Chicxulub impact crater dated back to the same era was found on the Yucatán Peninsula in Central America. The crater was caused by a meteorite of 12 kilometres in diameter. The original crater, now buried in the ground, may have been up to 200 kilometres in diameter and 20 kilometres deep.

The asteroid impact caused earthquakes, wildfires, landslides as well as tsunamis that could be up to a kilometre high. The material that rose to the atmosphere darkened the sky for a long time, lasting several years. Water vapour and sulphur compounds also rose into the atmosphere, resulting in fine particles that blocked out sunlight as well as acid rains that were harmful for living organisms. It has been estimated that even entire food chains suddenly collapsed.

Meteorite Impact

The massive impact explosion released gas and spread it around with the force of a violent tempest. The wind speed close to the impact location has been calculated at up to 1,000 metres per second. The blast wave bringing down trees, the tidal waves as well as the rain of hot rocks were deadly near the impact location and even hundreds of kilometres away. The meteorite impact caused earthquakes and massive landslides, and the hot meteorite fragments resulted in wildfires. The dust and chemicals rising to the air from the impact blocked out sunlight for several years. In the oceans, the photosynthesising phytoplankton died from the lack of light, killing off countless plankton-eating molluscs and consequently, mollusc-eating marine vertebrates. Some deep-sea organisms, shallow-water amphibians and some omnivores as well as decomposers survived. Terrestrial plants perished in extensive areas, resulting in the death of those herbivores that had survived the earlier devastation of the impact. Carnivorous dinosaurs eating the herbivores died accordingly.